Triac problem while switching an inductive load

[SIVAPRAKASHTRIDENT]

Hello.

I have a system in which i have to switch on the inductive load of 8 mA for 10 s.

It is working properly for sometime. But i had problem of triac failure (triac always on) after continuous usage

 

[srizbf]

what is the breakdown voltage spec of the triac ?

 

[KlausST]

Hi,

ACST2 is not suitable to switch a load current of only 8mA. It has an hold current of 0mA.

But this is not the problem of "always on".
The triac is overvoltage protected. So it should be able to handle the 8mA.

When it is "always on", do you need to replace it? Can you check resistance with a DVM compared to a good triac?

Klaus

 

[SIVAPRAKASHTRIDENT]

Hi,

ACST2 is not suitable to switch a load current of only 8mA. It has an hold current of 0mA.

But this is not the problem of "always on".
The triac is overvoltage protected. So it should be able to handle the 8mA.

When it is "always on", do you need to replace it? Can you check resistance with a DVM compared to a good triac?

Klaus

Hi,

When triac is "always on". the gate mt1 and mt2 are internally shorted. if i replace the triac it works fine.

 

[SIVAPRAKASHTRIDENT]

Hi,

dv/dt > 500 V

Insulation voltage 1500 Vrms

Igt 10 mA

di/dt 50 A/us

IH 10 mA

 

[KlausST]

Hi,

are you sure your load is only 8mA = 0.008A?

Klaus

 

[edsonmoky]

You can use a pulse transformer, positive pulse turn on and a negative pulse turn off the Triac.

 

[SIVAPRAKASHTRIDENT]

Hi,

Yes i am sure the load is 8 mA. I have measure it. The L value of inductive load is 1.5 H.

I am using a timer IC 4541 for giving the pulse.

 

[chuckey]

1. Put a snubber across your load because you do not know what the Dv/Dt is.
2. If the gate drive circuit has a high impedance source, the Dv/Dt can be coupled through to the gate via internal capacitance and destroy it.
3. Ignore the failed device, the failure mode is not on the spec sheet and so its source can not be guaranteed.
Frank

 

[SIVAPRAKASHTRIDENT]

Hi ,

Thanks for the reply

is there any other triac which will be suitable for this application

 

[KlausST]

Hi,

i just did a serach for low hold current triacs. I came on BT131-600. But it needs extra overvoltage protection...as far as i can see.

Or maybe opto-triacs like VOM160.


Klaus

 

[schmitt trigger]

The capacitor in parallel with the Triac/coil is not damped (it doesn't have a series resistor)

Therefore you have a large-Q tank circuit, which will oscillate wildly at turnoff. This could exceed the internal clamp ratings.

I know that the datasheet indicates that no external snubbing or clamping is necessary. But desperate problems require desperate solutions. Add the resistor to the snubber, and see if it helps.

What quadrants are you triggering the triac? It is not specified at the IV quadrant.

 

[SIVAPRAKASHTRIDENT]

I am using a posive gate pulse. So it would work in 3+ and 1+ quadrant.

Is there any way to find the dv/dt of an load.

I am using a inductive load. But the when triac fails the terminals are shorted inside. What could be the reason for the triac internal short. I have searched in net every where it was mentioned that it would be due to high di/dt.

Do we have to consider on di/dt for inductive load or is there any other reason for the internal short.

 

[KlausST]

Hi,

High du/dt makes the triac to incidently switch ON, but it does not destroy it.

With inductive loads you don't have high di/dt, at least not more than with resistive loads.

Klaus

 

[SIVAPRAKASHTRIDENT]

Hi,

Is there any chance of triac internal short due to increased junction temperature.

 

[schmitt trigger]

If you are applying a positive gate pulse, then you are operating in the I and IV quadrants. See the attached image.
On this device, triggering on the IV quadrant is not specified.

https://obrazki.elektroda.pl/3910349100_1449701735.png

 

[SIVAPRAKASHTRIDENT]

Hi,

Yes it is operating in the 1 and 4 quadrants. i am working with an AC load.

 

[SIVAPRAKASHTRIDENT]

Hi,

Will the the clamping voltage destroy the Triac.