# Where to get schematics about SCRs full-wave triggering circuits ?

#### betwixt

##### Super Moderator
Staff member
'Ground' in a simulation means the reference point that everything is measured from, it doesn't have to be a connection to the Earth beneath your feet.

There is no issue with placing the load in the cathode side as long as you observe the need to raise the gate voltage above the cathode voltage to trigger it. Keeping the cathode at 'ground' makes triggering easy because the same voltage is needed all the time. If you place the load in the cathode, as a voltage appears across the load the voltage needed on the gate rises, it still has to be Vgt above the cathode voltage.

Note that it isn't normal to place a diode in series with the gate. The component used there is usually a diac, the symbols are similar but their behavior is quite different.

Brian.

eagle1109

### eagle1109

points: 2

#### eagle1109

##### Full Member level 4
'Ground' in a simulation means the reference point that everything is measured from, it doesn't have to be a connection to the Earth beneath your feet.
Yep, OK but when I don't put a ground the output signal doesn't clear any sense and really different than anything know about rectifiers.

There is no issue with placing the load in the cathode side as long as you observe the need to raise the gate voltage above the cathode voltage to trigger it.
yeah .. well ! I don't know almost all circuits that put the load on the cathode, treat triggering as a normal signal that would be 5 or 12V.

But according to the working principle you mentioned, it should be set above any voltage that would be on the cathode; like, hmmm I'm doing a brain storming now trying to imagine how would the +ve/-ve signals of the power supply voltage that would work on either +ve/-ve pulses whether the load is on the anode or cathode.

I think of the thyristor as a power switch or a manual switch that is like the one in home to switch room light on/off. For this normal AC circuit, it would work if either we put the lights on the neutral or live lines.

Keeping the cathode at 'ground' makes triggering easy because the same voltage is needed all the time. If you place the load in the cathode, as a voltage appears across the load the voltage needed on the gate rises, it still has to be Vgt above the cathode voltage.

This is the ordinary scr output I used to know.

Note that it isn't normal to place a diode in series with the gate. The component used there is usually a diac, the symbols are similar but their behavior is quite different.
I got the idea from here:
https://www.electronics-tutorials.ws/power/thyristor-circuit.html

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I have 5 questions about these 5 points.

1. Are these values a universal standards across the world, not in all but in most power systems ?
2. Are 480 VAC system is the standard industrial voltage for primarily inductive loads ? of course could be to power other things than inductive loads; like, motors, huge fans, pumps .. etc.
3. Is the electrical noise caused by switching devices the same as the line distortion ?
4. Is a power factor of one hard to get in most cases ? like, commercial or industrial applications.
5. What's the difference between harmonics and distortions ?

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This is the file I got the info from:

#### betwixt

##### Super Moderator
Staff member
I got the idea from here:
The schematic shows a normal diode but the text calls it a 'trigger diode'. In a real circuit a diac would be used. A diac isn't like a normal diode, it doesn't conduct at all in either direction until a threshold voltage is reached, it then suddenly switches to being almost a short circuit. The idea is the voltage across the capacitor can build up at a rate decided by the series resistor and when diac breakdown voltage is reached, it suddenly discharges into the SCR.

Incidentally, the values in your schematic are not reasonable and they demonstrate a weakness in simulators. They obey the rules without comprehending the practicalities of it running in real life. A 240VAC 100uF capacitor is a big and expensive device, for example:
https://uk.farnell.com/kemet/c44afgp6100zg0j/cap-100-f-250vac-5-pp-can/dp/2664775?st=250VAC
Additionally, more than 40W of heat is produced in RV1 and RV2 !!!
RV1, RV2 and the capacitor are there to create a phase shift so the trigger point doesn't coincide with the instantaneous rise in the AC cycle. If you reduce the capacitance and increase the resistance you get exactly the same effect but with FAR less heat being produced. Consider something like 0.22uF as being a typical capacitor value and adjust RV1 & RV2 accordingly.

Brian.

eagle1109

### eagle1109

points: 2

#### KlausST

##### Super Moderator
Staff member
Hi,

GROUND is just a name. A name for the node which the designer (you) decided to be the voltage reference.
Theoretically any node in any circuit may be called GROUND. It does no necessarily need to be connected to EARTH.
GROUND has no function. It is just the "point of view" to other nodes / voltages.

A simple example.
Use two 9V batteries. Connect one "+" of the one battery with the "-" of the other battery.
Then you get three nodes
* the unconnected "-" of the one battery
* the unconnected "+" of the other battery
* the center node where both batteries are joined.

1) when you decide the center node to be GROUND, then you get a +/-9V supply, because the one free node shows +9V referenced to GND, the other shows -9V.

2) Don't rewire anything. Just call the free "-" node as GROUND. Then you get a +9V / +18V supply.

3) Don't rewire anything. Just call the free "+" node as GROUND. Then you get a -9V / -18V supply.

Klaus

- - - Updated - - -

Hi,

1. Are these values a universal standards across the world, not in all but in most power systems ?
2. Are 480 VAC system is the standard industrial voltage for primarily inductive loads ? of course could be to power other things than inductive loads; like, motors, huge fans, pumps .. etc.
3. Is the electrical noise caused by switching devices the same as the line distortion ?
4. Is a power factor of one hard to get in most cases ? like, commercial or industrial applications.
5. What's the difference between harmonics and distortions ?
1) These are industry standard control signals. They are not (only) related to SCRs. This is NOT the gate trigger signal.
They are widely used as (examples): as temperature measurement, voltage measurement, speed control signal....

2)It depends on country. Here in Europe we have a 230V/400V 50Hz mains system. Transmission voltage may be 10,000V, 20,000V or other high voltages ... all have 50Hz because they are transformer coupled. Industries may have their own power grid.

3) Noise, distortion, overtone, harmonics .... Let's go not into deep with this.
In short:
Noise mainly is used as all unwanted signals/frequencies in a system. Noise frequencies may be random
Distortion is when the waveform of a signal becomes modified.
Overtones = harmonics mainly describe the frequencies. They always are integer multiples of the fundamental frequency.
With a sinusoidal system (like mains grid, which works with sinusoidal waveform) the sum of overtone signals is called distortion.
--> Don't focus on that phrases, better focus on SCR and it's basic function.

4) I don't fully agree with this statement. But a discussion goes too deep for now.

5) see above

Klaus

#### eagle1109

##### Full Member level 4
The schematic shows a normal diode but the text calls it a 'trigger diode'. In a real circuit a diac would be used. A diac isn't like a normal diode, it doesn't conduct at all in either direction until a threshold voltage is reached, it then suddenly switches to being almost a short circuit. The idea is the voltage across the capacitor can build up at a rate decided by the series resistor and when diac breakdown voltage is reached, it suddenly discharges into the SCR.

Incidentally, the values in your schematic are not reasonable and they demonstrate a weakness in simulators. A 240VAC 100uF capacitor is a big and expensive device, for example:
https://uk.farnell.com/kemet/c44afgp6100zg0j/cap-100-f-250vac-5-pp-can/dp/2664775?st=250VAC
I want to know the type of the capacitor ? is it ceramic, electrolytic, polypropylene .. etc ?

There are cheap caps on aliexpress that work on high voltages +240V.

They obey the rules without comprehending the practicalities of it running in real life.
Yep I know that I know that simulation won't be the exact as a real life. It work on the rules that programmers put in and these rules don't contain everything in the real life.

Additionally, more than 40W of heat is produced in RV1 and RV2 !!!
Yes that's a serious issue, if the gate is secured from getting this high power, then it would be also a waste of power for triggering circuit that is meant to work in lowest power possible.

RV1, RV2 and the capacitor are there to create a phase shift so the trigger point doesn't coincide with the instantaneous rise in the AC cycle. If you reduce the capacitance and increase the resistance you get exactly the same effect but with FAR less heat being produced. Consider something like 0.22uF as being a typical capacitor value and adjust RV1 & RV2 accordingly.
I'm trying now and I'm really not following what is going on !! I'm just randomly changing the values of the capacitor and resistors, and also getting simulation warnings, I think because I'm delivering a very high current somethings when I try to change the phase shift. So I have to do a more balance to secure the gate from the high current to reach the point of starting to trigger the scr at 0 to 180 degrees.

I'm now starting the scr directly at like 80-90 degrees I can't get lower than that.

So now I have to do the following:
1. Secure the gate from the excessive current
2. Know how to trigger the scr at 0 - 180 angles.

----------------------------------------------------------------------------------------

Another thing, as it clear on the oscilloscope that there are a very high voltages on the triggering circuit.

I really have to balance that, I can put more high power resistors ! Could that be a start of a solution, then work on the rest of voltages to get the 50mA triggering current.

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I tried this circuit and got a lot of simulation warnings.

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How about this design too ? I think it's really interesting because the author posting it as a project with photos.

#### betwixt

##### Super Moderator
Staff member
I want to know the type of the capacitor ? is it ceramic, electrolytic, polypropylene .. etc ?

There are cheap caps on aliexpress that work on high voltages +240V.
Look at what happens if you set both potentiometers to minimum resistance - you effectively connect the capacitor across the incoming AC. That means the capacitor has to be rated for at least 250V AC, all the cheaper ones are rated for DC only.

Ask yourself: What is the purpose of the phase shift circuit and how is the phase shifted. If you work that out it will go a long way to understanding how the circuit works.

Things to note:
1. An SCR will only conduct in one direction so you will never manage more than 180 degrees conduction angle and in most cases a few degrees less.
2. A Diac in series with the gate will improve performance, try adding one in your simulator.
3. You don't need the extra resistor in series with the gate.
4. Don't believe everything you see on the internet! A lot of schematics are posted by experimenters or students but may not really work.

The bottom schematic is a rather complicated way of emulating a Triac. You can make a Triac by wiring two SCRs in parallel 'head to tail' but they need different firing signals. An alternative way to do this is to use a bridge rectifier in series with the load and to wire the SCR circuit across the + and - bridge outputs.

Brian.

#### FvM

##### Super Moderator
Staff member
As long as the SCRs aren't defective, the voltage across the capacitors can't exceed 8 to 10 V, because one SCR will trigger at this level.

eagle1109

### eagle1109

points: 2

#### eagle1109

##### Full Member level 4
Look at what happens if you set both potentiometers to minimum resistance - you effectively connect the capacitor across the incoming AC. That means the capacitor has to be rated for at least 250V AC, all the cheaper ones are rated for DC only.
How about these ones I found on Aliexpress:

https://www.aliexpress.com/item/32688789282.html?spm=a2g0o.productlist.0.0.59045cdf3PhRCR&algo_pvid=a0085918-88f3-43b4-a80e-14e3a2cc96b3&algo_expid=a0085918-88f3-43b4-a80e-14e3a2cc96b3-0&btsid=0b0a050b15854293646325210ed31e&ws_ab_test=searchweb0_0,searchweb201602_,searchweb201603_

and

https://www.aliexpress.com/item/33038298924.html?spm=a2g0o.productlist.0.0.59045cdf3PhRCR&algo_pvid=a0085918-88f3-43b4-a80e-14e3a2cc96b3&algo_expid=a0085918-88f3-43b4-a80e-14e3a2cc96b3-3&btsid=0b0a050b15854293646325210ed31e&ws_ab_test=searchweb0_0,searchweb201602_,searchweb201603_

and

https://www.aliexpress.com/item/32880393495.html?spm=a2g0o.productlist.0.0.59045cdf3PhRCR&algo_pvid=a0085918-88f3-43b4-a80e-14e3a2cc96b3&algo_expid=a0085918-88f3-43b4-a80e-14e3a2cc96b3-6&btsid=0b0a050b15854293646325210ed31e&ws_ab_test=searchweb0_0,searchweb201602_,searchweb201603_

Ask yourself: What is the purpose of the phase shift circuit and how is the phase shifted. If you work that out it will go a long way to understanding how the circuit works.
To control the output of the thyristor; like DC motor speed control. Mostly SCR is used to control the amount of voltage delivered, sometimes it's used as a switch. Very important in 3 phase rectification and also 3 phase motor speed control.

But I really don't know how this simple RC circuit works with the incoming AC signal. I'm also thinking of would it change the rate of voltage at triggering circuit if I either put the load on the anode or cathode ?

Like now when the load is at the cathode, the voltage at the triggering circuit is +100V, so if I go back and put the load at the anode

Things to note:
1. An SCR will only conduct in one direction so you will never manage more than 180 degrees conduction angle and in most cases a few degrees less.
Yeah I know that it's like a diode but I can control when it pass the voltage in forward bias.

2. A Diac in series with the gate will improve performance, try adding one in your simulator.
ok

3. You don't need the extra resistor in series with the gate.
I got that from the datasheet, but anyway the most important thing is to control the amount of current passed to the gate and the related phase control.

4. Don't believe everything you see on the internet! A lot of schematics are posted by experimenters or students but may not really work.
Yep .. pretty much. Even text books put theoretical designs or conceptual circuits.

The bottom schematic is a rather complicated way of emulating a Triac. You can make a Triac by wiring two SCRs in parallel 'head to tail' but they need different firing signals. An alternative way to do this is to use a bridge rectifier in series with the load and to wire the SCR circuit across the + and - bridge outputs.
Yep a bridge with scr at the output is like controlling the DC voltage. But a triac is controlling the whole AC signal both +/- pulses.

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GROUND is just a name. A name for the node which the designer (you) decided to be the voltage reference.
Theoretically any node in any circuit may be called GROUND. It does no necessarily need to be connected to EARTH.
GROUND has no function. It is just the "point of view" to other nodes / voltages.
Yeah it's the area where the lowest voltage is at, that would be the 0V. That really explains the natural physics of electrons and protons and the electromagnetic involved.

A simple example.
Use two 9V batteries. Connect one "+" of the one battery with the "-" of the other battery.
Then you get three nodes
* the unconnected "-" of the one battery
* the unconnected "+" of the other battery
* the center node where both batteries are joined.

1) when you decide the center node to be GROUND, then you get a +/-9V supply, because the one free node shows +9V referenced to GND, the other shows -9V.

2) Don't rewire anything. Just call the free "-" node as GROUND. Then you get a +9V / +18V supply.

3) Don't rewire anything. Just call the free "+" node as GROUND. Then you get a -9V / -18V supply.
I actually tried to use the LM741 with full +/- supply voltages to amplify a small signal but I didn't succeed, even I used another LM324 that has 2 op amps, but I didn't know how to run it. I don't really remember how I used the supplies but I remember that I used this method of dividing the voltages and connect the IC GND in the middle so I split the whole voltage and how I tested that I have both +/- voltages is that I put the black probe of the DMM at the ground level and the red probe at the +/- supply pins.

And I really had +/- volts, but also that didn't work !! I stopped everything and bought this simple amplifier:

and I remember it worked. But the people who worked with me in that project didn't continue the work so I left the project, as it was a simple one. And I posted a thread about it here in this forums, I think it was a year ago.

1) These are industry standard control signals. They are not (only) related to SCRs. This is NOT the gate trigger signal.
They are widely used as (examples): as temperature measurement, voltage measurement, speed control signal....
ok, so EEET people standardized these numbers in many things to get things in more simpler rating. Like the standardized values of resistors, capacitors, voltages, ... etc.

Yep I understand now.

3) Noise, distortion, overtone, harmonics .... Let's go not into deep with this.
In short:
Noise mainly is used as all unwanted signals/frequencies in a system. Noise frequencies may be random
Distortion is when the waveform of a signal becomes modified.
Overtones = harmonics mainly describe the frequencies. They always are integer multiples of the fundamental frequency.
With a sinusoidal system (like mains grid, which works with sinusoidal waveform) the sum of overtone signals is called distortion.
--> Don't focus on that phrases, better focus on SCR and it's basic function.
This was and still a complicated topic for me, because I don't work around these topics; like, my most work is with simple microcontroller projects. I don't know anything about harmonics and distortions.

But I know that there are things that could affect an electrical/electronic system, like, spikes, inrush current, distortion of switching, bouncing signals, EMI ... etc.

#### betwixt

##### Super Moderator
Staff member
As long as the SCRs aren't defective, the voltage across the capacitors can't exceed 8 to 10 V, because one SCR will trigger at this level.
FvM - not according to the schematic in post #42. With the pots at minimum resistance it places 240V AC across the 100uF capacitor.

To control the output of the thyristor; like DC motor speed control. Mostly SCR is used to control the amount of voltage delivered, sometimes it's used as a switch. Very important in 3 phase rectification and also 3 phase motor speed control.
Not quite true, if you take your half cycles and trigger at 90 degrees from zero crossing, you get peak voltage (turning on at the crest of the half-wave) but if you trigger sooner it will already be conducting as it passes the peak so the maximum voltage will be the same. What does change is the time it passes current and hence the average power it delivers to the load.

But I really don't know how this simple RC circuit works with the incoming AC signal. I'm also thinking of would it change the rate of voltage at triggering circuit if I either put the load on the anode or cathode ?
It does change the trigger voltage but the reason it is used is because it delays the wave shape at the trigger. Remember that a capacitor takes time to charge up and to discharge. How quickly that can happen depends on how much charging and discharging current you allow in and out of it. With AC applied through a resistance (the potentiometers), there will be a time lag between the AC rising and the voltage across the capacitor rising, as the AC drops, the capacitor voltage will drop after a similar delay. As you change the potentiometer values you control the amount of delay between the applied voltage and the capacitor changing and therefore the point at which the SCR will trigger. The same principle applies in single phase and multi-phase AC supplies although it is unusual to use RC phase control in 3-phase circuits.

I got that from the datasheet, but anyway the most important thing is to control the amount of current passed to the gate and the related phase control.
You can get the same phase shift with a smaller capacitor and higher value resistors. The higher value resistors will limit the gate current by themselves. Note that it is normal to have some fixed resistance and a single potentiometer so their combined value can't go down to zero.

Yep a bridge with scr at the output is like controlling the DC voltage. But a triac is controlling the whole AC signal both +/- pulses.
Wrong I' afraid!
Try simulating AC fed to a bridge rectifier through a lamp but with nothing at the bridge output at all. The lamp will not light. Now add a diode across the bridge output, anode to the positive side. The lamp will turn on and it will have AC across it. Substitute the diode with your circuit and you will see it is possible to control AC with an SCR.

Brian.

eagle1109

### eagle1109

points: 2

#### dick_freebird

Old SCR databooks sometimes had a lot of application circuits.

Here's a list of RCA app notes they called out back in 1978.

I'd bet that "application note scr trigger" would turn up a
hit or two. Or 5,640,000.

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eagle1109

### eagle1109

points: 2

#### FvM

##### Super Moderator
Staff member
FvM - not according to the schematic in post #42. With the pots at minimum resistance it places 240V AC across the 100uF capacitor.
Sorry, I was referring to a different schematic, on the post #45 bottom, the only meaningful schematic in this post. The circuits in post #42 and #45 with load at the cathode side have also other issues, they kill the SCR with a large negative gate-cathode voltage after triggering.

eagle1109

### eagle1109

points: 2

#### eagle1109

##### Full Member level 4
Not quite true, if you take your half cycles and trigger at 90 degrees from zero crossing, you get peak voltage (turning on at the crest of the half-wave) but if you trigger sooner it will already be conducting as it passes the peak so the maximum voltage will be the same. What does change is the time it passes current and hence the average power it delivers to the load.
I didn't understand that completely, it would take me time to understand specific process of triggering the scr.

It does change the trigger voltage but the reason it is used is because it delays the wave shape at the trigger. Remember that a capacitor takes time to charge up and to discharge. How quickly that can happen depends on how much charging and discharging current you allow in and out of it. With AC applied through a resistance (the potentiometers), there will be a time lag between the AC rising and the voltage across the capacitor rising, as the AC drops, the capacitor voltage will drop after a similar delay. As you change the potentiometer values you control the amount of delay between the applied voltage and the capacitor changing and therefore the point at which the SCR will trigger. The same principle applies in single phase and multi-phase AC supplies although it is unusual to use RC phase control in 3-phase circuits.
I think I go the idea, that it's about the time of charging the capacitor and of course with a big resistance, charging time is more and hence, triggering the scr with delayed time than triggering it at 0 degrees.

Fast charge = immediate trigger voltage at the gate which means the scr trigger at the start of the sine wave
Late charge = delayed trigger voltage at gate meaning the scr should trigger any point after 0 degrees

You can get the same phase shift with a smaller capacitor and higher value resistors. The higher value resistors will limit the gate current by themselves. Note that it is normal to have some fixed resistance and a single potentiometer so their combined value can't go down to zero.
I got this simulation, it's better but as soon as the scr conducts, I get simulation error.

Try simulating AC fed to a bridge rectifier through a lamp but with nothing at the bridge output at all. The lamp will not light. Now add a diode across the bridge output, anode to the positive side. The lamp will turn on and it will have AC across it. Substitute the diode with your circuit and you will see it is possible to control AC with an SCR.
Oh yeah I got the idea ... yep the diode would act like a switch to the circuit.

I tried the circuit but I have to add a resistor to avoid short circuit.

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Trying the diode circuit.

It'a working ok but I also have some concerns about the location of the ground.

When the lamps are off the simulation of course isn't complaining.

But when reach 1.1A, the simulation give me the warning ! Is it because I exceeded the lamp's max current ? As I guess there is only the voltage limitation and I adjusted it to 240V.

And have 24ohms resistance.

#### KlausST

##### Super Moderator
Staff member
Hi,

I got this simulation, it's better but as soon as the scr conducts, I get simulation error.
Didn't we recommend several times to put the load at tha anode side?

May I ask where you see the benefit when you repeatedly connect it to the cathode side?

Again: it just causes trouble ... and as FvM correctly mentioned it will kill the SCR (gate).
--> follow the recommendation, connect the load at the anode side and see what happens.

I still recommend not to do random trials and error. I don't think it gives a good learning curve.
Start with the most basic circuit. Learn to understand it, learn to calculate the part values, timing, voltage, current.

After that ... use the existing circuit and modify only one part at once...then run the simulation again and see what's the result.

Klaus

#### eagle1109

##### Full Member level 4
Hi,

Didn't we recommend several times to put the load at tha anode side?
Yes that's correct and I'm putting two simulations side by side, one has the load on anode and the other on the cathode.

May I ask where you see the benefit when you repeatedly connect it to the cathode side?
Because of scr output.

With load connected on cathode, I get the known scr output voltage portions.

But when I put the load on the anode, I get the opposite, the negative pulse that hasn't passed the scr and the rest of the voltage that hasn't passed the scr too. So the problem I don't get the shape of the output voltage that actually passed the scr.

Again: it just causes trouble ... and as FvM correctly mentioned it will kill the SCR (gate).
--> follow the recommendation, connect the load at the anode side and see what happens.
I started to understand this point because the voltages are so high on the cathode with respect to gate.

I still recommend not to do random trials and error. I don't think it gives a good learning curve.
Start with the most basic circuit. Learn to understand it, learn to calculate the part values, timing, voltage, current.

After that ... use the existing circuit and modify only one part at once...then run the simulation again and see what's the result.
ok thanks

- - - Updated - - -

Here's my new simulation with the load on the anode.

The voltage at the gate doesn't exceed 40V which is good.

But I can't get the scr to full conduct. I think I should remove the diac.

#### KlausST

##### Super Moderator
Staff member
Hi,

But when I put the load on the anode, I get the opposite, the negative pulse that hasn't passed the scr and the rest of the voltage that hasn't passed the scr too. So the problem I don't get the shape of the output voltage that actually passed the scr.
Yes, for sure. It does what it should do. It measures the voltage at the given node with respect to GND.
If you want to measure the voltage across the load, then you have to measure one node of the load with respect to the other node of the load.
= differential measurement. This is the same as with a real scope.

The proteus scope can do this. It has a function [A + B] = button on Channel A (and also [C + D]). This means it can ADD the voltage on input A and the voltage on input B.
But we want the DIFFERENCE. Thus we need to invert ONE input. The proteus scope can do this. See the [Invert] Button at each channel.

Do this:
* connect scope input A to the upper connection of the load
* connect scope input B to the lower connection of the load
* scope setup channel A --> press [A + B]
* scope setup channel B --> press [Invert]

BTW: I never used Proteus. I found the proteus scope info within one minute on the internet.
OK, I know it´s more easy to find such informations when one know "what to look for".
But it should motivate you to use the internet on your own.
And yes, I know reading manuals, datasheets, turorials... and so on is boring. But I see no way around it. Even we professionals must do it. Every day.
But I can tell you: the more often you do it the more familiar you get with it ... to focus on what you are looking for .. without the need for reading the whole (1000 pages of a) datasheet.

--> It becomes easier each day.

And: Talk to us. Seeing that you ignore our recommendations (to connect the load at the anode side) is not motivating. But as soon as we know why you do this .. we can understand it and find solutions.

Klaus

#### eagle1109

##### Full Member level 4
Hi,

Yes, for sure. It does what it should do. It measures the voltage at the given node with respect to GND.
If you want to measure the voltage across the load, then you have to measure one node of the load with respect to the other node of the load.
= differential measurement. This is the same as with a real scope.

The proteus scope can do this. It has a function [A + B] = button on Channel A (and also [C + D]). This means it can ADD the voltage on input A and the voltage on input B.
But we want the DIFFERENCE. Thus we need to invert ONE input. The proteus scope can do this. See the [Invert] Button at each channel.

Do this:
* connect scope input A to the upper connection of the load
* connect scope input B to the lower connection of the load
* scope setup channel A --> press [A + B]
* scope setup channel B --> press [Invert]
Yep I did it !

But:
1. I can't get the scr to full conduction.
2. I get simulation warning.
3. Gate current is 10mA where it should be > 50mA.
4. After some time I got simulation fatal error with this log:

ISIS Release 8.09.00 (Build 27865) (C) Labcenter Electronics 1990- 2019.
Compiling design 'C:\Users\wopre\OneDrive\Documents\programming\theory_simulation_proteus\power\scr_basic\scr_basic.pdsprj'.
Netlist compilation completed OK.
Partition analysis completed OK.
Simulating partition [299EDFF7]
PROSPICE 8.08.00 (Build 27367) (C) Labcenter Electronics 1993-2019.
Loaded netlist 'C:\Users\wopre\AppData\Local\Temp\LISA0191.SDF' for design 'scr_basic.pdsprj'
Simulation is not running in real time due to excessive CPU load.
Logging stopped - check times specified in Configure Diagnostics dialogue form.

BTW: I never used Proteus. I found the proteus scope info within one minute on the internet.

OK, I know it´s more easy to find such informations when one know "what to look for".
I really don't have a clear idea of what I'm doing, I thought power electronics would be easy. Of course when you have pre-manufactured lab board and I just have to connect some wires, then I would think I'm the king of power electronics. But the real stuff starts by trying to build a simple scr circuit.

Pre-manufactured lab boards have no benefits except to do your lab experiments and get your marks to pass the diploma program. But I get the diploma certificate with 0.001% knowledge of what is electronics circuits and how to design one.

But it should motivate you to use the internet on your own.
I search like often but not always, I mostly do my search about microcontrollers and little about electronics.

Now these days I find searching for power electronics is a relatively difficult as I'm downloading files, most of them are basic, and not getting to the real stuff.

And yes, I know reading manuals, datasheets, turorials... and so on is boring. But I see no way around it. Even we professionals must do it. Every day.

But I can tell you: the more often you do it the more familiar you get with it ... to focus on what you are looking for .. without the need for reading the whole (1000 pages of a) datasheet.

--> It becomes easier each day.

I have a lot of PDF files of manuals and datasheets, and lately I revised what I have and deleted the old files that I got the basics of it and don't need them very much and kept the rest that I haven't finished or didn't start exploring through its chapters.

And: Talk to us. Seeing that you ignore our recommendations (to connect the load at the anode side) is not motivating. But as soon as we know why you do this .. we can understand it and find solutions.
I thought connecting the load to the anode or cathode is just a matter of a style as many circuits on the web have the load on the cathode.

But still with the load on the anode, there's a high voltage on the gate when the triggering circuit is off. So how the voltages on the triggering circuit should be done ? I really have like a very little idea of how to design electronics circuits. I studied electronics for 5 years and I'm an electronics trainer for like 6 years and I'm still a noob in these stuff. I started to tell myself "Don't ever tell your self that you know something about electronics" seriously, I consider myself a hobbyist, not even a professional one or maybe in the middle. I start my day with some gaming, then some search on the web for my work. I don't have serious electronics projects.

#### KlausST

##### Super Moderator
Staff member
Hi,

well done

But:
1. I can't get the scr to full conduction.
2. I get simulation warning.
3. Gate current is 10mA where it should be > 50mA.
4. After some time I got simulation fatal error with this log:
1) typical with SCR and triacs. The gate needs some current to trigger, the diac needs some voltage to trigger, the capacitor needs some time to charge....
2) It´s no error, just a warning. Never used Proteus..
3) I don´t know why do you expect 50mA? And you don´t measure gate current, you measure capacitor input current: It is 202V / (4% * 50k + 10k )Ohms = 202V / 12k Ohms = 16.8 mA .. seems the voltmeter rounds down
4) what does "Configure Diagnostics dialogue form" say?

But still with the load on the anode, there's a high voltage on the gate when the triggering circuit is off.
How do you come to this conclusion?
Do some tests:
* Connect scope channel C to the SCR gate
* Connect scope channel D to C6
Show the results.

With a 10k resistor from SCR_Gate to SCR_Cathode you may reduce the effect of floating gate. But in a real circuit it usually is not necessary. (but sometimes it´s a benefit)

Klaus

#### eagle1109

##### Full Member level 4
1) typical with SCR and triacs. The gate needs some current to trigger, the diac needs some voltage to trigger, the capacitor needs some time to charge....

2) It´s no error, just a warning. Never used Proteus..
Yeah, first I get the warning, but after some time I get the fatal error.

I know that there are differences between the real and simulation, but I think warning means that there is something not quite right, sometimes it turns out the there's excessive current somewhere in the circuit.

error message of course means that the circuit contains a serious mistake in the design.

3) I don´t know why do you expect 50mA? And you don´t measure gate current, you measure capacitor input current: It is 202V / (4% * 50k + 10k )Ohms = 202V / 12k Ohms = 16.8 mA ..

Because this specific model of scr needs 50mA to start triggering.

But I think there's no relationship between the amount of current that goes into the gate and the phase control.

So when I try to change the pots at triggering circuit, there is of course more current, but that doesn't work on the exact point of the sine wave, it's actually the matter of charge/discharge timing on the capacitor into the gate.

So lower resistance = faster charge/discharge = more current into the gate << is the variation of current going into the gate a normal issue ? I think in this circuit, I can't control the amount of current going into the gate, if I want to control the power consumption of the firing circuit.

seems the voltmeter rounds down
I discovered another nice feature, I can change it to milliamps.

4) what does "Configure Diagnostics dialogue form" say?
The list of messages I posted. Yes it should list the specific problem and the location, that would be awesome in this wonderful simulator.

How do you come to this conclusion?
Do some tests:
* Connect scope channel C to the SCR gate
* Connect scope channel D to C6
Show the results.

With a 10k resistor from SCR_Gate to SCR_Cathode you may reduce the effect of floating gate. But in a real circuit it usually is not necessary. (but sometimes it´s a benefit)
Yeah I'm sorry I thought there would be +200V on the gate while I forgot that there is a switch open.

Also another note: I raised the voltage of the capacitor, as it's not necessary for that high rated capacitor. Because after designing the circuit I should know the max voltage that the capacitor should handle and that specific part of the design which is the triggering circuit.

So I guess, I have to balance the voltages which the scr start to conduct, to the part the get the scr to full conduction. Then cut the unneeded resistance and measure the max voltage on the capacitor. I hope it's a simple as that.

- - - Updated - - -

Now what is wrong in the situation ?

I get a warning, but I think the circuit is balanced.

Also another inquiry, how the scr started to conduct as the current drawn is 2.35mA ?

The DB4 main parameters on ST website says:

VBO : 32 V and 40 V
Low breakover current
Breakover voltage symmetry : 3V
ECOPACK®2 compliant
But the voltage is at 22V.

#### KlausST

##### Super Moderator
Staff member
Hi,

Also another inquiry, how the scr started to conduct as the current drawn is 2.35mA ?
Because this specific model of scr needs 50mA to start triggering.
As already mentioned: You don't measure the gate current.
--> If you want to know the gate current, then measure the gate current.

And we don't talk about steady current, we talk about a trigger current = a short pulse. You can't measure a short pulse with a voltmeter.--> use the scope.

Klaus

eagle1109

### eagle1109

points: 2

#### eagle1109

##### Full Member level 4
Hi,

As already mentioned: You don't measure the gate current.
--> If you want to know the gate current, then measure the gate current.

And we don't talk about steady current, we talk about a trigger current = a short pulse. You can't measure a short pulse with a voltmeter.--> use the scope.

Klaus
oh yeah .. thanks

really fundamental things.

I really should have thought of that .. it's an ac signal. And the value in the voltmeter is the rms value ! It's a huge different thing !

One huge benefit of the oscilloscope.

There are areas where an rms value would do, but I think much more depends of the actual signal.