"Good enough" calibration of DMM AC ranges? - Page 1
Forum section: electronics / general

author: c4757p
author_rank: Super Contributor
timestamp: December 06, 2012, 08:05:57 pm
content: I have a Fluke 8050A bench multimeter. All ranges are absolutely spot-on except the AC ones, which are pitiful. I tried to perform the calibration procedure in the service manual, but among the test levels it wants, it calls for 19V 10kHz and 100V 10kHz. I used approx. 18V for the lower one (my function generator wouldn't go any higher), and I have absolutely no way to get 100VAC at such a frequency. Did my best to adjust that range using mains voltage but it doesn't seem to have done much good. The two trimmers affect each other, so not being able to properly adjust the higher range has an impact on the lower one as well - and they're trimcapacitors, not resistors, so the frequency does indeed make a difference. Does anyone have any ideas, short of building a nearly ±150V amplifier, to get the AC ranges at least decently usable?

author: SeanB
author_rank: Super Contributor
timestamp: December 06, 2012, 08:17:49 pm
content: Audio output transformer will do the work. Use a scope to measure the output to get it close.

author: c4757p
author_rank: Super Contributor
timestamp: December 06, 2012, 08:30:33 pm
content: "Hi, I have a problem that can best be solved by transforming a voltage" "Use a transformer"Um... well... OK, my question didn't sound so stupid when I heard it in my head!Thank you.

author: tom66
author_rank: Super Contributor
timestamp: December 06, 2012, 08:35:17 pm
content: But what about the accuracy of the transformer ratio?

author: c4757p
author_rank: Super Contributor
timestamp: December 06, 2012, 08:39:00 pm
content: I'm happy with matching it to my oscilloscope, I don't need AC accuracy. I just need it not to be absolutely awful.It's OK if it's not exactly 100V - I can measure what it really is and adjust the meter to read that. I'm not going for cal lab accuracy here.Edit: See my next post, about it reading 13V for 100V 10kHz. I don't really care for it to give good AC readings, but I'd really, really like it to not say "yep, that's only 13V" for something definitely capable of shocking me.

author: c4757p
author_rank: Super Contributor
timestamp: December 06, 2012, 08:40:17 pm
content: Wowit was way out. I got a somewhat reasonable reading off AC line voltage (113V, the line runs about 126V here), but when I probed 10kHz at that voltage it read 13V. Yes, thirteen. Guess I'm definitely going to take the top off and twiddle a couple trimmers.

author: SeanB
author_rank: Super Contributor
timestamp: December 06, 2012, 08:41:18 pm
content: Not a problem, you do not often see people using audio transformers much, unless they deal with PA systems. You just need to drive it on the 8 ohm taps with a good audio amplifier that can give around 10W, and use the 100v line input as the output. The transformers are cheap, and are available from most PA installer suppliers.Failing that take a standard 120V to 6VAC mains transformer and use it in place of the audio one. Will be a little more lossy, but will give the desired voltage with around 5V drive from a 10W amplifier. The amplifier from a car stereo will work as the power driver with the signal generator providing the 10kHz sine wave to it.Use a peak detector and a capacitor ( simple 1N4007 diode, a 1uF 400V capacitor and a 1M resistor across the capacitor) and set the drive at 10kHz to 141VDC across the capacitor. Careful that this output can bite, even though powered by 12V.