I needed a high power dummy load that was a bit more health and safety friendly than my bucket of immersion heaters. I investigated lots of possibilities until I eventually found a Tesla Model 3 heater matrix on eBay. The Internet said it could draw up to 6kW, and being PTC, it shouldn’t catch fire if I forget to turn the fan on, which would look great on a risk assessment. So I went for it. 🙂
The lid is held on by penta-lobe screws with a tamper-proof peg in the middle. There was nothing in my collection of tamper-proof bits that would fit, but a sturdy flat-bladed screwdriver worked quite well after Dremeling a new slot or just jamming it in hard enough to break off the peg. 😀
Ooh, fancy! What does all this stuff even do?
There appear to be 6 separate PTC elements, each with low side switching by a 600V IGBT and non-isolated gate driver.
To the left of the IGBTs is a voltage divider and low side current shunt, and on the right an isolated CAN interface and DC-DC converter. Handling CAN communications and A/D conversion of the voltage and current signals, we have an 8 bit ST microcontroller (probably sharing its 0V rail with DC bus negative)
Of course I immediately set to work reverse engineering the CAN protocol so I could command it to connect its elements in 2 groups of 3 for 800V input. Oh wait that’s not gonna work 🙁 we need a hardware solution…
The heating elements were connected to the PCB by spring clips that released quite easily, but the PCB was stuck firmly to the aluminium enclosure with thermally conductive glue. I freed it with the old embedded programming trick of heating the enclosure with a heat gun and prying with a paint scraper. (sadly this doesn’t work any more in Python 3 😉 )
With the PCB removed we can see the terminals for the heating elements, looks as if some sort of spade terminal should fit nicely. (Also note the little nodule at bottom right which appears to be for cooling the CAN transceiver chip.)
And after a pulsating second half the score is Wago 221, CANbus 0. 😀
Do you know what values should be on the 4 pins of the control connector?
No, I removed the control board and didn’t use it.
Could you please measure the ohm value on each ptcheatier. Thank you!
Hi Thomas, measured at 7C temperature in my shed:
704, 787, 795, 797, 867, 819 ohms
Steve
Steve, how do you check the resistance of these elements? Simple case of putting ohm meter across both sides of the element?
Lots of these are failing worldwide and the fix is apparently to find the dodgy one and then simply remove it, refit it and away you go!
Hi Gavin, yes it is simple to measure like you say, you may even be able to get a resistance reading without removing the circuit board, though I can’t confirm this. I think you’d need to remove the circuit board anyway to remove and refit the elements, and this is the difficult part as it’s glued to the case with thermal epoxy.
Any ideas what the minimum voltage required for the heater to work?
I have a 144v battery bank I’d like to discharge and wondering if this tesla heater without the CAN board would be a good fit to discharge the batteries.
Hi Rob, I think this heater would draw a decent amount of power at 144V if you connected all 6 elements in parallel. I connected them in 2 series groups of 3 parallel and it draws over 3kW at 350V DC. It is still in its NTC region like this and increasing power as it warms up.