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Heavy duty DC relay needed

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.
So far the latest solenoid coil does not overheat at all. It just would not break the contacts until I made the modifications.
 
Maybe I’m dumb here. But can you not use the water heater/storage tank’s thermostat to act as a switch for a high/low industrial relay/contactor? I believe that’s what was described to me for a wood-fired off grid home heating/hot water system. Two hot water heaters on solar triggered two 12”x16”ish boxes with contractors on the wall shutting down solar.
I’ve not done it but was under the impression they were common parts?
 
Maybe I’m dumb here. But can you not use the water heater/storage tank’s thermostat to act as a switch for a high/low industrial relay/contactor? I believe that’s what was described to me for a wood-fired off grid home heating/hot water system. Two hot water heaters on solar triggered two 12”x16”ish boxes with contractors on the wall shutting down solar.
I’ve not done it but was under the impression they were common parts?
No sir, those controls are AC, they will fry using DC current.
 
12VoltInstalls was asking about using the thermostat to switch the coil (coil current can be AC) of a contactor that can switch DC.
The problem is not with the temp controller to set thermostat levels but it is with the solenoid to break the connection. So with using the factory thermostat you would replace the temp controller but would still need a solenoid to trigger because the factory thermostat just passes through energy. It cannot convert ac to dc.
 
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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.
You can use a DC relay for a vehicle, but for the coil, you need 12V.
 
A solid state DC relay, with plenty of headroom for open collector voltage and current rating seems the obvious solution, as DThames suggests.
They are readily available and cheap.

Mechanical relays are ill suited to your high current DC application. This is an understatement.

My home is straight 120VDC and 12VDC nominal so I have gone through a DC current switching education by expensive experience. Voltage and current in DC both increase the DC arc length. Welding on contact closing bounce is less common, but happens, but continued arc on opening of contacts with associated melt down is a sure bet if the contacts aren't rated for your max voltage and current. AC current self extinguishes at zero crossings at a maximum of 8 milliseconds, a entirely easier situation.
 
Mechanical relays are ill suited to your high current DC application.
That may depend on the risk. Solid State relays fail closed and if that closed state presents a risk then a properly sized Contactor designed to break under load may preferred.
Mechanical contactors are used in EVs for that reason. In this use case of water heater powered by solar the Solid State relay may be the most cost effective with little downside risk.
 
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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.

A mechanical relay or contactor will only be good for a limited amount of cycles; and that # goes down the higher the load it's switching.

So a couple things:

1) Make sure you are not overly frequently cycling the relay -- some cheap control systems will do this. I had a heat controller that would cycle it very rapidly around the set point - don't expect any sort of duty cycle limiting until you test it. You can use a TDR for this to control it yourself.

2) Consider a SSR of some type as they do not wear out with cycles like a mechanical relay/contactor. They do not like capacitive loads but it doesn't sound like this is a issue for this use case.
 
A mechanical relay or contactor will only be good for a limited amount of cycles; and that # goes down the higher the load it's switching.

So a couple things:

1) Make sure you are not overly frequently cycling the relay -- some cheap control systems will do this. I had a heat controller that would cycle it very rapidly around the set point - don't expect any sort of duty cycle limiting until you test it. You can use a TDR for this to control it yourself.

2) Consider a SSR of some type as they do not wear out with cycles like a mechanical relay/contactor. They do not like capacitive loads but it doesn't sound like this is a issue for this use case.
1. As far as cycling, normally the unit is on and then off when it gets too hot where it resets till then next day. I have the controller set a 10 degree difference window. A lot of times when I go to bed and the water heater temp is 140, it may loose 5 degrees overnight. The only time it cycle a couple times a day is when laundry is being done. Even then, once the unit turns back on, it normally stays on the rest of the afternoon till evening. And since we are talking about a 600 watt element it takes a couple of hours to get back up to temp again so the solenoid does not activate a great number of times daily.
 
This is what I used as far as the temp controller. The solenoid fried in a couple of hours but i like the controller.
The PV is constant current- at least wont exceed the max output of the panels -so possible to think of a different solution. Bridge the PV output!
One of these - $30 Max 300A Continuous Duty Solenoid Relay 12V DC Starter Solenoid...or similar with any of the other suggested no relays. Set up, in off/open state, anywhere between the + and - of pv output. This way no current is flowing through the relay in normal operation. Only when the temperature reached, coil is charged, relay closes. pv is bridged, insignificant current through heating coil !!! Setting the temp controller with a large difference between the high cutoff and restart,,,,say 180f on and 120f off there maybe only one cycle per day or none if the temp is never reached resulting in unlimited lifespan of the contacts.

This wouldn't work with batteries or main because of the unlimited current flow but would work on PV

Edit: attached a diagram for clarity-pv relay-1.jpg
 

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The tyco has a coil economizer built in. Voltage fluctuation no problem
 
Batteryhookup has great deals on contactors occasionally. I got a few Gigavac 500a units with short leads for $5 each.

 
No sir, those controls are AC, they will fry using DC current.
That was not part of the suggestion.

But either AC or D- I meant only as a trigger; typical AC to DC switch derate is 5%. Therefore a ‘stat rated at 20A, for example, could handle a 1A “load” so even milliamp DC to trigger the high load switch should be fine I’d imagine.

The point was there’s already a temp control there and you could use 120VAC with the right relay to handle it.
 
1. As far as cycling, normally the unit is on and then off when it gets too hot where it resets till then next day. I have the controller set a 10 degree difference window. A lot of times when I go to bed and the water heater temp is 140, it may loose 5 degrees overnight. The only time it cycle a couple times a day is when laundry is being done. Even then, once the unit turns back on, it normally stays on the rest of the afternoon till evening. And since we are talking about a 600 watt element it takes a couple of hours to get back up to temp again so the solenoid does not activate a great number of times daily.

Sounds like it's under control no pun intended ;)
 
I've thought about doing this but my plan was to put the solar modules in series and use a standard 240v water heater element. It feels like that would be easier but I can't say why as I'm for sure no electronics expert.

Six 60 cell 250 watt modules will be around ~180v and ~8.3 amps. It's still DC so you can't use the water heater's built in thermostat so maybe it's just a different version of the same problem that @GLC is having??
 
I have some older mercury contactors, this could be an option?
Since the contacts are enclosed there is no oxidation of the contact points, so they never worn out.

I believe those type of contactors are stiil available, see this MDI catalogue: https://www.mdius.com/pdf/CatalogZ.pdf
That's what I was going to mention.
They don't arc, and can be obtained with some seriously high current capabilities.
I'm going to try one on my system.
 
I've thought about doing this but my plan was to put the solar modules in series and use a standard 240v water heater element. It feels like that would be easier but I can't say why as I'm for sure no electronics expert.

Six 60 cell 250 watt modules will be around ~180v and ~8.3 amps. It's still DC so you can't use the water heater's built in thermostat so maybe it's just a different version of the same problem that @GLC is having??
The thermostat is just a relay. It can pass anything. Just keep it low.
Use a very low-current through it to power a really big mercury relay.
Let the mercury relay do the heavy-lifting.
Size the mercury relay control circuit for low-voltage.
None of these things, whether AC or DC, will burn-up the mechanical thermostat contacts.
 
I needed to automatically switch my PV array on and off with the potential for 100+ volts of dc and a current of nearly 30 amps. I ended up using 7 Mosfets wired parallel and it does the job very nicely. I was originally going to use a solid state relay but the resistance was to high. Here’s the thread documenting how I got there.

 
That's what I was going to mention.
They don't arc, and can be obtained with some seriously high current capabilities.
I'm going to try one on my system.
This would cure some problems.
Use the 220vac from grid to water heater, through the thermostat, for control.
Use the 100A throughput to turn-on / turn-off the DC element.
I'd use an NO instead of an NC though.
SKU: 100NC-220A
 
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