Triac Gate sensitivity

belkzak · May 16, 2021

Hi Electronics Guys,
I hope that you are all doing well,
Among a validation plan of my triac, I need to perform a Gate sensitivity triggering test at some defined levels (triggering currents), would you please advise an appropriate circuit to do that?

Thank you in advance

rjenkinsgb · May 16, 2021

Testing is simple - a variable resistor to the gate from a fixed voltage source, or a fixed resistor from a variable voltage source.

Put scope leads on the gate an A1 (the load terminal) and watch the effect as the gate is switched, increasing the current until you get reliable triggering.

However, the trigger effects will be dependant on the load, especially if inductive - and vary from device to device.
Even if you test a hundred devices, you never know if the next is slightly different.

That's why the manufacturers supply datasheets with the possible maximum and minimum of each parameter.

crutschow · May 16, 2021

For ease of testing, apply a small 50-60 AC signal through a small resistor (to get an anode current above its minimum hold current), to the Triac Anode.
That will reset the Triac every half cycle so you can see the triggering sensitivity in both quadrants (it's likely different in each).

belkzak · Jun 3, 2021

rjenkinsgb said:
Testing is simple - a variable resistor to the gate from a fixed voltage source, or a fixed resistor from a variable voltage source.

Put scope leads on the gate an A1 (the load terminal) and watch the effect as the gate is switched, increasing the current until you get reliable triggering.

However, the trigger effects will be dependant on the load, especially if inductive - and vary from device to device.
Even if you test a hundred devices, you never know if the next is slightly different.

That's why the manufacturers supply datasheets with the possible maximum and minimum of each parameter.

I have used a variable resistor to the Triac gate with a 5v square wave signale generator,
I am getting something weird, each time I decrease the resistance in order to get the sufficient current for triggering,Triac keeps getting triggered even when I slightly increase the resistance (decrease current) till I get to a specific value.

Do you know what is exactly this phenomenon?

See these two pictures taken during the test.

crutschow · Jun 3, 2021

belkzak said:
Do you know what is exactly this phenomenon?

Not without seeing a complete schematic of your setup.

belkzak · Jun 8, 2021

crutschow said:
Not without seeing a complete schematic of your setup.

This is the setup that i have used to measure Igt

crutschow · Jun 8, 2021

If V2 is line voltage that is not isolated, that is a dangerous setup.

belkzak · Jun 8, 2021

crutschow said:
If V2 is line voltage that is not isolated, that is a dangerous setup.

Please how do you isolate it

crutschow · Jun 8, 2021

belkzak said:
Please how do you isolate it

You need a transformer with the secondary isolated from the primary (not an autotransformer or Variac).

alec_t · Jun 8, 2021

Is your ammeter designed to give accurate readings of square-wave current?

belkzak · Jun 8, 2021

alec_t said:
Is your ammeter designed to give accurate readings of square-wave current?

Yes, it gives the average value for square signal ( Vp/2)
But anyways, i have also modified the setup to use 5v DC on V1 and 50v DC on V2

belkzak · Jun 8, 2021

crutschow said:
If V2 is line voltage that is not isolated, that is a dangerous setup.

Could you please explain how it can be a dangerous set-up? I'm fact i have tried with this experiment several times, and in some cases i get my circuit breaker shutting down, but other cases it works. Is it about which line wire is connected to MT1 ?

belkzak · Jun 10, 2021

rjenkinsgb said:
Testing is simple - a variable resistor to the gate from a fixed voltage source, or a fixed resistor from a variable voltage source.

Put scope leads on the gate an A1 (the load terminal) and watch the effect as the gate is switched, increasing the current until you get reliable triggering.

However, the trigger effects will be dependant on the load, especially if inductive - and vary from device to device.
Even if you test a hundred devices, you never know if the next is slightly different.

That's why the manufacturers supply datasheets with the possible maximum and minimum of each parameter.

Thank you for your reply,
I have attached the datasheet download from the website, my triac is Q8016RH4, could you please tell me what is the the minimum and maximum triggering current from the datasheet? is 35mA maximum or minimum?

rjenkinsgb · Jun 10, 2021

35mA is the minimum current to guarantee firing under the other conditions given in the datasheet.

Note that specific part is a "three quadrant" device and will not fire in quadrant 4; ie. with gate positive of MT1 while MT2 is negative of MT1.
To guarantee firing with both MT polarities, the gate must be drive negative of MT1 or to the same polarity as MT2.

The peak gate current is in the "Absolute maximum" ratings section; 2A for 10uS.

belkzak · Jun 10, 2021

rjenkinsgb said:
35mA is the minimum current to guarantee firing under the other conditions given in the datasheet.

Note that specific part is a "three quadrant" device and will not fire in quadrant 4; ie. with gate positive of MT1 while MT2 is negative of MT1.
To guarantee firing with both MT polarities, the gate must be drive negative of MT1 or to the same polarity as MT2.

The peak gate current is in the "Absolute maximum" ratings section; 2A for 10uS.

according to the experement that i have done, the triac has fired with only 10mA!!
Besides, can you explain what is this Igt in the following table ?

rjenkinsgb · Jun 10, 2021

belkzak said:
according to the experement that i have done, the triac has fired with only 10mA!!

The datasheet figure is the current required to guarantee firing for any device with that part number.

The datasheet covers many different types in a range & the part you show is an overview of the whole range.
The exact information is given for each individual type in the detailed specifications.

See the table on page 3 of the datasheet - "Electrical Characteristics (TJ = 25°C, unless otherwise specified) — Alternistor Triac (3 Quadrants)"

That gives the maximum gate current that could be required to guarantee firing, for each sub-type of the triac.
You say yours is the H4 suffix type, so that is in the column for 35mA

The gate current needed will vary from device to device and probably also on what quadrant is being tested - see the info at the lower left of page 3.

That's why the manufacturer provides data, so you can design a circuit that can provide the correct current or any parameters, to guarantee operation with any device ever made under that specific part number.

Ramussons · Jun 10, 2021

belkzak said:
This is the setup that i have used to measure Igt

View attachment 131806

Is your 100 Hz ( why 100 Hz?) source Synced to the Mains 50 Hz? If not, you will get non consistent readings. Remember that the Trigger current is also dependent on the voltage across the Triac.

belkzak · Jun 10, 2021

Ramussons said:
Is your 100 Hz ( why 100 Hz?) source Synced to the Mains 50 Hz? If not, you will get non consistent readings. Remember that the Trigger current is also dependent on the voltage across the Triac.

100 hz is just a random frequency chosen, and no, it is not synchronized, i don't see why should it be, the purpose of the test is just to detect at will current value will the Triac trigger.
One more question, how can the voltage across MT1/2 affect Igt?
Thanks.

belkzak · Jun 10, 2021

rjenkinsgb said:
The datasheet figure is the current required to guarantee firing for any device with that part number.

The datasheet covers many different types in a range & the part you show is an overview of the whole range.
The exact information is given for each individual type in the detailed specifications.

See the table on page 3 of the datasheet - "Electrical Characteristics (TJ = 25°C, unless otherwise specified) — Alternistor Triac (3 Quadrants)"

That gives the maximum gate current that could be required to guarantee firing, for each sub-type of the triac.
You say yours is the H4 suffix type, so that is in the column for 35mA

The gate current needed will vary from device to device and probably also on what quadrant is being tested - see the info at the lower left of page 3.

That's why the manufacturer provides data, so you can design a circuit that can provide the correct current or any parameters, to guarantee operation with any device ever made under that specific part number.

Under what circumstances the triggering current will take it's maximum value ?
Thanks

rjenkinsgb · Jun 10, 2021

Repeat:

rjenkinsgb said:
The datasheet figure is the current required to guarantee firing for any device with that part number.

Try a million devices and you may find one that only fires at 34.9mA

Triac Gate sensitivity

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