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Triac protection against short-circuit

femtotech

Hi!

I have a board with the basic triac driver circuits as described at page 8 (fig. 14) of MOC3052:

 

https://www.fairchildsemi.com/datasheets/MO/MOC3052M.pdf

 

The triac is a BTA204S-600D (600V 4A).

The load is a small electric motor (less than 250W) but there are no snubbers because the leakage is not acceptable for the customer.

 

I tested a resistive load up to 750W with no problems. The circuits works well also with lighter inductive loads, about 100W.

With the original one (250W) after few commutations the triac goes in short-circuit - I guess due to the back-EMF when disabling the triac.

 

I tried with a very big triac (25A, 250A peak) and there are no problems, but of course it's not a good solution.

I also tried with a TVS (275V) to clamp the VAC across the line (between L and N, and between T1 and T2) with no chance.

Also a snubber between L and N does nothing.

 

Any idea?

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  • I hesitate to make too specific of comments because I don't have enough information about your case.  If you post a schematic, maybe I can be more helpful.

     

    Typically, snubbers operate just as well across the switch or across the load.  If the load is normally off, we shunt the load with the snubber.  If it is normally on, we shunt the switch.  Clamps and snubbers do not preclude one another.  Using a clamp allows using a higher-Z snubber, sometimes no snubber.

     

    My first move would be to go to a non-sensitive-gate triac.  Or more probably back-to-back SCR.  Remember to de-rate!

     

    Basic Triac and SCR Projects and Circuits

     

    Check out the last illustration.  There are less ways to turn on back-to-back SCR v. TRIAC.  This helps the situation.

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  • I hesitate to make too specific of comments because I don't have enough information about your case.  If you post a schematic, maybe I can be more helpful.

     

    Typically, snubbers operate just as well across the switch or across the load.  If the load is normally off, we shunt the load with the snubber.  If it is normally on, we shunt the switch.  Clamps and snubbers do not preclude one another.  Using a clamp allows using a higher-Z snubber, sometimes no snubber.

     

    My first move would be to go to a non-sensitive-gate triac.  Or more probably back-to-back SCR.  Remember to de-rate!

     

    Basic Triac and SCR Projects and Circuits

     

    Check out the last illustration.  There are less ways to turn on back-to-back SCR v. TRIAC.  This helps the situation.

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