Posted: Tue Jul 19, 2011 2:14 pm
Something to keep in mind, is that you should be able to control a 240V supply with a standard 120V temp controller via one extra relay. All you have to do is buy an AC-controlled relay and use the standard controller to be an in-line switch on the bias side of the external relay. This way, you could still use the Johnson/Ranco controller for your other purposes (e.g. controlling a fermenting fridge) and have it available to control a 240V supply.
In an AC-controlled relay, the high power contacts will short circuit if an AC signal is applied to the control side (typically 30 to 180VAC). The Johnson controller is already able to control 120V AC signals, so you'd just be asking it to switch on or off the control signal on another relay, rather than asking it to directly switch on or off the supply power to a heating element. I'm imagining a very simple box with two high power 240V cords (one in, one out) and one low power 120V control cord, which would get plugged into the Johnson/Ranco outlet just like a fridge.
Of course, the downside to this approach is that most of the Johnson/Ranco controllers do not switch easily between heating and cooling modes. The ones I've played with require a jumper to be repositioned. I suppose you could use an externally-mounted 3-way switch between the jumper'd pins to move between heating and cooling modes.
If anyone wants to give this a try, I think I've got a couple of extra AC-controlled SSRs laying around.
In an AC-controlled relay, the high power contacts will short circuit if an AC signal is applied to the control side (typically 30 to 180VAC). The Johnson controller is already able to control 120V AC signals, so you'd just be asking it to switch on or off the control signal on another relay, rather than asking it to directly switch on or off the supply power to a heating element. I'm imagining a very simple box with two high power 240V cords (one in, one out) and one low power 120V control cord, which would get plugged into the Johnson/Ranco outlet just like a fridge.
Of course, the downside to this approach is that most of the Johnson/Ranco controllers do not switch easily between heating and cooling modes. The ones I've played with require a jumper to be repositioned. I suppose you could use an externally-mounted 3-way switch between the jumper'd pins to move between heating and cooling modes.
If anyone wants to give this a try, I think I've got a couple of extra AC-controlled SSRs laying around.