TRIAC self triggering

[belkzak]

Hi Guys,

I would like to verify the value dV/dt for my triaca, which is a very critical value for self triggering.
would you please advise how can i do that?
Is is the rate of change of the voltage across MT1 and MT2? where should i take the value of the slope?

thank you

 

[MacIntoshCZ]

Where else would you like to measure it? Yes between MT1 / 2. The oscilloscope will calculate the value for you. Or just do simple math wih analogue one. (V2-V1)/(T2-T1)?

 

[belkzak]

Where else would you like to measure it? Yes between MT1 / 2. The oscilloscope will calculate the value for you. Or just do simple math wih analogue one. (V2-V1)/(T2-T1)?

my triac is used to control the motor speed in a washing machine, i have visualized the signal between MT1/2 as you can see in the picture, at what time should i calculate the derivative (the slope), i mean dv/dt is not constant over time as you know !!

 

[MacIntoshCZ]

When spikes occurs.
This looks most critical:

 

[tomizett]

It's the maximum dv/dt (the steepest slope) wherever that occurs.

 

[rjenkinsgb]

I would like to verify the value dV/dt for my triac

You should use an R-C snubber network across the triac.

That both limits the DV/DT and helps ensure reliable triggering when controlling an inductive load.
Plus it reduces interference.

The only time the DV/DT value is really important is if some other part of the circuit is also switching, as with such as a three phase controller when you need to be wary of the rate of voltage change across devices that should not be switching, as other devices switch.

With a single device running from 50/60Hz power, the only time you get a fast edge is when it has triggered and it's not relevant!

 

[ChrisP58]

The dV/dt spec of a triac is the rate of change in voltage across MT1 and MT2 that the device will tolerate and NOT trigger. The gate is open for this test.

As such, the waveform in post #3 tells you nothing, as that shows triggered operation.

To test the dV/dt of the device, you need to apply a voltage pulse across MT1 and MT2 that has the specified rise time.
- The source of the pulse must source sufficient voltage and current that, if triggered, the device will stay latched.
- The pulse should stay on long enough to see whether the device triggered or not.
- You will need to test this in both polarities.

 

[belkzak]

The dV/dt spec of a triac is the rate of change in voltage across MT1 and MT2 that the device will tolerate and NOT trigger. The gate is open for this test.

As such, the waveform in post #3 tells you nothing, as that shows triggered operation.

To test the dV/dt of the device, you need to apply a voltage pulse across MT1 and MT2 that has the specified rise time.
- The source of the pulse must source sufficient voltage and current that, if triggered, the device will stay latched.
- The pulse should stay on long enough to see whether the device triggered or not.
- You will need to test this in both polarities.

Thank you for the reply,

I understand that you want to check the value mentioned by the constructor doing the test this way, which is really interesting, however, i have few question:

- I dont get what do you mean by the device stay latched, if triggering occurs.
- how to confirm that the device is triggered by making the pulse long enought!
- isn't is risky to apply a voltage between MT1/2 taking into consideration that it becomes a short circuit if the triac is triggered !

Thank you in advance.

 

[rjenkinsgb]

The device DV/DT rating is the safe maximum rate of voltage change across the main terminals when the device is NOT being deliberately triggered.

It's a value which may cause unwanted triggering if exceeded.

Think of the device having internal parasitic capacitance from MT2 to gate as a simple example:

If the rate of change of voltage across the power terminals is too high, that capacitance may supply a high enough internal gate current pulse to make the device falsely trigger.

That is a DV/DT problem.

As I've already said, in a basic single phase control circuit with no other parts causing fast, high-energy pulses across the device, it's irrelevant.

Just use a snubber, which is good practice anyway.

All the information you keep asking about is supplied by the device makers in the data sheets. It's there for a reason - so you do not have to derive the parameters and safe working limits for yourself!

 

[belkzak]

The device DV/DT rating is the safe maximum rate of voltage change across the main terminals when the device is NOT being deliberately triggered.

It's a value which may cause unwanted triggering if exceeded.

Think of the device having internal parasitic capacitance from MT2 to gate as a simple example:

If the rate of change of voltage across the power terminals is too high, that capacitance may supply a high enough internal gate current pulse to make the device falsely trigger.

That is a DV/DT problem.

As I've already said, in a basic single phase control circuit with no other parts causing fast, high-energy pulses across the device, it's irrelevant.

Just use a snubber, which is good practice anyway.

All the information you keep asking about is supplied by the device makers in the data sheets. It's there for a reason - so you do not have to derive the parameters and safe working limits for yourself!

Thank you for the clarification, I got the idea !

i know that the information i am asking about are mentioned in the data sheet, just as a part of a validation process, we need to verify them.

 

[rjenkinsgb]

we need to verify them.

No, you do not.

Unless you test every one of that part in existence, you can never know if a result of such a test is normal or an outlier.

You use the data sheet.
end.

 

[danadak]

A good reference book -

components :: ge :: 1972 GE SCR Manual 5ed : Free Download, Borrow, and Streaming : Internet Archive

From the bitsavers.org collectio n, a scanned-in computer-related document.components :: ge :: 1972 GE SCR Manual 5ed

archive.org

Regards, Dana.

 

[KeepItSimpleStupid]

You have to trust the manufacturer's datasheet.

 

[belkzak]

A good reference book -

components :: ge :: 1972 GE SCR Manual 5ed : Free Download, Borrow, and Streaming : Internet Archive

From the bitsavers.org collectio n, a scanned-in computer-related document.components :: ge :: 1972 GE SCR Manual 5ed

archive.org

Regards, Dana.

Thank you... Very helpful !