Thyristor-based load switching

Has anyone tried to use thyristors to replace contactors ? I have some beefy ones capable of 150A, but not sure what the best approach would be to implement the "off" circuitry.

This looks safe, but requires two thyristors :




Then there is this, same deal :



More here :
https://www.electronicshub.org/scr-turn-off-methods/

This is the one I am mostly wondering about :



How much current would that stop switch need to handle ? Seems like this would be easily implementable with a... contactor, but then it sort of defeats the purpose other than to save a few watts of power of having to run contactor as long as the load is on.

I think one of the reasons contactors get used is for safety's sake because they provide galvanic isolation

Like FETs, a common failure mode of Thyristors (SCRs) is shorting anode to cathode.

So...you would be unable to use the switch to disconnect the load, and if the purpose was a safety disconnect, it would be unsafe.

amberwolf said:

Like FETs, a common failure mode of Thyristors (SCRs) is shorting anode to cathode.

So...you would be unable to use the switch to disconnect the load, and if the purpose was a safety disconnect, it would be unsafe.

Click to expand...

And contactor can have the contacts welded and being unable to disconnect Solid-state contactors are built on thyristors though, correct ?

Punx0r said:

I think one of the reasons contactors get used is for safety's sake because they provide galvanic isolation

Click to expand...

That's not really a fact. More often than not contactors have the same coil voltage as the main voltage they're switching, so they're activated by directly applying pack voltage. There is nothing inherent in their design that provides for isolation if you're using the same power source - both contactor and thyristor may require an optocoupler if isolation is desired.

I meant isolation in the sense amberwolf said - when the contacts are open the load is guaranteed to be isolated. It's something I recall being told was required to meet an electrical saftey code - no solid state switches allowed.

Contacts can weld but contactors would appear to be much more tolerant of overload than solid state.

One issue with using an SCR is the forward voltage drop. It would be over 1v for any kind of bike sized load. While the SCR can handle this with an adequate heat sink, the heat sink would need to be very large and would get hot. Over 150W for a 150A load.

The other issue as you have noted is they are not easy to turn off. The switch in last drawing would need to handle the full current of the motor. You might as well use a IGBT and not need to worry about that.

A MOSFET would have much lower voltage drop and heating, which is why you see them used most frequently in controllers and BMS circuits.

cricketo said:

Solid-state contactors are built on thyristors though, correct ?

Click to expand...

AC contactors (SSR's) are built with thyristors; DC contactors are built with FETs or IGBTs.

Yeah, some valid points. Thanks guys!