Bridge Rectifier as Load for a SSR Circuit

Thread Starter

Xavier Pacheco Paulino

Joined Oct 21, 2015
728
Somehow, I wanted to control the power to a bridge rectifier. I had two ideas in my mind:
1- Using a power relay like this one driven by a transistor (is isolation maintained?)
2- Using a SSR, i.e, a power triac driven by a optotriac as shown below

In both cases, I would have to place a NTC thermistor to limit the inrush.
Now, the idea I chose was the SSR, no tested yet. But someone told me:

Zero crossing will work for the first 1/4 cycle, but then
a) There will be no current flow to the "zero-volt switch" IC for it to know when the zero-cross happens, because "2200uF" is charged to 170V & all bridge diodes are reverse biased.

b) Even if a firing pulse is given to triac @ zero-cross, it will not latch-on, because the available current is below "hold-on" current for Triac

Both solutions have their pros and cons, but I haven't decided which one can work better. It seems that the relay one is easier. But one has zero cross and the other don't.

I would like to hear your viewpoints.

 

ebeowulf17

Joined Aug 12, 2014
3,282
Somehow, I wanted to control the power to a bridge rectifier. I had two ideas in my mind:
1- Using a power relay like this one driven by a transistor (is isolation maintained?)
2- Using a SSR, i.e, a power triac driven by a optotriac as shown below

In both cases, I would have to place a NTC thermistor to limit the inrush.
Now, the idea I chose was the SSR, no tested yet. But someone told me:

Zero crossing will work for the first 1/4 cycle, but then
a) There will be no current flow to the "zero-volt switch" IC for it to know when the zero-cross happens, because "2200uF" is charged to 170V & all bridge diodes are reverse biased.


b) Even if a firing pulse is given to triac @ zero-cross, it will not latch-on, because the available current is below "hold-on" current for Triac

Both solutions have their pros and cons, but I haven't decided which one can work better. It seems that the relay one is easier. But one has zero cross and the other don't.

I would like to hear your viewpoints.

Why not simply switch the DC side (between cap and load) instead of the AC side? Seems like that would eliminate the problems you've suggested.

As for whether or not those problems are real, I can't say with confidence either way. My gut feeling is the SSR would work just fine, by my gut isn't always right, and I haven't done my homework on this one!
 

Thread Starter

Xavier Pacheco Paulino

Joined Oct 21, 2015
728
Why not simply switch the DC side (between cap and load) instead of the AC side? Seems like that would eliminate the problems you've suggested.

As for whether or not those problems are real, I can't say with confidence either way. My gut feeling is the SSR would work just fine, by my gut isn't always right, and I haven't done my homework on this one!
I want to try the SSR to see what really happens, at least to learn from it. If it actually works or not, that's what I don't know. One thing is true, the circuit is kind of uncommon. Not seen people implementing it, neither heard of someone telling that doesn't work.
 

MaxHeadRoom

Joined Jul 18, 2013
23,342
This is basically the method used in the Motorola TDA1085 controller IC for implementing variable speed of a Universal motor with a bridge rectifier..
Max.
 

Thread Starter

Xavier Pacheco Paulino

Joined Oct 21, 2015
728
Update about this:

I've tried the circuit in real life, and the capacitor charges extremely slow, as the triac is no effectively switching, i.e, the voltage across the bridge is really low.
 

ebeowulf17

Joined Aug 12, 2014
3,282
Update about this:

I've tried the circuit in real life, and the capacitor charges extremely slow, as the triac is no effectively switching, i.e, the voltage across the bridge is really low.
Do you have an oscilloscope? It would be interesting to see what specifically is going on.

Watching the TRIAC gate might be informative. Watching the DC load side would be good too, although I think to do that well, you'd need to run two channels in a differential mode, since the DC side is not isolated and no part of it is at ground potential.

I'm curious if the TRIAC is firing at every zero crossing as expected, but stopping abruptly, or if it's failing to even fire sometimes. All but impossible to tell what's going on with simple voltage readings.
 

Thread Starter

Xavier Pacheco Paulino

Joined Oct 21, 2015
728
Do you have an oscilloscope? It would be interesting to see what specifically is going on.

Watching the TRIAC gate might be informative. Watching the DC load side would be good too, although I think to do that well, you'd need to run two channels in a differential mode, since the DC side is not isolated and no part of it is at ground potential.

I'm curious if the TRIAC is firing at every zero crossing as expected, but stopping abruptly, or if it's failing to even fire sometimes. All but impossible to tell what's going on with simple voltage readings.
All I know for now is that Triac is never activated, although the control signal is always present. Sounds like the zero crossing is not working. I simulated the circuit with a no zero cross opto, and it worked. But I don't success simulating the circuit with zero cross. That's why I built it in real life, and it didn't work either.

The capacitor should be charging slow due to the leaking current passing through the RC snubber network. (I think, and could be wrong).

I will try to get readings from an scope. And if I don't manage to solve the problem in my circuit (already built in a PCB) I will completely remove the SSR and will use a electromechanical relay, which I know it will work for sure.
 
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