How many SSRs?

It seems like many of the schematics I've seen out there only control one of the 220 lines through an SSR. Wouldn't you want to control both lines with dual SSRs?

 

That's definitely an option - but with 220v, as long as one leg is "off", the circuit is off - the element will not fire.

If you want to go belt & suspenders, you can definitely put SSR's on both legs - or I've even heard of dual pole SSR's (where you have one SSR that you can run both legs through independently of one another, which may simplify your control scheme).

But for most of us, it's enough to have a dual pole electromechanical relay providing input to the elements, and have one hot leg as an input to that relay, and an SSR's output as the other input. That way, should the SSR fail in a closed position (essentially, leaving it stuck "on"), we just kill the relay and cut all power to the element.

 

Ok, I thought that if you killed one of the legs on a 240 element it was still running 110 essentially through the other. Maybe I'm confusing three conductor wiring with a four conductor wiring to the element now. I'm still working on my AC knowledge. 99% of what I deal with at work is DC. I like DC better. It's theory is much simpler

 

That might be a part of the issue - it's not actually 3 conductor wiring to the element. There's no neutral to the element, just the two hots and you ground the vessel.

If you haven't already, browse through theelectricbrewery.com. Spending an hour or three just scanning through Kal's step-by-step instructions and his design, even if you don't go with something to match his design, really helps wrap your head around a lot of the concepts.

 

So if you're running a three conductor to the element, what's the point of the 4 conductor GFCI into the box? Doesn't running three conductors to the element defeat the GFCI wiring? I'll check out Kal's stuff when I get home tonight.

 

The element, like any simple resistive heater, is 2-wire plus a ground. Those two wires in the residential environment are either a hot and a neutral (120VAC circuit) or two hots (240VAC) which is a split phase system of your home wiring. The third wire is a ground or exit path, for safety.

The "four" wire in the GFI feed includes all these wires (2 hots, one neutral, one ground) which a controller might use and distribute to different circuits (eg elements).

BTW, the circuit is a loop - interrupt the loop anywhere and current ceases to flow. You can use 2 SSRs but they may not play nice together unless purposed for it.

BTW#2, AC and DC are essentially no different with respect to wiring.

 

DC is way easier. Hot wire, and not hot wire 🏻

 

Just for a little more clarification on "why 4-wire to the control panel": typical 3-wire 240v setups include 2 hots and a ground wire. 4-wire setups add a neutral - which is vital if you want to include any 120v equipment in your panel. Want to control your pumps? You need a neutral and one of the hot legs from the 4-wire 240 to complete that circuit (ground too, of course). And most of the electromechanical relays that are commonly used feature a 120v coil, so you need a hot and a neutral just to switch stuff on and off.

 

strats is right, though you could build a 240VAC only panel easily enough. Get 240V pumps and relays.

Regarding "hot and not hot", I suggest you go ahead and grab a live AC neutral wire while standing in puddle of water with bare feet. You will find it very "not hot"!

 

Is that your budget minded multimeter

 

Ya. Don't even lie and say you don't check 9V batteries with your tongue!

 

I'm to lazy to do that. I just toss me out if the dates getting old.