Hot tub Part 2: Need to switch low voltage AC with low voltage DC from PID
Above was my 1st post about my hot tub project....
Now I have installed my PID controller and thermocouple only to find the SSR I got won't turn off after it's activated. I'm pretty sure it's because the SSR is rated for a min of 24 volts AC on the output side (DC input), and the voltage I'm switching is only 7 volts AC. The 7 volts AC is the control voltage the hot tub uses to trip the high voltage contactors inside the electrical panel. The way this tube is wired, it's a lot safer and easier for me to switch the control voltage instead of the heating/pump high current/voltage. I need a solution for a "low voltage" SSR... I am at a loss on what to use. The PID output is 8v DC and was not strong enough to trip an auto relay. My PID also has 2 "alarm" relays which would be sufficent, but they don't work the same way as the SSR terminals do (if they do I don't know how to set that up). Any ideas on how or what I need to get this going? Hope this made sense and thanks for your help!
You might want to verify that it is AC across the contact side of the SSR. SSRs usually use TRIACs or side-by-side SCRs to switch the load and they rely on the zero crossing to turn off. The 24v rating could also be your problem, I have never tried running one that low.
If the PID output is strong enough to drive a small relay coil, If would probably be easiest to just add another relay instead of an SSR. Remember the clamp diode, the PID controller may not appreciate the inductive spikes.
Originally Posted by KI6LNG
More than likely because AC SSRs often need a resistive load in order to turn off. An inductive load, such as a transformer coil, or the coil on your relay, can prevent the SSR from going into the off state.
Try placing a resistor across the SSR output that will, when combined with the load already presented by the relay coil, not exceed the amperage rating of the SSR, but will draw enough current resistively to allow the SSR to turn off.
Rather common problem for the SSR circuit designer, actually.
Some interesting information here; http://www.omega.com/temperature/z/pdf/z124-127.pdf.
Consider use of a buffer transistor between PID output and load. Common practice.
Originally Posted by KI6LNG
NPN or NPN Darlington, the TIP130 is a good choice that can handle considerable current in these applications. One transistor and one or two resistors is all it would take and then you can switch an electromagnetic relay at the output.
I was actually able to find a setting in my PID thast allows using the relay as the control output, so no ssr needed. Now the pid relay is taking the place of the thermostat in the control circuit. Thanks for the help guys!