Building a fume extractor, measuring voltage from fan case to earth - Page 1
Forum section: electronics / projects

author: jtw008
author_rank: Newbie
timestamp: July 08, 2020, 05:02:43 am
content: Hey guys, hoping I could get a little advice. I made a soldering fume extractor fan using a 240 V fan. The fan only had connections for hot and neutral. I was checking everything to make sure it was safe. I checked the prongs of the plug to the fan body and got open circuit. Then I plugged it in and turned it on, and measured from the frame to earth and I'm reading almost 60 V, which doesn't seem ideal. If I use the LoZ voltage measurement mode it's only around 7.5 V, but I'm not sure what that means.I had a spare fan, so I wired it up separately and am getting about the same result.Should I just use a three lead power cable and connect the frame to Earth?Thanks

author: jtw008
author_rank: Newbie
timestamp: July 08, 2020, 11:56:27 am
content: When I measure with my meter in low impedance mode the voltage drops down to about 7 V, and the current from the chassis to ground is zero, so it seems safe. I might chuck on a three lead power cable and ground the frame to be on the safe side.

author: Ian.M
author_rank: Super Contributor
timestamp: July 08, 2020, 12:39:38 pm
content: There's a significant leakage current from Line to the fan frame, more than there should be from coils with adequate insulation on a stator connected to the frame. A possible source of the leakage current might be capacitors to the frame in an EMI filter, designed with the assumption the frame will be grounded.Therefore, if you cant either cover all exposed metalwork, or isolate it from the fan frame, you *SHOULD* ground the fan frame for safety.

author: jtw008
author_rank: Newbie
timestamp: July 08, 2020, 01:06:59 pm
content: Thanks for that Ian. That sort of makes sense, I'll grab a more appropriate power cable for it tomorrow. If there's capacitors to the frame and it's not just a ghost voltage, why do I measure no current from the frame to earth?

author: Ian.M
author_rank: Super Contributor
timestamp: July 08, 2020, 01:17:16 pm
content: How sensitive is your meter? Most non-grounded plug-in devices are permitted up to 0.75mA leakage current, and if the leakage is a small fraction of that (as would be expected for a fan intended to be used as a component of a larger device), it may be below the resolution of the AC current range you were using.Alternatively, if you know the specified input impedance of your meter's low impedance mode, you can work out the current from the measured 7 volts.

author: jtw008
author_rank: Newbie
timestamp: July 08, 2020, 01:29:52 pm
content: Ah, gotcha. So 7.5 V / 2.1 kΩ = 3.6 mA. I have the eevblog bm235 meter. I'm not sure how to interpret this page of the manual - does this mean in the mA range the minimum it can read is 60 mA?Edit: I just tried it with the meter in uA mode and it's still reading zero...

author: Ian.M
author_rank: Super Contributor
timestamp: July 08, 2020, 02:15:29 pm
content: Hmm. It looks like their low-Z voltage range auto-adapts the impedance to the input voltage so that a calculation based on the initial impedance isn't trustworthy. Try connecting a 10K resistor from fan frame to ground then measuring the voltage across it with the meter in its normal Hi-Z AC V mode.