proper SSR usage

After seeing a few small debates on the usage of SSR's on 240v elements, I really would like to clearly understand the usage of dual SSR's. I do like the idea of fully switching both legs of power, of course it does sound more "correct" than switching a single leg (even when wiring a double pole relay before the SSR).
So if possible I'd love to hear from someone with experience with this application, as I'll be soon building. I want to switch 5500watt 240v elements, which is 23 amps.

Am I correct in assuming that I can use a 25 amp SSR on each hot leg, and wire the dc control in parallel between the two? Or should I buy one of the more expensive dual SSR's?
Also is 25 amp too close to the max amperage? 40 amp is the next step up if running two singles. If I go with one dual SSR, I think I have to buy a 110$ 50 amp model.
And is this just a waste of money? What kind of event would need to occur for a single SSR to be a dangerous thing?

Thank you very much in advance for any help and info,
Cheers!

Budzu said:

... I want to switch 5500watt 240v elements, which is 23 amps.

Am I correct in assuming that I can use a 25 amp SSR on each hot leg, and wire the dc control in parallel between the two? Or should I buy one of the more expensive dual SSR's?
Also is 25 amp too close to the max amperage? 40 amp is the next step up if running two singles. If I go with one dual SSR, I think I have to buy a 110$ 50 amp model.
And is this just a waste of money? What kind of event would need to occur for a single SSR to be a dangerous thing?

Thank you very much in advance for any help and info,
Cheers!

Click to expand...

25A is a bit close, it isn't that much more to go to 50 Amps.

Brew Beamer will disagree with me on this one, but both ways will work and I feel both are equally safe. I have nothing against SSRDs (dual SSRs) other than I don't put complete faith in solid state relays.
I prefer one SSR and a contactor.

I use a contactor like THIS to switch both legs using an element off/on illuminated panel switch.
Then after that I put an SSR on one leg and then run that to the element.

It's a little cheaper and I like hearing the contactor engage, the coil also buzzes a little bit kind of like a constant reminder. Plus, I personally feel safer with a mechanical disconnect there. Doing it this way, both lines are de-energized when not in use.
I think Brewbeamer will agree with me that it isn't safe to have line voltage hanging out there on one side when the element is not in use.

Here is a link to a drawing I did for Brewpastor, (you ever do anything with it BP?).
http://www.mediafire.com/file/iidmmtlmymt/BP-ElectricalXPS.xps
It is for two elements switching between 110 and 220 for high/low output.

I see, thanks CR! So I understand, that contactor is switched on and off using a 120v panel switch?
My plan was to use this relay, http://www.grainger.com/Grainger/wwg/search.shtml?searchQuery=1EJH6&op=search&Ntt=1EJH6&N=0&sst=All
and run a 24vac transformer to run my panel switches.

Would the contactor play the same role as the relay?
It would indeed simplify my layout, eliminating the transformer.

Thanks, very helpful!

Yes, that relay would work fine in place of a contactor. 30A is plenty of overhead.

If I ever have the opportunity to redo my control circuitry I would use 12-24VDC. It can be just as lethal is 24VAC but I feel a lot safer working around it.


This is just my thinking and I could be very wrong but 24VAC is at RMS. Meaning at the peak of the cycle, voltage is greater than 24V. At this point the voltage can break through the resistance of skin and jolt you.

I would throw a rectifier on that transformer and put a large filter cap or two on the output.

I used one dual SSR to switch both legs of the 240VAC with my PID. I use another dual to individually select which of 2 elements are powered.

In front of the SSR's, I also put a 2-pole 30A mechanical switch that I was surprised to find at Home Depot. This lets me manually turn off the elements regardless of what the PID is doing.

The 3PDT switch allows me to select which of the two thermocouples and elements are being driven by the 1 PID. Since I only have a 30A ckt, I can't use 2 elements at the same time, so

This contraption took its maiden voyage this morning. I expected fire, but it worked exactly as I had hoped. Weird.

passedpawn said:

This contraption took its maiden voyage this morning. I expected fire, but it worked exactly as I had hoped. Weird.

Click to expand...

Congratulations mate!

I keep a fire extinguisher next to mine. A little faux logic tells me that as long as I have it there, there will not be a fire .

I see yah got a fuse in there protecting from over current should you accidentally turn both elements on. Just a suggestion but, an interlock would be a better option.

Thanks. Took me 2 days of drilling/grinding/wiring to put it together.

Actually, the fuse isn't in there. I put a 3P3T switch in there (there is an OFF position between the 2 ON postions) so I am certain there is a break-before-make with the switch. So, I'm pretty sure both elements can't come on at the same time.

BTW, I used a very heavy metal outdoor junction box and skipped the heat sinks. Box gets pretty warm, but all went well. I think I will mount the box on another chunk of aluminum to make sure the heat gets spread out.

Ah okay.
You know, I have never had a problem with the heat sinks on mine getting warm. I picked up the din rail mounted deals at automationDirect.com rated for 40A. Not sure if it is the SSR itself or if the 40A has a lower on resistance, have an inkling that it is the ladder of the two.

Lower on resistance. Same as any FET. I should have gotten those 40A SSRs, but I found a nice deal on the 25's.

Thanks for the post passedpawn, and congrats on your system I was able to find the dual SSR's you used and they are very affordable. They also can handle up to 40 amp max evidently. 25 is the RMS rating.
So you supply your PID with 120v and it supplies the 12vdc for the control on its own am i right? Haven't looked into those as I'll be using the brewtroller.
Thanks and cheers

Yes, PID drives the SSR (if you get the right one from Auber). I did have to add a small DC supply for the 2nd SSR as I drive it manually through my switch.

They will handle the 40A (i.e., 20A per section in the dual). You will need to add a big heatsink. It WILL get hot.

Man, I'll take that back, I put 25 Ampers in for my 4500W element.

These HERE are the ones. A little pricey, especially if you plan to use two, but they work really really well and stay cool, the built in heat sink and din rail mount is what sold me.