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:
- As efficient as possible
- Able to provide 200-250mA at 3.3V (maybe 5V) (Power consumption of the ESP8266: http://bbs.espressif.com/viewtopic.php?t=133)
- As cost effective as it can be (I want to build large amount of devices, so the cost matters)
- The electrical isolation is clearly not required. The efficiency is much more relevant
Ok, I don't want to go through all of the possible solution (Resistive, Capacitive, Linear, Isolated and non-isolated switcher, etc.) now. I want to buy/build some, and test them.
For comparing the performance of the power supply variants I desperately need a constant current DC load.
First I was looking around the forums, markets, to find a solution. I was looking for two kind of devices:
1. A cheap one, what can be used as low current load, and not heart to put it aside when I need a pro thing
2. A professional one, what can be used later on, for other projects.
In the first category I found none. Looked around the ebay, aliexpress, tindie, but all of the things I seen there was not suitable for the task or not fall into the first category.
In the second category I found the Maynuo M971x series:
There are quite good reviews about it on the net, but my actual needs doesn't justify $400-600 spending (depending on model it will became the amount together with shipment and customs fees).
So my decision is to build one.
I said before, that I didn't find a suitable cheap one. It is true for the already built models, but on the other hand I found the device what I can build upon. The re:load 2 from Arachnid Labs. http://www.arachnidlabs.com/blog/2013/02/05/introducing-re-load/
Why I not just bought it?
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).
Finally I decided to change the box's original front panel to a 3D printed one. The main reason behind that I didn't want to work on drilling/tweaking the holes on it.