Using the RD TC66 USB tester as a cheap ($30) 5 ½ digit multi-meter?   - Page 1
Forum section: electronics / testgear

author: theHWcave
author_rank: Regular Contributor
timestamp: March 18, 2021, 01:33:40 pm
content: Hi guys, I know that RD and their USB testers have a bad reputation for spamming the forum. This is not one of these hidden advertising posts. I bought a TC66C with my own money and put it through its paces. I am posting this because I am very impressed by its accuracy and resolution compared against my trusty HP34401A.In fact in is so good that I am now using it as a general multi-meter (of sorts) for data acquisition with its remote control capability (including Bluetooth if you get the C version) and its internal recording capability of 1440 voltage and current measurements. Especially neat is that it measures both voltage and current simultaneously. The big caveat: its DC only (of course)As a start: all the stuff below and measurements assume that you keep the PWR and PD switches in the OFF position and power the TC66 externally through its micro-USB port, either from a PC or a power bank.The TC66 can measure 0..30V with 0.1 mV resolution (probably by oversampling the INA226) and I confirmed that in my tests. It does 0..5A with 0.01mA resolution. I don’t have the means to verify that current resolution but it is certainly at least 0.1mA and 0.01mA is quite possible if they are using the same oversampling trick of the shunt voltage directly instead of relying on the INA226 to convert it to current internally.The TC66 accuracy suffers in the low millivolt and milliamp somewhat because of the fixed offset voltages for bus and shunt voltages in the INA226 chip that does all the measuring. Because of that, the TC66 accuracy gets better the higher the measured values.For volts, it is within 0.1% from 15-30mV onwards and better than 0.02% from above 1.4V.For current it is within 0.5% from 30mA upwards, within 0.1% or better from 250mA and better than 0.05% from 400mA. At higher currents 1A+ the accuracy is an impressive 0.02%. I can only measure currents with that accuracy for <2.5A. For higher currents I used the next best meter I own, the Brymen 869s as reference. The 869s has worse specs that the TC66 so I can’t give definitive numbers for the range from 2.5 to 5A, but from the test it seems that the accuracy of the TC66 doesn’t change, or possibly it changes in the same way the BM869s does so the differences stay the sameThe input resistance for volts isn’t that great but workable, I measured between around 200K to 400K. The burden voltage is a fantastically low 0.015V/A which is better than any of my other meters.I put a little box together to make using the TC66 in a non-USB environment more convenient and a little safer because it adds some moderate protection against accidental polarity reversal and over-voltage, either of which would kill the TC66. It has a switch to turn that off, because the PTC causes a measurement error of 0.1% (it forms a voltage divider of 1:1000 with the input resistance of the TC66). It is also necessary to turn that off when doing current (or power or resistance) measurements. The fuse, that is active in that mode protects only against serious over currents of 15 or more amps. The TC66 will still get fried at 10A before the fuse will blow. You may want to put a 3A or even 2.5A fuse in, if you limit your current measurements of 5A to a few minutes.Schematic, parts list and some additional info is here:https://github.com/TheHWcave/TC66/blob/main/Using%20the%20TC66%20as%20multi-meter.pdfA video about the built:https://youtu.be/iZ5JjA6DoZgMy review of the TC66:https://youtu.be/rOlhibDUJgsIf you suffer from insomnia, even more details on USB-C, INA226 and TC66:https://youtu.be/L70gtJ9hzfQ

author: Algoma
author_rank: Frequent Contributor
timestamp: March 18, 2021, 04:18:27 pm
content: RDTech puts some pretty good engineering effort into their products, given their target price point. I'm curious what they could do given a more mid-range target for their power supplies and test equipment, Their latest RD6006P power supply even has an extra decimal point in the current range (0.0000)They seem to have the engineering to produce an actual full function multimeter, or perhaps even Linear power supply, even if its a hybrid with their existing switching supplies.

author: theHWcave
author_rank: Regular Contributor
timestamp: March 18, 2021, 09:15:45 pm
content: That is true, but I am always sceptical when I see super resolutions. It is easy in software to just show a crazy amount of digits that are not justified by actual resolution and accuracy. For example, there is a general trend in USB testers (RD included) to show more and more digits. It's kind of an arms-race against competitors. Who has the most digits on the display? This is the reason I was very (!) sceptical when I got the TC66. But the accuracy and resolution claims seem to be actually true. The only way I can think of is that they do quite a bit of oversampling of the INA226 to boost the ADC from native 16 to 20 bits or more.I don't know how the RD6006P works and whether the digits are real. I'd love to get my hands on one but can't justify the expense. I have the DPS5005 which works a treat (with far less resolution though). So far it does all I need from a USB controllable power supply (I use my own software to control it).