Sunday, August 30, 2015
Calibrating a cheap crappy tire multimeter
Anyone living in Canada is likely familiar with CT products carrying the Mastercraft branding. They are significantly overpriced to allow for heavy sale discounts of 50-60% that usually happen a few times a year. About 10 years ago I bought a Mastercraft model 52-0052-2 auto-ranging multimeter when it was on sale for C$20 (about US$15 at current exchange rates). I use it for low-voltage, and have another multimeter with a current clamp that I use for household (mains) 120/240V testing.
I've been using it a lot over the last few years, and started getting a feeling that it was reading a bit high, based on readings from 3.3 and 5V regulators, and even from comparisons with battery voltage readings. After testing a batch of TL431 voltage references, I was able to confirm that it is reading between 0.6% and 0.7% high. I also ordered a couple 0.05% REF5050s, which will allow me to double-check my TL431 measurements.
After opening up the meter, I found there are 8 trimmer pots, and no indications on the board as to which of these adjusts the voltage. I eventually found a discussion on EEVblog that indicates the meter is made by the Hong Kong company Colluck, and is spec'd for 0.8% accuracy on voltage readings. I have what DIPlover calls the old model, which measures 9cm wide and 18.5cm long. While I still couldn't find documentation on how to adjust the calibration, I did find a review of another cheap multimeter that had several trimmer pots, and the pot labled VR1 was the voltage calibration trimmer. I figured I had little to loose by trying the same pot on my meter.
In case VR1 didn't adjust the voltage, I needed to adjust it back to it's original position. I used an ultra-fine sharpie to mark a small line on the top and the base of the trim pot so I could locate the original position. I used a TL431 which was reading 2.513V, which I though should read around 2.496V, connecting it to my meter probes with a hook probes. With the first small adjustment of the pot, the voltage reading went up to over 2.53V, so I had the right pot. The sensitivity was a bit of a problem, as the tiniest adjustments I could make were undershooting and overshooting. After several tries, I got a reading of 2.496V, which I think is within 0.1%. With a couple of REF5050's it should be possible to calibrate it to +- 1mV, or 0.04%. But given how sensitive the trim pot is, I won't touch it as long as it is within 2mV.