diy solar

diy solar

Use AC water heater thermostat to switch milliamp 12V DC relay?

soylentgreen

New Member
Joined
Apr 23, 2021
Messages
88
I'm cleaning up the wiring in my Class B RV where a previous owner did something of a hash job: they added a water heater, and took 120VAC wired from the inverter to a SPST switch on the DC panel then snaked a 120VAC extension-cord wire around to a tiny resdinential-style water heater. There were badly soldered connections, wire nuts in a few places (loose!), absolutely no fusing, etc.

Also, I really didn't like the idea of switching 120VAC on a panel otherwise meant for DC.

I've already redesigned the system using a 12VDC opto-isolated relay so that the 120VAC switching is done at the 120VAC panel, and the "water heater on" switch is now only a 12VDC trigger relay. Here is the DC/AC relay board I'm using: https://www.amazon.com/gp/product/B00LW15F42

My DC relay board actually has two relays, so I realized I could make the system even safer by eliminating one of the 120VAC runs, and using the 2nd relay to switch 12VDC through the thermostat, rather than running 120VAC through the thermostat.

The water heater has a standard "59T" therm-o-disc thermostat as shown here: https://climate.emerson.com/documents/bimetal-59t-66t-section-en-us-5469062.pdf

The problem: I understand that most water heater thermostats are designed to switch high voltage AC only, and if you try to use them to switch high voltage DC, they will burn out, due to arcing. This makes sense.

However, I would be switching low voltage DC - about 12V at 40 milliamps for the DC relay trigger circuit.

Question: do you think a 59T / 4090 style Therm-o-Disc water heater relay would be able to switch this low voltage / low amp DC circuit without issues?
 
Sure. No problem. There is a general rule-of-thumb for AC switches used for DC: Within their amp rating, 120 VAC switches can safely switch up to about 30 volts DC.
 
Excellent - I've wired it up, and let it run through a few thermostat on/off cycles, and it's working great so far. (y)
 
Back
Top