Monday, March 4, 2013
Water Heater and Solid State Relays Part 2
Part 1 of this is here <link>
I wandered out into the garage to admire my new SSR controlled water heater and noticed that the indicator led was on. It was during the period when the water heater should have been turned off and that can't be allowed to happen. I drug out the ladder, voltmeter, and screwdriver and dug into it. There was no power getting to the SSR control circuit, so it shouldn't have been on, but the indicator led showed there was voltage on the output. Sure enough, there was a little over 100V on the output. Could this brand new SSR have failed already? Fortunately, everything was fine. I just didn't read enough about the devices. They have transient suppression on them that is built right into the module. This transient suppression will allow a little voltage to be seen on an open circuit. So, the water heater had shut off by opening the switch that operates the heating element. The SSR was off, but the suppression circuitry was allowing a little voltage through. Nothing to worry about, but how to test it to be sure?
See, strange readings and unexpected results often happen when you're dealing with 240VAC circuits that use power at 15 - 25 amps. Leakage of a few milliamps is expected and allowed for in this kind of circuitry for various suppression techniques. The relay that used to do this job was a real mechanical open circuit, not a semiconductor.
Testing turned out to be simple; just drain enough hot water out of the tank to kick the mechanical heater switch, and then measure the voltage while the SSR was off. Sure enough, when the element switch closed, the voltage on the output dropped to virtually nothing. Everything was actually fine. The little led indicator just doesn't draw enough current to suck it down to zero and will continue to glow. I also noticed that the led was dim when the SSR was off and bright when the SSR allowed current through. It's hard to tell the difference between the two levels, and I may look into the indicator at some point and see about correcting this. It may just need a resistor in line or something.
No, I didn't waste the hot water. I needed a bath anyway.