Metering DC Current with DC fans and Thermoelectric (peltier) Doesnt Work? - Page 1
Forum section: electronics / beginners

author: yanir
author_rank: Regular Contributor
timestamp: August 12, 2011, 09:38:03 pm
content: Hi, I want to measure the total current drawn by two 12vdc fans and and thermo-electric device (also 12VDC) running together. This is a common setup for a thermo-electric module. The thermo-electric device has a heat sink and fan on the hot side and "cold sink" and fan on the cold side.I put my meter in series with the thermo-electric device alone and got a 3.3A reading. When I combined this with the fans (which work). I didn't get a reading and the fans didn't spin. I don't know if the thermo-electric device was drawing current (but I assume it wasn't). When all devices are connected to the 12VDC power supply they all work.What's up with that? Shouldn't the meter allow the current to flow freely even if it can't take a reading? What is the proper way to measure DC motor current draw, alone and with other devices?

author: Psi
author_rank: Super Contributor
timestamp: August 12, 2011, 11:18:19 pm
content: There's no problem measuring all devices at once. From the DMMs perspective it's the same as measuring one device.Maybe try it again, there was probably a bad connection somewhere.Note: You may not get much of an increase over the 3.3A when you add the fans, Fans don't draw much unless they are huge.

author: amspire
author_rank: Super Contributor
timestamp: August 13, 2011, 12:11:21 am
content: There are three things that could be happening.1. Peltier devices work by reversing time within the junction, allowing heat to flow in the reversed direction. If you accidentally cross the reverse time vector with the real time vector (don't cross the beams !!!!), you can get a time-space discontinuity across the devices sending the fans each side into parallel universes. All that remains are two useless virtual fans. A Flux Capacitor would solve the problem - I just can't find the design details anywhere.2. The protection fuse has blown on the hi-current range of the multimeter.3. When you are trying to test the total current, somehow you are not correctly completing the circuit.I am not sure all these solutions are likely, so it just leaves the obvious one.I hope you can get those fans back.

author: amspire
author_rank: Super Contributor
timestamp: August 13, 2011, 12:47:00 am
content: I am sure I got that description of the Peltier effect right.The other explanation involved with heat and charged carriers always seemed a bit to unbelievable.If something very weird has happened, like the fuse was blown, it will be very obvious - the hi current input of the multimeter will not work at all any more, so if you repeat the test with just the Peltier device, it wouldn't work either.

author: Psi
author_rank: Super Contributor
timestamp: August 13, 2011, 01:55:23 am
content: You may get some odd effects when you disconnect the power. The fans may continue to run for a little while.Since peltiers can work in reverse and produce electricity if there's a temperature difference between each side.

author: NiHaoMike
author_rank: Super Contributor
timestamp: August 13, 2011, 10:55:41 pm
content: Quote from: Psi on August 13, 2011, 01:55:23 amIt's pretty difficult to get any meaningful amount of power from a TEC. There is a gas water heater on the market that uses a TEC (heated by the pilot light) in reverse to power a microcontroller and stepper motor for digital control, but that's only a few milliwatts at most.

author: yanir
author_rank: Regular Contributor
timestamp: August 13, 2011, 11:34:48 pm
content: I didn't notice any current coming out of the tec when I cut the power.My meter's fuse didn't blow. One of the fans had a connection problem so that may be to blame. I'll check again on monday.The TEC draws 3.3A when on, and the air got cooled about 10 degrees to 63 degrees F. Not super efficient. But the cold side wasn't properly isolated from the hot, so more testing is needed.A compressor based refrigerator (undercounter style) that I have draws 2.5 amps when running. Way more efficient but not ascool, (terrible pun).