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

[eagle1109]

Hello,


I'm working on Proteus to build SCR triggering circuit and then put some example loads; like, a motor.

I actually don't have a deep understanding of power electronics, I know they are used to big electronic control; like controlling heavy machines that need variable AC voltage.

Of course there are a lot of other applications; they are used in power transmission, heavy drills, compressors, ac motor control, energy conversion, UPS ... etc.

I'm really interested in doing a starting project; like controlling ac motor speed.


But on the web, there are various examples of circuits that driver SCR for a certain purpose. I don't know where to start, I built some examples on Proteus, but when the simulation crashes I realizes that I don't know what I'm doing and why I'm doing it, when I try to modify the circuit I know that I don't know how it works in the first place.

Here's my efforts to do something:

 

[BradtheRad]

Start with the simplest possible simulation to trigger an scr. Apply increasing voltage to the gate (referenced to ground). See what level of current triggers it to conduct.

Try putting the load in different locations, to see which affects triggering, or which affects turnoff, etc.

As for how to trigger at any desired point on the supply sine, your middle schematic might be in the right direction. However I believe the load should be close to the top of the scr.

Also look at the programmable unijunction transistor (PUT, made from two transistors). It's cousin to the thyristor.

 

[eagle1109]

How about examples, are there any manuals or websites that have really good examples with values. So I can proceed to get a grasp on these stuff.

In our course we have the following types of circuits, I'm copying the photos from Google to explain which examples we have in our course book:

1. First circuit, single pulse


2. Two pulse:


3. three phase half wave:


4. three phase full wave:


all that in the text book, but it doesn't teach us how to trigger one !

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These circuits are in the lab manual, but because the lab is equipped from this company with their manual that has more circuits and experiments than the ones in the text book. So there's a little contradiction, but not a big problem, I can mange the course the way I want. But I want to do a good job and deliver the best amount of information I can.

This is the company:

• https://www.hera.de/en/electriclab.html

But I'm surprised that our board isn't on their website now, maybe it's obsolete for them now.

This is our board:


This is the manual:

View attachment 570.058.001 Key to Principles of Power Electronics.pdf

All of them are experiments, everything is behind the board, I don't know what firing circuits they use and how to develop one, this board in my opinion doesn't teach a lot, just easy experiments.

 

[KlausST]

Hi,

Every SCR has different trigger requirements, thus:
* you will find no manual, example, website with common values for all SCRs
* you need to read the datasheet of the SCR you want to use
Generally a datasheet should give all informations (values) you need to know.

Klaus

 

[BradtheRad]

Since it's not easy to find thorough instructions about triggering SCR's, it implies they're not the easiest devices to work with. Each must be triggered individually in a circuit, and attention must be given that current falls low enough through each SCR so it turns off reliably.

Diodes are simple to use so they're used more often.

So if a designer prefers to use SCR's then evidently it's because he sees sufficient advantage in them so the control circuitry isn't such a bother to build and test.

 

[betwixt]

None of the schematics in the first post will do anything useful, either because the connections are incorrect or the values are wildly out of proportion.

I would suggest starting with a simple phase control circuit, search for 'lamp dimmer' and see if you can find one similar (but not the same as) the third schematic in post #1. SCRs are only diodes with a trigger to turn them on, the trick is how to produce the trigger and when to apply it.

Brian.

 

[KlausST]

Hi,

Generally I avoid SCRs for usual circuits.

But for high power, high voltage applications the SCR still have their place.
For a chemical plant we build a current loop control for up to 6000A RMS and up to 2500V RMS. Here we used SCRs. I couldn´t find any suitable other switching device.
For controlling the gate we designed a fast high power driver. The driver is on a 160x100mm PCB since it needs an extra power supply with sufficient creepage distance for the 2500V. It is controlled via a POF for isolation and to avoid inductive influence when swtiching up to 10kA.

The circuit surely is not suitable for a 10A SCR or similar.

Klaus

 

[eagle1109]

Hi,

Every SCR has different trigger requirements, thus:
* you will find no manual, example, website with common values for all SCRs
* you need to read the datasheet of the SCR you want to use
Generally a datasheet should give all informations (values) you need to know.

Klaus

Yep, that pretty much what an engineer is about. That every device has its own values, and so if there are no particular example for that device, then the engineer has to design the circuit which depends on a lot of understanding / experience for the engineering & the fundamental requirements for that system circuit.

Working with circuits is pretty difficult for me because I don't know how to design one and why. Where to put a resistor, where to put a capacitor .. etc. I know that a resistor is used to lower the current, divide the voltage, protecting certain branch in the circuit .. etc.

A capacitor is also used for various purposes; like, decoupling an ac signal, smoothing the ripple voltages, working as a tank for charging/discharging .. etc. An inductor is used also for filtering purposes and increasing branch current.

I know how a transistor should work basically, and many other electronics components but I don't know how to design an electronic system.

So my experience about power electronics is really simple.

I think my most goal now to study is firing circuits, also the different types of power systems that uses certain types of power components and why they are used in that system, which ratings are used, voltages, current .. etc. Power electronics is a whole different world, I thought I know something about it but when I tried to follow the designs I found on Google I realized I really need to study the basics.

 

[KlausST]

Hi,

we could help you through this. Step by step.

Best is if you tell us what you want to achieve (requirements, specifications).

I'm really interested in doing a starting project; like controlling ac motor speed.

when I read this my toughts are:
* Why AC
* what voltage
* what motor current
* why SCR
* ... and so on.

With some of these informations I´d then go to a distributor´s online catalogue to find a suitable SCR.
Then look at the SCR vendor´s internet site to find the datasheet and application notes.
Then you have a lot of informations, schematic, calculation examples...

We (the forum) could do an example design .. but you need to give your requirements.

Klaus

 

[eagle1109]

Since it's not easy to find thorough instructions about triggering SCR's, it implies they're not the easiest devices to work with. Each must be triggered individually in a circuit, and attention must be given that current falls low enough through each SCR so it turns off reliably.

Diodes are simple to use so they're used more often.

So if a designer prefers to use SCR's then evidently it's because he sees sufficient advantage in them so the control circuitry isn't such a bother to build and test.

Absolutely ! Totally agree with you. They are not easy. It amazes me the complex machinery used in factories for manufacturing, in different sizes and functions, some are really small that are used for small manufacturing purposes to fabricate little things that are easy to work with, some machines are so huge/powerful for heavy things; like robot arms.

But SCRs are really special thing, that to our diploma program is really difficult thing to understand because its applications are a lot of course, but why SCRs are used in that particular system is difficult to explain; like, SCRs that are used in solar or wind power generation systems has pretty difficult circuits that are even more difficult to find on the internet at least so far.

We can explain simple things like microcontrollers and connecting some sensors would be relatively easier than explaining power electronics, but that is maybe because I have like no experience with power electronics.

I found this video on YouTube that I really liked, but also that is a complex system, I don't even know how the power components are connected in that system.

 

[KlausST]

Hi,

I don´t agree that SCRs are that complicated.

The function is very basic )it may be used in other configurations, too):
* anode --> load current
* cathode --> load current and return path for the gate current (reference)
* gate

From the beginning the scr is high ohmic (A to C) OFF
When you apply gate current that is above I_GT then the SCR becomes low ohmic (A to C). Load current will flow. ON
Then you may remove the gate current (or not). The SCR still will be low ohmic....
...until the load current (= A current, = C current) drops below a certain level (hold current = I_H). Then the SCR will become high ohmic, preventig load current. OFF. (This automatically happens when AC (current) crosses the zero line)

That´s it. Basically.

Just as an example: read TIC116 datasheet.

Klaus

 

[eagle1109]

Hi,

I don´t agree that SCRs are that complicated.

The function is very basic )it may be used in other configurations, too):
* anode --> load current
* cathode --> load current and return path for the gate current (reference)
* gate

From the beginning the scr is high ohmic (A to C) OFF
When you apply gate current that is above I_GT then the SCR becomes low ohmic (A to C). Load current will flow. ON
Then you may remove the gate current (or not). The SCR still will be low ohmic....
...until the load current (= A current, = C current) drops below a certain level (hold current = I_H). Then the SCR will become high ohmic, preventig load current. OFF. (This automatically happens when AC (current) crosses the zero line)

That´s it. Basically.

Just as an example: read TIC116 datasheet.

Klaus

Yep that's not a very complicated datasheet, simple thyristor with clear ratings.

I know the basic operation of a thyristor, traic. Diac is not very obvious to me so far, I really don't know why a diac would be used to trigger a traic ? Maybe if the designer don't want the traic to be triggered by any voltage and the diac would put the threshold voltage to trigger the triac in both directions.

Also one information that is not clear to me until this moment, which is the traic trigger method, is it that the traic could be triggered by a +ve & -ve signals OR it only triggered by a +ve signal in both directions ? But I know traic is the best device when we want to control an ac device with both +ve & -ve voltages.



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For example, on the following link they say 2 resistors are often connected to the gate. Is that really important ?

The source link:
https://www.electronics-notes.com/articles/analogue_circuits/thyristor-scr-triac/circuit-design-basics-primer.php



Also, on the following link they mention this:

Within photographic flash lights where they act as the electronic switch to discharge a stored voltage through the flash lamp, and then cut it off at the required time.

Link:
https://www.electronics-notes.com/articles/electronic_components/scr/what-is-a-thyristor.php

So my question about this particular point, when using the thyristor with camera flash light, is the thyristor the most appropriate device for this application ? A power transistor or mosfet won't be maybe better ?

 

[KlausST]

Hi,

Diac is not very obvious to me so far, I really don't know why a diac would be used to trigger a traic ?

One does not need a diac to control an SCR, but it helps in some circuits.
A diac usually is used in combination with an RC trigger delay.
An RC causes a time constant tau = R × C, but only if there is no significant load current.
But an SCR gate draws current. The gate current may be low enough not to trigger the SCR, while at the same time avoids the charging of the capacitor.
But with the diac there will be no significant current...the capacitor charges ... until a certain voltage depending on diac and gate voltage), then it immediatly becomes low ohmic...and a lot of energy stored in the capacitor will instantly flow through the SCR gate to trigger it safely.

Don't want all at once: (SCR, triac, diac, triger modes ....). Slow down, let it settle...


Klaus

 

[BradtheRad]

diac would put the threshold voltage to trigger the triac in both directions.

Correct. Lamp dimmers started early to be a common role for triacs decades ago. Where there was a diac, schematics often substituted a neon bulb. NE2/NE2H type were readily available at that time. They fire at a consistent volt level in either direction (60-80V). As a result it's easy to dial a potentiometer so the triac turns on at a precise point on the AC mains waveform.

(I never saw a real diac. None were available at Radio Shack, nor in electronics catalogs I looked at over the years.)

I built a triac project to adjust fan speed. The control circuit was only a potentiometer (about 4M ohm). I realized it was risky to use with house voltage, but it seemed to work as well as the complicated control circuit printed on the back of the triac package I bought at Radio Shack.

The fan was a squirrel cage type on a wood stove. It drew so little current that I could do without a heat sink on the triac. My speed control worked for several years. However it broke the day I attached a window fan.

 

[betwixt]

Just for interest, I had to repair a dead UV water sterilizer unit recently and had to trace it's schematic from the PCB. It used a diac in a clever way to operate an audible alarm:
Rectified AC (230V) from the input fed the main PSU + and also a high value resistor, I think it was 1M. From the bottom of the resistor there was a 100uF capacitor to the negative rail. Across the capacitor was a diac and buzzer in series. Also across it was a transistor biased from the PSU output. Under normal operation, the PSU output kept the transistor conducting and in turn that kept the capacitor discharged. If the output failed (as it had on this unit), the transistor stopped conducting and the capacitor charged through the resistor until it reached diac breakdown voltage, at that point the capacitor discharged through the diac into the buzzer and made the alarm noise.

A simple four component pulsed buzzer, with a transistor to mute it when not needed.

Brian.

 

[eagle1109]

I found this information on the following link:
https://instrumentationtools.com/basics-of-thyristor/

Merits of SCR:
1. SCRs with high voltage and current ratings are available.
2. On state losses in SCRs are reduced.
3. Very small amount of gate drive is required since SCR is a regenerative device.


Demerits of SCR:
1. Gate has no control after the SCR is turned ON.
2. External circuits are required to turn OFF the SCR.
3. Operating frequencies are very low.
4. Snubber circuits are required for dv/dt protection.


Applications of SCR:
1. SCRs are used for controlled rectifiers.
2. AC regulators, lighting and heating applications.
3. DC motor drives large power supplies and electronic circuit breakers

Also this is my current simulation, I don't know why the SCR isn't working ! The trigger pulse is exactly at the start of forward biased voltage.

 

[betwixt]

If that was a real circuit the optocoupler would be embedded in the ceiling by now.
However, the problem is the gate voltage has to be relative to the cathode to trigger it. Your design can only produce any gate current if 'R8(2)' is at a high enough voltage for R8 to conduct enough current to trigger the gate. With a typical SCR it might have to be 50V or more!

Place your load (R1,R2) in the anode side, connect the cathode to the 'ground' side and wire the optocoupler output between the top of X1 secondary and the gate, with a pull down resistor to sink a tiny leakage current. You also need a resistor in series with the LED side of the optocoupler to limit the current it can draw.

Brian.

 

[eagle1109]

My last simulation.

 

[KlausST]

Hi,

Your post#16.

I asked to slow down. Keep it simple, go step by step.
The trouble is sure to follow.

Also this is my current simulation, I don't know why the SCR isn't working ! The trigger pulse is exactly at the start of forward biased voltage.

The TIC datasheet has the load connected to SCR anode, but you connect the load to the cathode.
I told the cathode is the return path for tha gate current (reference).
Now you connected the load to the cathode, thus the voltage at this node varies with SCR state and load current...it is not stable
Your drive gate voltage does not refer to SCR cathode, instead it is referenced to GND (this is not what the datasheet says).
******
--> start with simple circuit like given in the datasheet:
* supply
* SCR (cathode connected to GND)
* load (connected to anode)
Then drive the gate with respect to cathode (= with respect to GND, since cathode is connected to GND)

Use amperemeters to measure gate current, load current
Use voltmeters to measure V_gc, V_ac
= measure the situations, voltages, currents like given in the datasheet

*******
You schematic has several other isssues. Hint: measure the LED current of your optocoupler. In real world it surely will explode with a huge sound and fire.

Klaus

Added:
Just saw your post#18. Much better now.
But I miss some important informations:
* diode type
* SCR type
* transformer output voltage

Klaus

 

[FvM]

Circuit #18 will probably work in a simplified simulation circuit, less likely with real components.

Power SCR need a certain amount of trigger current that can't be provided by an analog opto coupler. For the intended phase angle control, you want to generate a trigger pulse with variable timing. Circuit #18 isn't particularly suited for this purpose.

A versatile method to provide isolated gate signals for multi phase SCR circuits are trigger transformers. Another popular means are opto couplers with triac output like MOC3020. They can be utilized for SCR triggering with an additional rectifier diode. There are also opto couplers with SCR output like Toshiba TLP748.

Post #1 is asking for variable AC voltage, but most of the presented circuits have DC output. You understand the difference?

Phase angle control isn't well suited to operate AC motors will variable speed, except for brushed "universal" motors.

The term "full-wave triggering" in the thread title is misleading. It's mainly used for controllers that switch full sine waves on and off in contrast to phase angle control which is cutting part of the wave.