Using the TC66 USB tester as a $30 5½ digit precision multi-meter 

I totally agree with out. Sadly I don't have a 3d printer. Maybe one day .. (we can dream...). PS yes I know there are Makerspaces etc. but with the pandemic and all that and anyway it would be quite a trip each time...

Theoretically the chip datasheet says -0.3V is the max. reverse voltage but I also found it handles a bit more if the duration is short. In my protection circuit the reverse current is limited by a diode, which is at 0.8V for a full blown reverse application a bit more than what is allowed according to the datasheet. In your case, the current will read just fine because it can measure current in both polarities. The fact that you saw 1.1A means some internal protection diode shunted the excess voltage to ground, suffering a high current and possibly heat damage in the process. If the duration was short, it may have survived. If not, your TC66C may still measure voltages but the protection diode is now blown, meaning the next time there won't be any protection and at the very least the TC66C's voltage measurement circuit will likely die or show bad values. I assume you have tested the TC66C with a known good voltage and it is still showing the correct value?

@@TheHWcave "In your case, the current will read just fine because it can measure current in both polarities." I don't quite understand what you mean here. Unlike you I just cut a good usb c cable and am using the positive and negative wires to measure the voltage (using the micro usb port to power the tc66) or using the tc66 connected directly to a PD charger (set to 3.7V) for current and power measurement from, for example, an electric brush motor. Both the voltage and current measurements appear to be the same as before, so I don't think any permanent damage has occurred. I would also like to know if you recommend using the voltage measured by the tc66 to calibrate a cheap multimeter in the same price range? Thanks!

@@KaMyKaZii Sorry, what I meant is that the chip (INA226) inside the TC66C is capable of measuring currents that flow in either direction (indicating the direction with an arrow) but that chip cannot measure negative voltage. So, assuming the TC66C was powered from micro USB when you measured the reverse voltage, the current flowing through the protection diode would show on the display because the chip can measure it but the (negative) voltage cannot be displayed. If the TC66C seems undamaged by the experience, it would be fine to use as reference for calibration of DC volt and current of a cheap multi meter.

Indeed, a hidden gem. Glad you confirm my measurements how good that little thing is. I have used it and its recording function many times now but never for USB stuff which normally doesn't need that kind of accuracy.

You are correct in that low - ohm resistance measurement is definitely not the strength of the TC66 because of the silly display formatting, but it is doable even for very low < 1 Ohm resistances which are notoriously hard to measure with the best multimeters. In the TC66 case we use Ohm's law and need all the digits of the volts and current readouts (and a calculator) to get a surprisingly good result. The fact that it works so well (0.24 Ohm vs a 4-wire measurement by an 34401A showing 0.235 Ohm) is a testament how accurate the volts and current readouts are. I understand that you want to know more about the basic V and A accuracy. Please checkout my two other TC66 videos where I cover this. The first one shows the results of my accuracy measurements while the 2nd one goes into a lot more details on how it manages to be so accurate

It uses the Texas Instruments INA226 chip. I made a whole video about it here: ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-L70gtJ9hzfQ.html

@@TheHWcave I should have known :-) your always very thorough.

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