I like heated seats, especially in a high altitude cruiser with tandem seating. I also like heated socks, vests and gloves, but sometimes you don’t want to have to get specially dressed to go flying—and having all that stuff to fit different passengers isn’t really an option, so having heated seats is a nice touch for airplanes where it is tough to get engine heat to the back seat. I have a heated seat in both of our single seat airplanes and it makes for a comfy ride, especially in winter. Because the little jet has NO engine heat, it is pretty much essential at cruise altitudes.
I had the seats for our F1 Rocket project made with automotive seat heaters installed, and they came with the wiring harnesses usually provided for cars, all of the wiring is PVC insulated – not something I really want in the cockpit, especially in a heating device. I also want to use different switches, and the lengths of the harness might work for a car – but not for the airplane. So some (most) re-wiring is required. In order to do that, I really wanted to understand what the switch and relay were doing, so I pulled out the voltmeter and some clean sheets of engineering paper and started sketching.
It turns out that the electrical designers are pretty clever. These ubiquitous heaters for car seats have two pads, one for the bottom cushion, and one for the back. You have two plugs, once for each cushion, and they are identical. I guess in a naive sort of way, I assumed that “Low” heat powered one pad, and “High” powered both – but have never noticed that you just get one cushion or the other in our existing planes, and it doesn’t seem to matter which cushion gets plugged into which harness port.
So how to they make it “high” and “low” with just a single SPDT switch and SPST relay? With a clever little wiring scheme, the two switch positions put the two heater pads into series or parallel! In “High”, the pads are in parallel, and in “low”, they are in series – and Ohm’s law tells us that with the same voltage, the current (and power) is going to be half in low. Dang clever the Yanks.
