Measuring current through a DC fan - Page 1
Forum section: electronics / beginners

author: wardenclyffe
author_rank: Contributor
timestamp: June 16, 2011, 04:21:01 pm
content: Hey guys, I am almost embarrassed to ask this question, but is there some special way to measure current through a DC fan? I pulled a fan out of my older stereo that is rated at 12Vdc, 0.8A to use elsewhere. I hooked it up to a 12V wall adapter just to see if it still worked and it did. When I tried to measure current with the meter in series with the power rail, the fan would not operate and the meter read 0A. The fan worked fine when I tried it at 12V again and using lower voltages as long as the meter was not also connected. I don't have much knowledge at all about motors and fans so I was wondering if someone could tell me what I am doing wrong here.Help is always much appreciated!!

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: June 16, 2011, 04:25:16 pm
content: 1) Open the DMM and check the fuse inside, probably is burned out.2) Post a picture of your DMM.

author: Simon
author_rank: Unknown
timestamp: June 16, 2011, 05:57:18 pm
content: Presumably the meter was set for current and you had the probes in the right sockets

author: Zero999
author_rank: Super Contributor
timestamp: June 16, 2011, 06:47:39 pm
content: Quote from: Simon on June 16, 2011, 05:57:18 pmOr maybe not and that's the problem.This could be worse, some nubes try to measure the current capacity of their power supply/battery by connecting their meter set to measure current across the output terminals.

author: sacherjj
author_rank: Frequent Contributor
timestamp: June 16, 2011, 06:50:48 pm
content: If you had the probes in the Voltage position, you introduced a very high resistance in line, which would make this happen. (The open circuit for a current blown fuse is an even higher resistance which is also a good guess.)

author: wardenclyffe
author_rank: Contributor
timestamp: June 16, 2011, 07:22:09 pm
content: If I managed to do that, and the more I think about it that is probably what happened, I think I may just quit this electronics stuff altogether. Thanks, guys.

author: sacherjj
author_rank: Frequent Contributor
timestamp: June 16, 2011, 07:36:10 pm
content: Quote from: wardenclyffe on June 16, 2011, 07:22:09 pmThe only one to be really worried about is measuring voltage with the current setup. My dad killed my Heathkit DVM with that move. They didn't have a 10A fuse, just a 10A trace that line voltage vaporized.

author: w2aew
author_rank: Super Contributor
timestamp: June 16, 2011, 08:39:39 pm
content: Another possibility is that the drop-out voltage of the DMM in current measurement mode might be too large, and not leave enough voltage for the fan to start/run. This could especially be the case if the fan is a Brushless DC motor which includes some driver electronics buried in the fan. Try measuring current on the highest current scale on the meter to minimize the dropout voltage...

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: June 16, 2011, 11:00:35 pm
content: People you are replying to some one who had introduced him self in this forum as" my name is Chris and I am a recent graduate in electrical engineering. "Oh well yes quit that stuff .. save your self

author: Mechatrommer
author_rank: Super Contributor
timestamp: June 16, 2011, 11:15:26 pm
content: better to take phd?

author: Semantics
author_rank: Contributor
timestamp: June 17, 2011, 12:49:19 am
content: Quote from: wardenclyffe on June 16, 2011, 07:22:09 pmEh, don't be so hard on yourself. One of the first things I remember doing was measuring a car battery voltage except I didn't pay attention and grabbed a meter with probes in the mA sockets. Oooh yeah, too cool for school, yo.Think of it as a $7 lesson learned: cost of a new fuse, and move onwards.

author: MrPlacid
author_rank: Frequent Contributor
timestamp: June 17, 2011, 01:07:37 am
content: Quote from: wardenclyffe on June 16, 2011, 07:22:09 pmYou can't. Do you think you can just toss your interest out the window that easy? Soon, you'll have a compulsive disorder and end up buying boat loads of multimeters like the rest of us here. Haha... this is your future and you can not change it!