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Heavy duty DC relay needed

GLC

Solar Enthusiast
Joined
Aug 26, 2020
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771
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Texas
So i have been feeding my hot water heater with straight pv to a DC water heating element for a while. I have been trying to use a temp controller to keep water temps below 150 degrees. Works good and sometimes too good. With longer summer days my pv panels on the west side of my house generate power late into the evening. I have tried several relays but they have all failed. I am only pulling 20 amps and max 38 volts. Best results i have made is taking a decent relay increasing the spring pressure and also bending the contacts so there is more travel to separate the contacts to stop the dreaded green flame disconnect. Anybody working with some better relays.
 
So i have been feeding my hot water heater with straight pv to a DC water heating element for a while. I have been trying to use a temp controller to keep water temps below 150 degrees. Works good and sometimes too good. With longer summer days my pv panels on the west side of my house generate power late into the evening. I have tried several relays but they have all failed. I am only pulling 20 amps and max 38 volts. Best results i have made is taking a decent relay increasing the spring pressure and also bending the contacts so there is more travel to separate the contacts to stop the dreaded green flame disconnect. Anybody working with some better relays.
search the word "contactor" here on the forum. You'll find what you need.
 
. I am only pulling 20 amps and max 38 volts.
Automotive relays give bang for bucks. 36v may be a problem. 24v may do??
I do have 12v 70amp nc relays with the contactor on the pv side (22v) of PWMs, These activate when the v is high and i is low. What v is your coil side?
Even though your max is 20amps ,,,,how is your array setup? One or two over rated nc relays on a couple of panels may drop the voltage significantly.
 
I'll be following this thread as I would like to do something similar in my 1200 gallon storage tank tied to my wood boiler.

One idea I'll toss out: What about reducing the number of modules to the point where you are balancing your production with your needs thus eliminating the need for a thermostat?? Of course that idea has at least one glaring problem which is that you'd have to remember to shut the system down if you weren't going to be home for extended periods.
 
What is your relay control voltage?

Have you looked at solidstate relays?
 
Automotive relays give bang for bucks. 36v may be a problem. 24v may do??
I do have 12v 70amp nc relays with the contactor on the pv side (22v) of PWMs, These activate when the v is high and i is low. What v is your coil side?
Even though your max is 20amps ,,,,how is your array setup? One or two over rated nc relays on a couple of panels may drop the voltage significantly.
I have the coil side voltage coming off of my MPPT controller. It is a 24 volt controller but pretty well stays in the range of 27.5 volts at its max. I have fried some auto relays already. The last one being made for tractor trailer heating units. Unit was rated for 80 amps and 24 volts but fried within a couple of hours. Array is set up - 3 panels at 305 watts each direct wired parallel to DC water heater element. I believe that they are 38 volts and 8 amps open circuit.
 
I'll be following this thread as I would like to do something similar in my 1200 gallon storage tank tied to my wood boiler.

One idea I'll toss out: What about reducing the number of modules to the point where you are balancing your production with your needs thus eliminating the need for a thermostat?? Of course that idea has at least one glaring problem which is that you'd have to remember to shut the system down if you weren't going to be home for extended periods.
The thing is, what I have found out with trying different configs of number of panels and parallel, three 305 watt panels wired parallel with a 600 watt DC thermostat gave me the best bang for the buck meaning solar power converted to hot water heating. The element that I use is a 36 volt 1200 watt dual element. I connected 3 panels facing south east and three panels facing north west. The ones that face south ease, I do not worry about. But, if we do not use very much hot water during the day, those north west panels go to town. There is no problem at all going 150 degrees and up. I do have a temp monitor that goes off telling my wife to shut that string down at 150. I would rather get a relay working that would do it automatically. i think that I have one working but I had to alter it by increasing spring pressure to break the contacts and created a greater distance of movement between the contact points when the solenoid tries to disconnect the power. By doing this it can move the contactors away more quickly and father away to kill that dreaded green flame.
This is the one that I am currently playing with.
 
Are you asking the relay specs on the current relay that I am using?
Control voltage, I should have said "coil voltage".

Solidstate relays are modules with no moving parts, no contacts to burn

Solenoid type (this one is 24v coil) might work for you if you want to stay with mechanical devices.
 
Control voltage, I should have said "coil voltage".

Solidstate relays are modules with no moving parts, no contacts to burn

Solenoid type (this one is 24v coil) might work for you if you want to stay with mechanical devices.
Currently being feed off the 24 volt controller and stays appx 27.5 volts.
 
It is a 24 volt controller but pretty well stays in the range of 27.5 volts at its max. I have fried some auto relays already. The last one being made for tractor trailer heating units. Unit was rated for 80 amps and 24 volts but fried within a couple of hours.
There's part of your problem, you're running above the voltage limit. Get something rated for 48 volts and we'll above the amperage needed.

The other issue you're going to run into is that you are opening and closing the really under load, so it's going to arc on every opening and closing. I'd recommend making sure you get something that is rated to open and close under load. Or choose a relay/contactor that is rated well over your actual current (maybe 2x-4x max current)
 
There's part of your problem, you're running above the voltage limit. Get something rated for 48 volts and we'll above the amperage needed.

The other issue you're going to run into is that you are opening and closing the really under load, so it's going to arc on every opening and closing. I'd recommend making sure you get something that is rated to open and close under load. Or choose a relay/contactor that is rated well over your actual current (maybe 2x-4x max current)
Unless he has issues with the coil burning up, running 28v on a 24v coil will not contribute to switching contact failure. A 48v coil may or may not pull in at 27v-28v. That would need tested with any specific relay/contactor unless the specifications state the coil range.
 
Unless he has issues with the coil burning up, running 28v on a 24v coil will not contribute to switching contact failure. A 48v coil may or may not pull in at 27v-28v. That would need tested with any specific relay/contactor unless the specifications state the coil range.
I read that voltage at the switches voltage.
That looks interesting. Checking to see what I may can get one.
Something like those gigavac contractors should work well.
 
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