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

Thread Starter

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
 

Thread Starter

sam68

Joined Mar 2, 2017
41
I think I can do it this way but would like someone to help out;
here is a $4 dc to ac 25 amp relay. If I use my old 12v solar panel and connect the output to a POT and then into the relay DC input, would adjusting the POT (voltage) then turn on the space heater when the solar energy reaches a certain level?
 

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Chris65536

Joined Nov 11, 2019
139
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.
At any given moment, your solar panels are producing a certain amount of power, not energy.

I assume there would need to be a "bandwidth" so it wouldn't cycle on/off every few seconds.
That's called hysteresis. Bandwidth is something different. Your solid state relay is designed to be only on or off. That one says it will turn on somewhere around 3vdc, but who knows exactly where. You should have some kind of comparator in there that switches the relay on and off. But why are you wasting valuable solar electricity on resistance heating? How many KW do you have? A purely thermal system would be better.
 

Thread Starter

sam68

Joined Mar 2, 2017
41
Thanks for your reply. Good to know about hysteresis - i think that is solved because an electric space heater doesn't waste energy turning on/off. Usually, I agree with your idea that solar is wasted on resistance heating but in my case, i think it makes sense.
1. I live in Ca where they only pay me at the end of my 'solar' year, 3.7cents per kW that I have sent to them but they charge me from .22 to .57 cents for the Kw they send to me. Since i send them between 100 and 200 KW a month, that is only 3.70 to 7.20 dollars they are giving back to me. 2. Therefore, at that rate of .037 cents, it makes since to use the extra kw instead of gas to heat a room to build heat during the solar day. 3. i could get a battery system but the initial cost of all the components, fees and replacements are too high. 4. Since i'm a hobbiest, I like to do things even if I just save a little money.

So back to the initial idea. I think the idea of using an existing micro solar panel to feed a voltage to the $4 relay is the way to go and even better if there is a pot just before the relay. Or would this be an even better way = on a sunny day, use a multimeter to see the voltage coming out of the panel and adjust it the voltage by covering part of the panel to get to the voltage i want the relay to turn on at 3 volts? Would either of those ideas work?
thanks
 

ElectricSpidey

Joined Dec 2, 2017
779
I wouldn't be caught dead using a 4$ 25 Amp relay.

Your idea will work, just be sure to use a properly rated pot, set it up as a divider.

Using the pot will require that the panel produces more than 3 volts.
 
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Thread Starter

sam68

Joined Mar 2, 2017
41
Oh, ok do you have a recommendation? and if you don't mind, what rating pot do I need (max voltage = 14 v and less than one amp) is produced by the solar unit. Also, I don't know what you mean by a divider - I have use audio pots with three leads. Would the wiring be the same?

Maybe I don't need a pot by setting this up the 2nd way in my last post = partially shade the solar cell unit so it only produces a voltage over 3 volts (or whatever your recommended relay minimum is) which then activates it. Do you think that would work? Either way, it would be nice to have the circuit and reccomendations of the first paragraph above too.

Thanks
 

ElectricSpidey

Joined Dec 2, 2017
779
A 5k half watt pot should do the job. (linear)

As far as the relay is concerned, I was just advising against a cheapo Chinese unit, but the choice is yours, I’m sure you can find an exact replacement at a supplier like Digi-Key, but the price would be much higher.

Below is a pot set up as a divider.

A_Divider.jpg
 

Thread Starter

sam68

Joined Mar 2, 2017
41
Great, just what i need. What about this 25 amp 9$ relay from amazon. although the space heater will be at the most 10 amps. And what about my idea of bypassing the pot by shading that 12 volt solar panel unit so it produces the minium on voltage of the relay when the sun energy goes over a certain level?
thanks
 

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Chris65536

Joined Nov 11, 2019
139
What about this 25 amp 9$ relay from amazon.
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.
 

Bernard

Joined Aug 7, 2008
5,335
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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Thread Starter

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
 
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.
 
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