# Solar circuit = 110V relay to turn on when a variable amount of solar energy is available

#### sam68

Joined Mar 2, 2017
41
I want to turn on a 110v electric space heater when the sun is producing a certain amount of energy because I have solar panels. The "receiver" unit would be outside and the "Relay" unit would be by the space heater. There might be a unit like this on ebay but I don't now the name to search for as I'm a circuit novice or if not, then what components/circuit would I need to do this. I think I could use old small solar light feeding into a variable circuit (pot) that once the voltage was at a certain value, it would trigger a 110v relay on. I assume there would need to be a "bandwidth" so it wouldn't cycle on/off every few seconds.

I'm a circuit novice so if there is not a unit on ebay (what name would I search for) then what components/circuit would I need to do this.

Thanks for any help

#### sam68

Joined Mar 2, 2017
41
I think I can do it this way but would like someone to help out;
thanks

#### ElectricSpidey

Joined Dec 2, 2017
1,458
thanks

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#### ElectricSpidey

Joined Dec 2, 2017
1,458
The bypassing idea sounds good.

I'm not going to comment on the relay again, you will have to make that call.

#### Chris65536

Joined Nov 11, 2019
257
What will the solid state relay do when you are right on the voltage threshold of turning it on or off? Will it flicker on and off? Die in a puff of smoke? Burn out your inverter? I don't know the answers, but I would want to know if I were doing this.

#### sam68

Joined Mar 2, 2017
41

#### Bernard

Joined Aug 7, 2008
5,741
Just a sketch to put on paper what I've heard. It is nice to know what open circuit ,OC, V & short circuit , SC, current of the solar panel is. Make R1 so that SP V is about 70% of OC V.
Change 1K5 ( 1.5K )R3 to R4, 100K to R5. Pos. feed back is single ended and is about 10%.
Ry drive current is unknown?

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#### sam68

Joined Mar 2, 2017
41
thanks Bernard, but that is all greek to me as i have very little electronics background. i'm sorry you spent the time on the circuit- hopefully someone else can use it from this thread. think I'm going with just using my small solar unit, shade it a litle and connect it to the dc/ac relay to close the circuit (i know that much) when the sun's energy goes over a certain threshold.

Thanks for everyone that conributed

#### KeepItSimpleStupid

Joined Mar 4, 2014
4,468
Solar cell voltage tells you very little. The inverter changes the operating point.
There is essentialy aa few points of interest.
V(open circuit)
Short circuit current = Voltage =0; a measure of intensity
V(max power)
I(Max power)

Power happens to be V*I.

it would be cool id you had a way to measure excess power.

I designed instrumentation for a published paper on demand side management back in the 1980's. One application was to heat water without an inverter in the summer. The other was ventilation.

You might be better off measuring I of the panel, but it doesn't tell you much. It;s telling you what your using.

You might be much better off putting a single solar cell outside to use as a sensor at short circuit current and calibrate it to Watts/sq-meter. Now if you could measure V and I of your panel, you would have produced power.

Properly scaled, you would have excess power.

The control scheme would be a lot more complicated. Excess power turns the system on and a small amount of excess power turns it off. Remember that yur heater has a thermostat too. Turning the system on, means ENABLEING the heater.

Look at Siemens LOGO as a controller. The simulator is free.

https://www.pololu.com/category/118/current-sensors has some nice current sensors.

Not a real easy project. So you measure the solar intensity and your solar output in Watts. When the excess wattage becomes grater than a certain amount, you enable the heater. When it becomes say less than 50W turn off the heater.

it might be more complicated than that, and it depends on what your loads are and if they are constant.

Taking the project even further, Pulse Width Modulate the heater so it uses nearly all of the excess power, so the other loads don't matter.