SSR/heater coil Question

ccfoo242 · May 16, 2015

Why do heater coils only need to be controlled on one leg of the 240v circuit? It seems like one side would get turned off/on but the other side wouldn't. But I don't know much about the

This is the diagram I've been using as a reference but I think all the others are the same. I'm trying to understand as much about this as possible.

trimixdiver1 · May 17, 2015

Because the flow of electricity is being interrupted. Say your powering a 240v light bulb, a switch is on one of the 120v legs, you open the switch and power stops flowing light goes out. You will still have 120v at the unswitched leg, but unless you give the power somewhere to go, ( close the switch or ground the unswitched leg or touch it) power can't flow.

Now the reason this probably seems weird is because a 240v breaker has two poles or switches and when it is tripped or switched off both 120v legs are denergized.

ccfoo242 · May 17, 2015

OK thanks, I almost get it now.

I also found this on stackexchange: http://diy.stackexchange.com/questions/33602/why-do-240v-circuits-not-require-neutral

But where I'm still confused it where does the return voltage go? In a 120v circuit it returns in the neutral wire (right?). If both wires are hot in the 240v circuit and no neitral, how does that work? I get the feeling the answer has something to do with phase but I haven't found an article that explains it like I'm 5.

trimixdiver1 · May 17, 2015

ccfoo242 said:
OK thanks, I almost get it now.

I also found this on stackexchange: http://diy.stackexchange.com/questions/33602/why-do-240v-circuits-not-require-neutral

But where I'm still confused it where does the return voltage go? In a 120v circuit it returns in the neutral wire (right?). If both wires are hot in the 240v circuit and no neitral, how does that work? I get the feeling the answer has something to do with phase but I haven't found an article that explains it like I'm 5.

On a 120v AC circuit the voltage oscillates at 60hz or 60 times per second (AC=alternating current)

So the neutral is a path for the circuit to be complete, amperage is carried on the hot leg.

On a 240, you have 2 120v hot wires, each wire is 180deg out of phase, so you have 240vac 60 times a second. The amperage is split among the two 120vac wires. The return is the other hot.

doug293cz · May 17, 2015

trimixdiver1 said:
On a 120v AC circuit the voltage oscillates at 60hz or 60 times per second (AC=alternating current)

So the neutral is a path for the circuit to be complete, amperage is carried on the hot leg.

On a 240, you have 2 120v hot wires, each wire is 180deg out of phase, so you have 240vac 60 times a second. The amperage is split among the two 120vac wires. The return is the other hot.

You've got the voltage part correct, but current flow (amps) is equal in all parts of an unbranched circuit loop, even tho voltage can vary at different points in the circuit.

Think of the current flowing thru the element as water flowing thru a narrow diameter tube, and voltage as the pressure of the water (we'll only look at the 120V analog, as negative pressures don't really exist.) Valves are like switches, except that an open valve allows flow, whereas a closed switch allows flow. The tube restricts the flow more than the connection hoses, so the pressure drops between the input side of the tube and the output side. The flow however is equal everywhere in both hoses and the tube. You can cut off the flow by putting a valve on the input side of the tube, or the output side. Either location will interrupt the flow when the valve is closed. If the valve is on the input side, the pressure in the tube is zero when the valve is closed. If the valve is on the output side, the pressure in the tube equals the pressure in the input hose when the valve is closed.

Brew on :mug:

SSR/heater coil Question

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ccfoo242

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doug293cz

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