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

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

You are still not grasping the fundamental properties of components.

No, a capacitor with AC applied across it does not alternately charge at one end then the other, as already stated, the charge is ACROSS THE ENDS, regardless of polarity. If you apply AC to a capacitor through a resistor, it will charge in the same polarity as the applied voltage but with a delay because of the resistor restricting the flow of current into it, as the applied AC drops below the capacitor voltage, the current will flow the other way, from the capacitor to the source, until there is sufficient voltage in the opposite polarity to start charging it again. Your calculation of 8.3mS is 1/60 seconds or 60Hz but there is a flaw in your thinking, the circuit operates on HALF AC cycles so the AC will drop through zero and turn the circuit off in half that time (4.15mS) and even then it assumes you are delaying the trigger until the last moment just before it would turn off anyway. In practice, the time delay would be anything between 0 and 4.15mS.

In DC circuits where you see capacitors across the supply rails they are still being used to store charge but for a different purpose. Most active devices like the 16F887 in your picture do not draw a steady current. They contain thousands of transistors, many of them switching on and off at high speed and as they do so drawing different currents. It follows that if the power to the device is restricted by resistance or inductance in the power supply feed, the voltage will drop according to the current. It would result in the device power fluctuating at high speed which would almost certainly cause a malfunction. The capacitor does exactly the same as in the SCR example, it stores charge when the voltage across it is above what it already holds and it releases the charge when it holds more than the voltage applied across it, in other words it works like reservoir for the power. It doesn't produce power by itself but you could think of it as being a little rechargeable battery placed at the point where demand is greatest. Incidentally, the supply capacitors are missing from the bottom picture so it may not work!

The other use you mention on the 555 timer is two different examples of capacitor usage. The timing capacitor does not discharge by itself, it charges from the supply through the resistors and that voltage is monitored inside the 555 by a comparator. When a certain voltage is reached (remember that the series resistor controls how fast the voltage can rise) an internal discharge path in the 555 is enabled and the capacitor voltage drops again. When a lower voltage is reached, the 555 turns the discharge path off and the capacitor starts to charge again. The cyclic charging and discharging is what produces the oscillation. The other mention of the capacitor to reduce noise again uses the same principle that charge is held
across the capacitor but in that position it serves a different purpose. The control pin on a 555 sets some internal circuits that determine its switching characteristics, the pin is a voltage input so anything on it will have an effect on the frequency. Here a capacitor is used to keep the voltage steady (reduce noise) by drawing or releasing charge from the control voltage source to help to hold it at fixed voltage, at lease over a short time period.

Brian.

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

Hi,

we are now at over 100 posts in this thread .. and still talking about the very basics of electronics. Voltage, current, resistor, capacitor....

Thus my well meant recommendation:
If you want to go deeper in electronics, please find a (good) school, maybe in your country maybe in another country.
Maybe even online electronics courses could help. There are online tutorials and videos that may help.
Donīt look for poor informations from hobbyists, they often are wrong and incomplete.
Look for high quality informations from semiconductor manufacturers, semiconductor distributors, universities... they usually are for free. Donīt be afraid of "unknown technical terms", bite through them, because they are helpful.
Use the internet search to find high quality informations.
Maybe you have some fellow with electronics knowledge..

A forum canīt replace school. We canīt do private teaching here, as there are more than 600,000 members.
I have to do my main job to earn money, and the forum moderator is just a hobby job for free.

Good luck
Klaus

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

Originally Posted by KlausST
Hi,

we are now at over 100 posts in this thread .. and still talking about the very basics of electronics. Voltage, current, resistor, capacitor....

Thus my well meant recommendation:
If you want to go deeper in electronics, please find a (good) school, maybe in your country maybe in another country.
Maybe even online electronics courses could help. There are online tutorials and videos that may help.
Donīt look for poor informations from hobbyists, they often are wrong and incomplete.
Look for high quality informations from semiconductor manufacturers, semiconductor distributors, universities... they usually are for free. Donīt be afraid of "unknown technical terms", bite through them, because they are helpful.
Use the internet search to find high quality informations.
Maybe you have some fellow with electronics knowledge..

A forum canīt replace school. We canīt do private teaching here, as there are more than 600,000 members.
I have to do my main job to earn money, and the forum moderator is just a hobby job for free.

Good luck
Klaus
Thanks .. you've been a lot of support to me and I know that you and Brain paid much patience that really exceed the limits.

I really get the mistakes of asking trivial questions, I should have focused on the main topic.

Of course I can learn from eBooks but because I'm a bit busy currently I less often try to read them.

So let's leave the fundamentals for my duty, it's not the forum's job to do that.

But how about the last simulation ? Is it near to be proper ?

- - - Updated - - -

Originally Posted by betwixt
You are still not grasping the fundamental properties of components.

No, a capacitor with AC applied across it does not alternately charge at one end then the other, as already stated, the charge is ACROSS THE ENDS, regardless of polarity. If you apply AC to a capacitor through a resistor, it will charge in the same polarity as the applied voltage but with a delay because of the resistor restricting the flow of current into it, as the applied AC drops below the capacitor voltage, the current will flow the other way, from the capacitor to the source, until there is sufficient voltage in the opposite polarity to start charging it again. Your calculation of 8.3mS is 1/60 seconds or 60Hz but there is a flaw in your thinking, the circuit operates on HALF AC cycles so the AC will drop through zero and turn the circuit off in half that time (4.15mS) and even then it assumes you are delaying the trigger until the last moment just before it would turn off anyway. In practice, the time delay would be anything between 0 and 4.15mS.
In DC circuits where you see capacitors across the supply rails they are still being used to store charge but for a different purpose. Most active devices like the 16F887 in your picture do not draw a steady current. They contain thousands of transistors, many of them switching on and off at high speed and as they do so drawing different currents.
This is a serious information, I've found on any website or a forum during my interest in electronics. So it's a benefit of joining this forum.

It's because the chip is drawing different currents ! I've read different PIC datasheets, some of PIC16F877A, many of PIC18F4550 and a little of PIC18F46k22. And I have a strong idea if I go to PIC18F46k22 and read in power section that I won't find this information.

It follows that if the power to the device is restricted by resistance or inductance in the power supply feed, the voltage will drop according to the current. It would result in the device power fluctuating at high speed which would almost certainly cause a malfunction. The capacitor does exactly the same as in the SCR example, it stores charge when the voltage across it is above what it already holds and it releases the charge when it holds more than the voltage applied across it, in other words it works like reservoir for the power. It doesn't produce power by itself but you could think of it as being a little rechargeable battery placed at the point where demand is greatest. Incidentally, the supply capacitors are missing from the bottom picture so it may not work!
Very important information ... thank you so much.

The other use you mention on the 555 timer is two different examples of capacitor usage. The timing capacitor does not discharge by itself, it charges from the supply through the resistors and that voltage is monitored inside the 555 by a comparator. When a certain voltage is reached (remember that the series resistor controls how fast the voltage can rise) an internal discharge path in the 555 is enabled and the capacitor voltage drops again.

When a lower voltage is reached, the 555 turns the discharge path off and the capacitor starts to charge again. The cyclic charging and discharging is what produces the oscillation. The other mention of the capacitor to reduce noise again uses the same principle that charge is held across the capacitor but in that position it serves a different purpose. The control pin on a 555 sets some internal circuits that determine its switching characteristics, the pin is a voltage input so anything on it will have an effect on the frequency.
I think the most magical feature is the discharging effect, when ever a voltage reached a level to trigger certain component over the threshold voltage and so, a capacitor or constant voltage supply through a switch could trigger that device or pin. So the capacitor works as a very fast tank that charges and discharges.

If the capacitor doesn't discharge, then won't have this world of electronics :)

Here a capacitor is used to keep the voltage steady (reduce noise) by drawing or releasing charge from the control voltage source to help to hold it at fixed voltage, at lease over a short time period.

Very clear thanks :)

But also I'm interested in my last simulation. Is it ok ? How about my conclusion and analysis of the 3 currents ?

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

Your last simulation is also incorrect.
A simulation is not a panacea. It is only helpful if you know what you are doing in the first place.

Again as Klaus mentioned earlier, I also respectfully advise you to fill your knowledge gaps first.

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

Originally Posted by schmitt trigger
Your last simulation is also incorrect.
OK, could you tell me which parts are ok and which are not so I work on improving the weak areas ?

A simulation is not a panacea. It is only helpful if you know what you are doing in the first place.
Absolutely, I also mentioned before that simulation won't 100% meet the real world parameters.

Again as Klaus mentioned earlier, I also respectfully advise you to fill your knowledge gaps first.

But the problem, that they mostly have conceptual circuits, so I don't know which values to choose.

Also the conceptual circuits most often to my experience don't have all the required components. Just the main ones as they are the point of the topic.

Also there are a lot of text and many are beyond my understanding, but I can understand some simple parts.
I delete some that really don't have a deep detailed chapter that talks about SCRs, how to drive them, parameters, circuit examples .. etc. Because they eBook just goes through a lot of theoretical concepts and topics and when it comes to a title that have something to do with SCRs it just an conceptual circuit with many equations that I don't understand.

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I actually have learn a lot in this thread, you don't know guys how this thread means to me. I'm really interested in getting out of this thread with robust knowledge about the title. But I really don't have to rely on many details I ask about.

I have to focus on getting the main keys and I should put the designs that really could interest anyone. Anyway, I'm still trying to find a good source of information.

I think applications notes and datasheets are better than heavy eBooks for my needs.

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Again !! Thank you all so much for all the support all the recent days I really enjoyed learning from you all.

I will post for God's wish anything that I think is meaningful in this thread the later days that is much better that what I was trying to achieve in earlier posts.

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