Using a SSR with Triac

Is the SSR rated for dimming service?

Hi, Andy. I'm not sure where you're coming from here. You're saying you're already using a triac for something, and also want to drive an SSR with the same signal? Why not just drive a second triac with the signal driving the gate of the first triac?

By the way, you can use an SSR for lamp dimming as long as it isn't a "zero crossing" drive model. Those wait to turn on their internal thyristor at the zero crossing of the AC line, which means by definition they won't do phase control.

I'm not sure this is responsive, though. Could you please explain your problem in more detail?

Thanks Chris

"AIMatrix" wrote in news:M2Xnh.1704$312.1300@trndny02:

it's ok by me...

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control

Hi Chris; I am trying to make an add on for an existing light controller that has 8 triac outputs for dimming. It is limited to 8 amps max per channel or 15 amps total. I want to plug a high power SSR in to it using a separate mains supply to get a

20 amp output. This is only needed some times and on random out puts so I would rather not change the existing controller. I have been looking at this for days now and have not found any info directly pertaining to this but have found the best high power switching is done with inverse parallel SCRs, but have not had any luck figuring out the gating. Most of the circuits that I have found just leave the gates hanging with no other circuitry. Some show the two gates connected together thru a triac, but non show phase control with 110VAC.

Finely I found a vender that makes a random turn on SCR SSR with 110VAC input, but they are not common and are hard to get and expensive. Not knowing if it will really work for what I want it to do a rather not spend the $30 to find out.

I rather roll my own out discreet components but have hit a wall with the gating circuitry.

Can you be of any help??

Thanks Andy

Yes it would be a random turn on SSR like a Crydom A1225-10.

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so happy to hear that...

Let me see if I understand what you want to do:

------------- | Existing |____________________ | Controller |8 outputs to lights \\ | Includes |____________________/ | Dimming | -------------

You want to plug in to one of those outputs and do this:

Output (AC)----+---------+ | | [SSR] [Light] | | Output (AC)----+---------+

The SSR needs to operate whenever the light is on, even if the light is dimmed. Assuming that is correct, all you need to do is sense the voltage to the light, and use the sensed voltage as a signal to a circuit. The circuit merely needs to detect the signal and turn on a triac/SSR/SCR's whatever when signal exists. You could AC couple the signal from the a wall wart or current transformer to a comparator and use the comparator output to control whatever. You need a separate power supply for the comparator

The following assumes your existing controller can handle inductive loads: Plug an AC output wall wart (in parallel with the light) into the controller. The output of that wall wart will provide input to the separately powered circuit - in other words, you are not using that wall wart to power the second circuit. The wall wart is just to isolate and reduce the voltage.

If your existing controller cannot handle inductive loads: Make an "extension cord" with a line cord and plug and an electrical 4" junction box and receptacle. One lead of the line cord passes through a current transformer. ------- AC --------------| | | | | o | | | Ground ----------| 0 | | | | | | |-------+ | o | | ------- [C | T] | AC ------------------------------+

The 2 wire output of the CT gets connected to a resistor. AC couple the signal developed across the resistor to the comparator.

Ed

That used to be true and was common practice for stage dimmers 30 years ago. It doesn't really apply these days.

To fire this circuit needs a 3 winding pulse transformer.

ago.

Homer; could you explain this?? Would it work if controlled by a triac??? Andy

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Ed; You are close, but I would not have a light on the input side of the SSR and the SSR would have its own supply.

one of 8 outputs from light controller ----> SSR -----> 20 amp output controlled by light controller ^ I 20 amp supply

Thanks Andy

You need to use a pulse circuit, like a UJT or the like. You can do it with two transistors. Look in your library for a GE hobby or SCR manual. But really, a triac is way easier.

triac???

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Homer; Thanks on the tip of UJT I will look into it and yes I would love to get my hands on the GE SCR book. I have seen many references to it over the last few days researching the problem. According to the library index the closest copy is in an university 90 miles away. Could not find an online copy but they may still be available to buy. Andy

Try for an interloan. I do see them on eBay also.

Right - the SSR gets *power* from its own supply. It is *controlled* by sensing power from the existing controller. The wall wart or current transformer AC coupled to a comparator should do it. If you use the CT, you must have a load powered by your existing controller on the channel you are sensing, but since you will have no light there, you'll need to provide a load. And since you would have to provide a load, you don't need a CT - just use a resistor divider and sense the voltage across the smaller resistor:

---------- | Existing |---[1K]---[1K]--+ |Controller| | | Neutral>|---+---[100R]---+---[.1uF]---> to comparator ---------- | input | +--------------> to comparator ground Neutral side of controller output.

Use 10 watt resistors for the 1Ks, and a 5 watt resistor for the 100 ohm. The 1K's will dissipate ~ 4 watts each and the 100 ohm will dissipate ~ .4 watts, worst case. Use a transformer type (not a switcher) supply for the DC to the comparator.

Ed

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Do You think that would work with the phase controlled dimming ???

light

Yes - and good that you ask. But better than what I think is building it and seeing for yourself.

You need a small DC wall wart (and an AC wallwart to sense the voltage, if your existing controller can handle that load), a comparator and support components, so we're talking a few dollars in parts to drive your SSR.

You will find that it turns your SSR on when the sensed voltage rises above the reference voltage, and encounter the problem that it only works during the + half cycle. Hopefully, when you figure out the comparator circuit, the solution to that problem will occur to you and you'll learn something plus get a real sense of accomplishment in solving that problem (for less than a dollar). The learning experience is priceless. I got the sense that you would enjoy this from your desire to roll your own. I can post the solution if you prefer.

Ed