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

1. ## Re: Where to get schematics about SCRs full-wave triggering circuits ?

A diac is in fact a triac, with internal resistors such that it breaks down to a volt or so ( as long as there is sufficient current to keep it on ) once the off state firing voltage is reached

2. ## Re: Where to get schematics about SCRs full-wave triggering circuits ?

Originally Posted by KlausST
Hi,

When the switch is open, then node may be seen as "floating". Floating means "undefined" .... anything can happen.
That's why I wrote "2) the switch is of no help. It makes things more difficult.".

An "open switch" is not the same as "Zero volts".

Klaus
Here's another simulation I did, and I think the diac here is continuously in conduction mode, the effect of oscillating as Brain told me that it should happen but it isn't so I think the simulation abilities here isn't perfect.

My understanding to what Brain told me, is that when the capacitor is charged up with the amount of voltage that should trigger the diac, it discharges and the diac conduct. As the capacitor is charging up again the diac should go back to off state. But that didn't happen, so anyway I don't know how to do it.

But I can change my experiment to use the UJT circuit or run the diac with an AC voltage source. What you think I should do ?

Here's the simulation.

The source voltage is 100V, R3 has a drop voltage of 38.9V the rest is 61.1V and R9 is also 38.9V. So should the voltage that conduct the diac ? so 61.1V - 38.1V = 22.2V. I don't know in the datasheet they mention that DB4 has breakover voltage from 35 - 45V.

Edit:
I increased the capacitor value so be able to observe the rise/fall of output voltage. If I return it to 100nF, the rise/fall would be so thin and zooming in the time on the scope actually diminish it and I can't go back.

3. ## Re: Where to get schematics about SCRs full-wave triggering circuits ?

Hi,

My understanding to what Brain told me, is that when the capacitor is charged up with the amount of voltage that should trigger the diac, it discharges and the diac conduct. As the capacitor is charging up again the diac should go back to off state. But that didn't happen, so anyway I don't know how to do it.
Please tell which of Brianīs posts are you talking about. Canīt remember he talked about oscillation. But maybe...

Klaus

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4. ## Re: Where to get schematics about SCRs full-wave triggering circuits ?

With 100V and 10K in series it is possible the current through the Diac is sufficient that it doesn't turn off. That's why I wanted you to try 100K as well so you could see how it works. Instead you have added another 10K resistor where it would limit the discharge current from the capacitor and stop it oscillating.

Please go back to where I first described the Diac across the capacitor, with a 100K resistor and all you need to monitor is the voltage across the Diac/Capacitor with the oscilloscope. Set it to single shot with positive edge trigger, DC coupling and try a 1 second sweep time.

Brian.

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5. ## Re: Where to get schematics about SCRs full-wave triggering circuits ?

OK, I only got one pulse, then it continues to have 22V output.

I just changed the capacitor value, to have more obvious charging time.

It charges until 42V ti drop to 22V contiuously.

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Originally Posted by KlausST
Hi,

Please tell which of Brianīs posts are you talking about. Canīt remember he talked about oscillation. But maybe...

Klaus
In #74. He didn't say oscillating exactly :)

He said a sawtooth waveform .. and I also got the idea before that, that he wants to tell me about the simplest demo to observe the working principle of the diac.

6. ## Re: Where to get schematics about SCRs full-wave triggering circuits ?

Hi,

Eventually we see the voltage at the capacitor rising until the diac triggers.
But - according datasheet - I can't see why it should stay at 22V. I don't even expect near 22V.

Out of curiosity I will add the DB4 to my shopping cart. It will take some (many? In times of corona) days...

In the meantime you may try another diac .... not from the same library.

Klaus

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7. ## Re: Where to get schematics about SCRs full-wave triggering circuits ?

Yep, it should work in reality. But I believe the simulator didn't cover this effect.

I found it here:
http://www.bristolwatch.com/ele/diac.htm

It's the diac relaxation oscillator, there's also another one for the UJT.

So at this point, I think I should continue my work on triggering the SCR, and try to modify the AC circuit to get the proper diac triggering pulses.

I have now different versions of triggering in my mind:
1. Ordinary RC circuit
2. RC with DAIC
3. RC with UJT
4. Zero crossing detection connected to a UJT

These methods I guess are the most basic triggering circuits, so I should start here and until I develop a good experience about triggering systems.

Of course the world now has more advanced digital triggering systems for the sophisticated systems that have SCR or the other power devices.

8. ## Re: Where to get schematics about SCRs full-wave triggering circuits ?

Racing Klaus.....

This is a DB3 Diac, 100K resistor and 100nF capacitor:

and this is what I see on the scope:

Brian.

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9. ## Re: Where to get schematics about SCRs full-wave triggering circuits ?

Hi,

I have now different versions of triggering in my mind:
1. Ordinary RC circuit
2. RC with DAIC
3. RC with UJT
4. Zero crossing detection connected to a UJT
Form the previous discussion you already should know that 1) not useful ... at least problematic
2) is good for analog phase angle control
4) is good for digital phase angle control

I recommend to go on with 2).
* fixed R, fixed C as timing control
* diac to get proper gate trigger
* measure the gate current with a scope

Don't longer use try and error method.
Calculate the values.
* R to get less than 250mW of power dissipation.
* C to get a useful phase angle

* measure the gate current with a scope
* then compare the scope results with the SCR datasheet limits and decide whether or not you need a current limiting gate resistor.

Show us what you do ... step by step.

Klaus

•

10. ## Re: Where to get schematics about SCRs full-wave triggering circuits ?

Originally Posted by KlausST
Hi,

Form the previous discussion you already should know that 1) not useful ... at least problematic
2) is good for analog phase angle control
4) is good for digital phase angle control
OK that's really good to know.

I recommend to go on with 2).
* fixed R, fixed C as timing control
* diac to get proper gate trigger
* measure the gate current with a scope
This is my next task to do.

But there's a problem in this regard which is that I can't control the amount of voltage that goes to the gate, thus I can't control the amount of current which should be constant during all phase control trigger process.

Don't longer use try and error method.
OK, I really have to get theoretical based calculations otherwise I won't get to the goal signal/current

Calculate the values.
* R to get less than 250mW of power dissipation.
* C to get a useful phase angle

* measure the gate current with a scope
* then compare the scope results with the SCR datasheet limits and decide whether or not you need a current limiting gate resistor.

Show us what you do ... step by step.
Yep I should do the rc tau calculation.

Just a quick experiment from the DB4 datasheet. I did their test circuit, they didn't put what voltages/currents I should get. But this is what I got on the simulation.

But also at the same time, I got a simulation warning as I just adjusted RV2 to 38% so 500k * 38% = 190k ohms.

Also why the signal on the capacitor and after the diac aren't in phase with source signal ? I really didn't the goal of this circuit.

But anyway I'm going back to the basic RC gate trigger. But I have to solve the phase/potential of the signal.

11. ## Re: Where to get schematics about SCRs full-wave triggering circuits ?

I think we are going nowhere fast.
I can't confirm what your simulator tells you because I don't use Windows and as far as I know Proteus doesn't run in Linux. However, it seems to behave strangely and maybe the Diac model is incorrect. The picture in post #88 clearly shows what happens in real life and apart from using fixed components and a DC source, the schematic is essentially the same as in your last simulation. Where the sawtooth waveform suddenly drops is where the Diac conducts, if it had a 20 Ohm resistor in series with it like in your schematic, it would show the pulse as in the data sheet. I'll add a 20 Ohm resistor and show you the result tomorrow (it's too late at night here to start building things!).

Brian.

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12. ## Re: Where to get schematics about SCRs full-wave triggering circuits ?

Originally Posted by betwixt
Racing Klaus.....

This is a DB3 Diac, 100K resistor and 100nF capacitor:

and this is what I see on the scope:

Brian.
The signal appears to be really stable. So this would a very/low cost oscillator for a lot of electronic systems. Where could this oscillator be used and how famous it's in current designs ?

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Originally Posted by betwixt
I think we are going nowhere fast.
I can't confirm what your simulator tells you because I don't use Windows and as far as I know Proteus doesn't run in Linux. However, it seems to behave strangely and maybe the Diac model is incorrect. The picture in post #88 clearly shows what happens in real life and apart from using fixed components and a DC source,
Yep I'm not trusting the simulator like 100% it's at the end of the day a simulator that doesn't cover everything in life.

My goal from this thread is to develop good triggering circuit for half wave controlled rectifier and then try to develop another one for full wave.

I'm considering now to use a diac and a ujt for triggering.

the schematic is essentially the same as in your last simulation. Where the sawtooth waveform suddenly drops is where the Diac conducts, if it had a 20 Ohm resistor in series with it like in your schematic, it would show the pulse as in the data sheet. I'll add a 20 Ohm resistor and show you the result tomorrow (it's too late at night here to start building things!).

But they are using AC voltage and I was just testing their circuit. But it doesn't matter I'm trying to get back to my diac test circuit ..

13. ## Re: Where to get schematics about SCRs full-wave triggering circuits ?

Hi,

But there's a problem in this regard which is that I can't control the amount of voltage that goes to the gate, thus I can't control the amount of current which should be constant during all phase control trigger process.
Quite confusing statement.
"I can't control the amount of voltage that goes to the gate" --> no comment
"thus I can't control the amount of current" --> why not?
"which should be constant during all phase control trigger process." --> constant? Why do you think it should be constant?

Also why the signal on the capacitor and after the diac aren't in phase with source signal ?
You use an RC circuit with AC source and wonder about phase shift ... really?
What do you expect from us?

Btw: where in the thread can we see that you are able to use Ohm's law?

Klaus

14. ## Re: Where to get schematics about SCRs full-wave triggering circuits ?

Hi,

I'm working on this simulation.

I just have a question about the best way to measure the gate voltage and thus calculating the gate current.

Is it enough to measure the rms voltage drop on R1 and then divide that by 0.7071 to get the peak voltage.

So to my calculations:

I(rms) = 0.34V / 10 ohms = 34mA

VR1(rms) = 0.34V >> VR1(p) = 0.34V / 0.7071 = 0.48V

Then, Ig = I(peak) = 0.48V / 10 ohm = 48mA

For ensuring calculations, I(peak) = I(rms) / 0.7071 = 34mA / 0.7071 = 48mA

But with the scope I'm getting that the voltage difference across R1 is 20V. So which is correct ?

Edit:
I forgot to mention the things I added to this circuit.
1. I added a diode to prevent negative voltages across C1 and also since I don't need the negative voltages to have negative triggering pulses which would needed in a TRIAC circuit.
2. I've put a series resistor 4.7k as I found it the most appropriate to get maximum current of 48mA at the gate which is the max rated current for this SCR.

- - - Updated - - -

I just thought of adding a voltage divider at the gate, is it a good idea ? I can split any incoming voltage the the balance I want.

According to the datasheet, if the gate voltage is at max which is 1.5V then the resistor that should be used in series with the gate is 33 ohms, so I tested that and I think I understood a little of how to get close to what the datasheet is telling me.

I used the voltage divider with the gate series resistor and got the required voltage/current values.

•

15. ## Re: Where to get schematics about SCRs full-wave triggering circuits ?

Hi,

Issues:
I just have a question about the best way to measure the gate voltage and thus calculating the gate current.
* The gate voltage is meaningless (as mentioned in posts before, several times)
* you can't calculate the gate current from the gate voltage (at least not in a usual way). Gate_voltage is not proportional to gate_current, non ohmic behaviour, nonlinear
* your scope channel C needs to be inverted (also mentioned before) to get the gate current
Is it enough to measure the rms voltage drop on R1 and then divide that by 0.7071 to get the peak voltage.
No. This relationship (0.707) is only true for non distorted, sinusoidal waveforms without DC offset, which the gate current not is.
* The RMS current is not of interest when triggering an SCR, only the peak current. It can be measured with the resustor and differential scope mode, thus we recommended this method. There should be a peak. The peak triggers the SCR. Before the peak there should be almost zero current .... and is not much of interest. After the peak there may be any current (as long as it does not destroy the gate): zero, negative, positive, low current, high current ... it simply does not matter.
But with the scope I'm getting that the voltage difference across R1 is 20V. So which is correct ?
You measured the sum, not the difference. So your result is not correct.
But at least here you are close to a useful value: invert channel C.
1. I added a diode to prevent negative voltages across C1 and also since I don't need the negative voltages to have negative triggering pulses which would needed in a TRIAC circuit.
First we focus on "safe triggering an SCR" ... in future we will avoid malfunction. Then the diode may be of help. ...later
2. I've put a series resistor 4.7k as I found it the most appropriate to get maximum current of 48mA at the gate which is the max rated current for this SCR.
The maximum allowed gate current is far higher than 48mA.
I just thought of adding a voltage divider at the gate, is it a good idea ? I can split any incoming voltage the the balance I want.
No good idea. How often did we mention before that the gate voltage is not much of interest....I wonder why you still insist in talking about gate voltage....
According to the datasheet, if the gate voltage is at max which is 1.5V then the resistor that should be used in series with the gate is 33 ohms, so I tested that and I think I understood a little of how to get close to what the datasheet is telling me.
The 1.5V is not the allowed limit, the 48mA is not the allowed limit, thus the result is wrong. Not only the result is wrong, but also the way you used. With your way you calculate the internal resistance (although the gate has no resistive behaviour) ... but need the value of an external current limiting resistor.
I wonder where you have the "48mA" from. When I do a search at the datasheet PDF on "48" there are no hits.
and I think I understood a little of how to get close to what the datasheet is telling me.
I would be pleased if I could agree...

"48mA" maybe is the "max gate trigger current" ... not the "max gate current" ( which is 8A in my datasheet) .
The trigger current may vary from part to part (like the size of feet with humans) and it may vary with conditions like temperature.
Some may trigger with 30mA, many will trigger with 40mA, most will trigger at 45mA, but all of this type of SCR at all conditions will trigger with 48mA. Means your external trigger circuit needs to deliver at least (minimum!) 48mA of trigger current to safely trigger these SCR in all conditions.

Klaus

16. ## Re: Where to get schematics about SCRs full-wave triggering circuits ?

Following on from post #91:

I actually used 22 Ohms in series with the Diac but that is irrelevant for this argument.
You will notice the same sawtooth waveform, it rises as the capacitor charges through the 100K resistor then the Diac triggers causing the capacitor to discharge through the diac and 22 Ohm resistor. The bottom trace is across the 22 Ohm resistor so it shows a voltage proportional to the Diac current. I am actually running it from 64V DC because thats the maximum my bench supply can manage but it works from anything from around 40V upwards.
Note carefully that the 100K resistor from a 64V supply cannot pass more than 0.64mA yet the current pulses are at least 7V high which across 22 Ohms means the current must be 318mA. What is happening is the capacitor is providing the diac current, not the 100K although a tiny amount does still come through the 100K as well.

A Diac is bi-directional and symetrical, if you use AC instead of DC it will still work but the pulses will be mirrored when the polarity is reversed. You would have to take into account the AC frequency and the RC time constant, obviously if the supplied voltage doesn't have time to charge the capacitor before it reverses and discharges it again, the Diac breakdown voltage may not be reached.

It is used commercially as a very low cost oscillator but the frequency depends on the applied voltage (hence capacitor charging time) and the tolerance and stability of components which for high voltages may not be too good. It is used for warning lamp flashers, audible alarms and the likes but is far too unstable for most uses.

Brian.

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