When it comes to old tech I am a bit of a hoarder. I don’t like the idea of working (albeit obsolete) technology ending up on the scrap heap. The other day I had an idea; re-purpose the housing and power supply for something else. I decided to re-purpose an old blue ray player into an enclosure for one or my raspberry pi projects. I will be putting a separate post up about that soon so check back if you want more information.
Whist taking on the project I realized how how useful old players might be. They have a power supply inside, a front panel with a display and buttons and a rear panel with connection on.
The rest of this post details how to reverse engineer a power supply. I am assuming that the device has a separate PCB for the power supply.
If you are reading this and are tempted to have a go yourself please take the following warnings. You will be getting close to mains voltages that can kill you. I am not going to take any responsibility for any personal injury or death you may cause to yourself or others.
My advice is to make sure you are plugging your projects into an RCD protected circuit as this minimises the chance of getting a deadly shock. Work on your project when someone else is around and make sure they know what to do if you did get shocked.
find what the outputs do
The most important part of this step is to keep the device in tack and all interconnect boards connected. I made the mistake of ripping my device apart only to have to re-connect everything.
Before doing anything else just look at what you have in front of you. Read the writing on the PCBs and look up anything you don’t understand. On the two units I have opened up to re-use almost all the wiring is labeled so the next steps are just a sanity check but still very much worth doing. PCB labels can be like comments in code, misleading, critic and out of date.
With your intact innards, the next step is to find a ground somewhere on the device. There are a number of approaches to this stage. Set your multi meter to continuity mode and try to find two or three points that all share a common rail. Ground rails often tend to be tied to the chassis some way so the screws that hold pcbs down are a good start. Connectors on the rear are likely to have a pin or shield tied to ground. It is not in common for push switches to have a one side connected to ground and often heat sinks will be grounded. My first step to to use the chassis and double check it to one of the locations mentioned above.
Now we have ground, or at least and educated guess at it, we can start to find the positive rails. Set your multimeter to volts dc and connect the negative probe to the ground point you found. With the positive probe measure the voltage of any outputs from the psu and make a note of the voltage. I had to do this step twice, once with the unit in standby and once when it was turned on.
The DVD player in this example is a nice one for a first go as it doesn’t have any fancy power features. The dc outputs are on all the time the player is on, even when it’s in standby.
I simply checked the outputs on here with a multimeter to check the labels where correct. They where so I now have an enclosure with 12V & 5V power rails.
The blue ray player in this example was a little more tricky. It has a switch line back to the power supply pcb that activates more outputs when the unit comes out of standby. This is one way the designers reduce standby power consumption.
When I first inspected the power supply i noticed a number of the outputs had the letters ‘sw’ in front of some of the voltages. I suspected this meant switched and this was proved when I tested with a volt meter. All the labels on the PCB where correct.
The next step was to find the input pin that operated the switched outputs. Again the labels on the board where the biggest help here. A pin marked ‘ata’ was the odd one out and had an unusual voltage 3.8v when the unit was on, opposed to the 12v & 5v I had seen everywhere else. At this point I cut that wire and checked the voltage when the unit was in standby and when it was in full power mode. I measured the voltage on both sides of the cut wire and could see that the 3.8v was coming from the main PCB not the power supply PCB and this was only present after the on switch had been pressed. This confirmed to be that the ATA line was indeed the line that enabled the switched outputs.
One final test was to loop the ata pin to a 5v pin and see if the switched outputs came alive. They did!
A fancy on/off button
I could have used a toggle switch in order to make full use of the second example. I wanted to take it a step further and mimic what the on off switch did originally.
I used a d state flip flop with the inverted output fed into the data input and the clock connected to the on off switch. This meant the data out line would change state every time I pressed the switch.
I made the flip from from NAND gates, mainly because I had a few 4001 chips lying around. Below is the final logic schematic.
The input pin on this was connected to the switch (normally open closed to ground) and a 10K pull up resistor. I connected the output from this circuit to the ata line of the power supply pcb and it worked. A single press on off switch.
This second power supply provided enough power to drive a Raspberry PI and a CD ROM drive.