Most of the electronic load projects on the net are designed for the few hundred mA and several Amps range. I needed something what can go down to few mA-s to be able to test power supply performance will be used for my sensor projects.
So I modified (deeply) the Arachnid Labs re:load 2 for my purposes
AC (~16-17V) plugpack PSU harvested from a Draytek router
Discrete Semiconductors / Power Transistors and MOSFETs
10 Turn 10K Pot
Amplifier and Linear ICs / Operational Amplifiers
The next big thing I want to work on is my home automation project (but just after I finished some of my actually running thing, like the build siren).
There is a well established central management - OpenHAB - available. Controlling it from whatever you want, also not a problem (Web, iOS, Android applications are available).
Building remote sensors, actuators nowadays is easy. Plenty of it can be found on the internet based on cheap IoT platforms like ESP8266 modules. Connecting different kind of sensors, relays, SSRs, just a piece of cake. Powering this remote thing is much trickier, even the mains line is available. The primary goal of my project to save energy. Consuming much power to measure some device power consumption is really bad idea.
So before I begin the project I'd like to find the most efficient way of powering mains connected sensors (remote sensors without mains connection is a different question - battery powering, energy harvesting, etc. - and I want to deal with it later time). There is a handful of way to design a power supply for this. The requirements:
Because it was designed with different requirements. The low current capability and the precision control was not a requirement, but the protection and the self powered operation was.
My design is:
- Powered from independent supply (this time I'll use an AC wall adapter - leftover from my ancient Draytek routers)
- Internal 12V DC supply (higher voltage is useful from many points: OpAmp, selection, optional cooling fan, drive non logic level MOSFET, etc.)
- Changed the current feedback resistor to a much higher value. This decrease the noise and the OpAmp offset problems. On the other side I get 2V voltage drop at 2A load, and I don't want to use this for anything bellow 3.3V. The second drawback is the heating. If the resistor gets hot, it's value will drift by the given 300ppm/degree value.
- Changed the MOSFET to a normal cheap n-channel one (to a type I found in my parts box)
- Added the capability to compensate the OpAmps offset voltage (0.6V negative supply and some trimmers)
- Added range switching capability (0-200mA, 0-2A)
- Changed the normal potentiometer to a 10 turn one
Here is the final schematic:
Built on BreadBoard:
The PCB design:
I was able to finish it in the last three days (picture at the top).