I did better than that, Dave. I was just hoping someone had a quick answer.
I first tried the 10A input on my DVM but must have blown its internal fuse at some point in the past - I need to check that now too. I then put an amp clamp on it so as not to blow up my DVM. It checked out around 200 mA. This was pushing the low end of the amp clamp in its lowest (40A) range. But, as it was "safe" at less than my DVM's V,Ω,mA input limit of 320mA max, I was able measure the current directly.
The stack I checked had a steady state current draw of about 135 mA at 18 VDC with an initial reading around 180 mA. The stack started to smoke around 10 - 12 VDC, with decent smoke at about 16 VDC and excellent smoke at 18 VDC. Current draw was about 100 mA at 12 VDC. The resistance must increase quickly as the stack heats up. It measures around 33Ω when read directly with the DVM. A LGB 5006/5007 combo was the power source for the testing.
For those who may be curious, I'm planning to use an ESU 54610 LokPilot V4.0 Multiprotocol decoder and 54670 "Power Pack". The specs are 1.1A continuous for the motor, 250 mA max per function output with a total function output of 280 mA. (FL, RL, F1 and F2). I would have used a LokPilot XL but wasn't sure it would fit. It was a challenge soldering the three leads for the 54670 to the decoder (fat fingers, unsteady hands, poor eyesight, and tiny wires and connection pads) but I think it's OK after checking with a 10x loupe. I don't believe I fried anything by applying too much heat but the proof will be in the testing!
You obviously know this subject well and must have groaned to yourself when you saw my simpleton's response! Good luck with your trial but I should say that anecdotally you would normally allow for up to 0.5A for smoke and (incandescent) lights.
at your 12v readings, you have a resistance of 120 ohms your 18v readings calculate out at 133 ohms your cold resistance of 33 ohms makes sense, since it is not "converting" oil to vapor... that takes more energy, so resistance goes up when vaporizing oil, requiring more power...
the laws of physics come into play, the energy to vaporize oil must come from somewhere. Note at startup you are drawing a bit over 1/2 amp, so it is my opinion that you are risking your decoder.
I'd personally use a relay for safety, or you could add a 33 ohm resistor in series, and run at a higher voltage so your current draw at cold would be from 66 ohms, not 33, cutting your initial current draw to a max of more like 1/4 amp, within your decoder's spec.
I agree that I'll be pushing the limits of the decoder. Once I open up the the U43 and see how much space I really have I may put in a relay and use track power for the smoke. By the way, I hooked up a spare motor and my LokProgrammer to the decoder. The decoder was recognized and updated its firmware and I was able to change the address, set some preliminary function CVs and the motor runs. Hurray! It seems to have survived my fumble fingers!
Also, Santa seems to like his new LokPower V4.0 Micro. Here he is pumping away. All his effort results in about 200 mA max - well within the decoders limit.
With only 2 pickups on each rail and no skates he still gets around OK.
Edit added: Thanks Greg for the series resistor suggestion as I don't have a relay at hand and am eager to get to the U43 this weekend.
Yep, a relay will certainly be less expensive than a new decoder! If you use one, be sure to put the "snubber" diode across the coil... and perhaps a resistor in series to limit inrush upon activation, again the nonlinear nature of the load.