Relay alternative

bruef · Joined: 20 Feb 2014 Posts: 40 Location: Wisconsin

I have a theoretical question. Could a person use a couple of mercury switches in place of the combination SSR's and relays/contactors? I realize they can be pretty pricey but if someone were short on space or just looking to minimize the number of components, they should work, shouldn't they? And if one did build the panel that way, would one still need the heat sink(s)?

**kal** · Posted: Fri Apr 18, 2014 9:45 pm Post subject:

Mercury switches are switches that open/close based on orientation. They look like this:

In the above position it is off. Tilt it 90 degrees such that the wires are at the bottom and the mercury falls and creates a connection path between the two wires, closing the switch.

They're used a lot in older thermostats, at the end of the coil that expands/contracts with temperature, like so:

Maybe you're thinking of something different?

Kal

bruef · Joined: 20 Feb 2014 Posts: 40 Location: Wisconsin

Aha - I just double checked and I was using the wrong term. What I had in mind was a mercury contactor. I own a sausage business and my commercial smoker uses a mercury contactor to fire the heating element (here's the specific model http://www.mdius.com/pdf/SpecSheets/relays/330NO-120AU.pdf ). It cycles the element on and off gently like an SSR - without the physical "clunk" of a standard contactor.

James Edmonton · Posted: Sat Apr 19, 2014 5:52 am Post subject:

Hello,

From what I read of your diagram you could perhaps replace the magnetic relay with one of these but not the SSR. They have different functions. I could see how this could work in a smoker or something that was basically designed to simply hold a temperature by cycling on and off.

The SSR gives you a much greater ability to control the temperature and power usage of the element. It essentially works like a light dimmer, turning the light on an off many times per second to control the light level. Essentially the same thing happens with the SSR and the heating element - it can in effect vary the power to control temperature and so forth.

As I see it - your unit works more like a freezer thermostat - which is full on or full off. On until the desired temperature is reached then off until it is needed again.

As such I am not sure how the PID would control it. It might in theory work, but I wouldn't expect it to work well. You may be able to replace the relay with it - that might work fine, but I don't think you could replace the SSR.

I am not an expert on these things, but from the information you provided I don't think it is the way you may want to go. Others may chime in with better information.

Have a great day!

James

**kal** · Posted: Sat Apr 19, 2014 1:13 pm Post subject:

Interesting. I had never heard of a mrercury relay until now. Like James said, I think in theory it could work but I'm not entirely sure the cycle time would be fast enough as they're not high speed devices.

According to Wikipedia:

bruef · Joined: 20 Feb 2014 Posts: 40 Location: Wisconsin

Interesting- yes I think you're both right as I burnt out one of the mercury contactors a couple years ago and the factory rep said I could just replace it temporarily with a regular contactor until I could get a proper replacement (can't leave the standard contactor installed indefinitely as it would wear out quickly from the frequent cycling on and off). The mercury unit cycles on and off more gently - without the physical "clunk" of the moving parts inside the regular contactor. This is needed because when the smoker is running, the heating element can cycle on and off as often as once or twice a minute.

**kal** · Posted: Sun Apr 20, 2014 3:55 am Post subject: