Solid State Relay concern

[Henry Tjernlund]

Hello. I asked this question on a forum years ago and can't recall if it was this one and what the answers were.So here it goes again.

I am using an SSR to turn ON and OFF 120v AC lights for stage/movie effects. So there is the 120v AC side, and the 6v DC side. So an operator with a simple push button can flicker a light On and OFF. This light can be anything from 100W to 1000W. All that is within the specification of the SSR. That is pretty straight forward so far. So here is the question.

I am using a regular household wall light toggle switch to (bypass/short past) the SSR so the operator can just turn the light on. I seem to recall that I was told that this was fine. And so far it has worked fine. But I still have this nagging concern that I am applying the same voltage to the IN contact and Out contact of the SSR's 120v AC side and that this might somehow damage it. I am not sure that I am being clear in my question.

 

[Harald Kapp]

No problem there, this is fine. An AC rated SSR doesn't "know" about hot and neutral or plus and minus etc. Bypassing it with another switch (or another SSR for what it's worth) simply applies the same voltage on both sides (as you correctly identified), making the voltage across the two terminals 0 V - this is considerably less stress for the SSR than withstanding AC 120 V.

 

[AnalogKid]

To expand on that, semiconductors are called semiconductors because they conduct in only 1/2 of the possible directions. Regular old wire in a circuit doesn't know which end is more positive or negative than the other, and doesn't care. Current moves in both directions with equal facility. But a semiconductor conducts current in one direction only, determined by the material and impurities in the body of the device.

This is a problem when using semiconductors to switch alternating current, because the direction of the current ... wait for it ... alternates. So the output stage of a solid state relay has a special semiconductor device called a TRIAC that appears as two devices connected backwards and back to back. The device can conduct current in both directions, and just like a wire it doesn't care which way it is hooked up. To be clear, a TRIAC is not just like a wire in all aspects of its operation, but it is good enough in a well designed circuit.

ak

 

[Harald Kapp]

the output stage of a solid state relay has a special semiconductor device called a TRIAC

... or two MOSFETs back to back, with the same effect.

 

[AnalogKid]

Or two SCR's. Keeping it to the most common term for the perceived level of the OP.

ak

 

[AnalogKid]

... or two MOSFETs back to back, with the same effect.

I'm playing with a lamp dimmer based on power MOSFET's, partly because I have a pile of them left over from something. I thought I had a cool idea until I found patents going back to the eighties.

ak

 

[Henry Tjernlund]

Thank you very much. I used to tinker with electronics when young, 40 years ago. Though had never worked with SSRs until making these boxes.

Here is a short clip I made some years ago with an attempt at lighting effects.

The first was creating a welding effect. I had more tests planed but didn't follow through because I noticed a problem with flickering lighting and certain cameras like the DSLR used here. The problem is called "rolling shutter." The CMOS sensor doesn't processes the exposure all at once. It reads the sensor top to bottom (or vice versa) in a pass that takes a little bit of time. With the light flashing on and off like that some of the frames are only partially lit. Thus you get this up and down jumping of the light which looks awful.

The clip also has a red and blue police strobe lites which are controlled by a micro, er... controller. But I was using those colored party CFLs which aren't instant on/off.

I have access to a newer camera that claims limited rolling shutter problems. And I've been starting to buy LEDs bulbs to see of they work better. So I dug out these SSR control boxes to give the idea another try.

Now, I have a couple more SSR related questions which are somewhat distinct. Should T continue asking them here, or start a new thread?

Thanks again.

 

[Harald Kapp]

Now, I have a couple more SSR related questions which are somewhat distinct. Should T continue asking them here, or start a new thread?

It depends on how distinctthese questions are. It is possibly a good idea to create a new thread.

 

[Henry Tjernlund]

Thanks. Turns out that the next question may have already been answered in a archived post. One on SSR current leakage which explain that some SSR have a "snubber" circuit. Which might explain why when I have the LED bulb plugged into my box, but the SSR is OFF that the LED bulb is very slightly lit.

A proposed solution, which I have yet to try as my stuff is packed up in storage boxes, is to plug in some small current using item, light a night light, in parallel to the bulb.

Thanks again.

 

[Henry Tjernlund]

Something to my interesting to my use of movie lighting.

http://www.bbc.com/news/science-environment-35284112