Tau Labs Power Distribution Board
First of all, apologies for not being very active on this blog since the beginning of the year – I am way too busy with tons of projects going on, and too little time to write about them.
This is a short article about a small board I just designed for a Tau Labs quadcopter I am building. One of my 2015 new year resolutions was to slowly get back into UAV design work after a nearly 2 year hiatus, and so far I am sticking to that one. Which also explains my introduction in the previous paragraph!
The quadcopter I built is based on the popular “ZMR250” carbon fiber frame that can be found on many Chinese sites (“Bang Good” or “Good Luck Buy”) for about $30. For that price, you are of course getting something a bit rough, but build quality is actually surprisingly OK.
One thing that is missing from those frames, though, is a proper power distribution board (PDB): while a PDB is not a hard requirement, it makes wiring a lot cleaner and the quad more reliable. And it is also a good place to put a current and voltage sensor, as well as a good quality power supply for the electronics.
For this reason, I decided to design exactly that, using the very popular 36mm square format that we all know and love.
As mentioned above, this Power Distribution Board does the following:
- Pads for up to 8 motors (double-sided)
- Built-in current and voltage sensor (G/V/I pins at the bottom)
- Built-in 5V 3A switching BEC, with high quality capacitors/inductor and very low ripple current. (pins on the upper left, “+” marked next to positive 5V output.
In its default configuration, the Voltage sensor is setup to measure 0 to 17V. It can be built with a current sensor that can measure up to 40A, or one that can mesure with up to 20A with better accuracy.
The PDB was tested with 4 12A ESCs on my ZMR250 with absolutely no overheating.
Current measurement accuracy
One figure that rarely gets mentioned on PDBs or current sensors, is their accuracy – though this is actually super important! The main reason is that there is no cheap way I know of to create a current sensor that will be accurate at both low and high current draws. My PDB is no exception: you can expect more than 95% accuracy on current measurement above 10A on the 40A version, and 95% accuracy above 3A on the 20A version – the sensor is usually pessimistic, so you know actual energy spent is a bit lower. Under 10A though, accuracy degrades fairly fast, and the sensor won’t really measure current draw under 1A with any sort of accuracy. Again, this is exactly the same as 99% of all sensors you can find on the market.
In real life though, since current draw on a multicopter will usually be well above 10A while you are flying, this is not an issue. But if you are thinking of using the current sensor for measuring standby current draw, this is probably not the right device.
Wiring of the PDB should be fairly self-explanatory, but just in case, here is a nice diagram to make sure there is no ambiguity – please give me feedback if you think it is not clear enough!
Configuring on TauLabs
You should connect the PDB voltage and current measurement pins to the Analog inputs of your flight controller. This PDB was tested on Sparky, Sparky2 and BrainFPV and will not output more than 3.3V under normal use.
The voltage measurement bridge gives you 188mV/V , and is very precise.
The current measurement output gives your 62 mV/A in 40A configuration, and 135mV/A in 20A configuration, and as mentioned above, is precise above 3A in 20A mode. You can also work on the offset to get better readings at lower current consumption.