Analyzing and Hacking a Power Supply Board

Tonight I am going to describe the repurposing of a power supply board from an obsolete dental X-ray machine. Before I start, however, I want to digress and go back 60 years to the beginnings of my interest and enjoyment of electronics.

The man who got me off to a good start was my Dad. Now you might guess that he was also involved in electronics or at least had an interest in the subject but this is not the case. My Dad had no electronic or mechanical skill whatsoever. If he had any ability with pounding a nail or working with a screw driver it was not evident. He was a salesman and his job demanded that he travel around the state usually within a days drive of home. His lack of electro-mechanical ability however did not stop him from recognizing that there was an interest in me. His way of encouraging me was to stop at second hand stores and bring home, old crank telephones, old tube radios, and any other piece of technology that he could find. He did not know or understand the fascination that I had for these things but it made him happy to feed my interest. Today when I sit in front of a circuit board and begin the process of analyzing it for potential further use and then modify it or disassemble it I am always reminded of the life long happiness my Dad helped me discover in electronics. Thanks Dad!

A few days ago I mentioned I received a box of scrap circuit boards. I have chosen one that came from a Gendex Panoramic X-ray Machine. This x-ray machine which was still perfectly functional was thrown out before its time, due to obsolescence. Technology is racing ahead so fast that equipment does not have time to get old and wear out before it is obsoleted by new technology and new regulations. In this case the Gendex X-Ray Machine was designed to work with film and presently the new technology machines produce better pictures with much less radiation by using a digital sensor and computer processing. This board is only one component board of the power supply for the x-ray.

The first challenge is to figure out the circuit well enough to power up the board. I begin with the individual components. The section on the right had 2 power transformers, a relay, 2 bridge rectifiers, a couple fuses, some capacitors, and 3 linear voltage regulators. This was obviously a linear power supply and from the compliment of regulators it should have plus and minus 12 volts as well as 5 volt output.

In the middle of the board there was another transformer, bridge rectifier, and 2 large caps. Primary and secondary voltages were printed on all the transformers which is very nice and not that usual. Based on this I suspected that there was a source of 30 plus volts available from this middle section of the board.

The far left of the board had still another transformer but the secondary of this one left the board on one of the headers and was apparently intended to power a different circuit board.

After tracing the input power circuit that would feed the primaries of the transformers I discovered that the board was equipped with a low voltage (12 volt) circuit that would turn on mains power to primaries of the transformers. The source of this 12 volts is the smaller transformer which is designed to have power to its primary continuously so that the machine can be turned on and off.

I was now ready to power the board and continue the process to decide if any part should be repurposed or if I should just disassemble it for parts.

Powering a board like this can be very dangerous as the voltages involved can easily kill a person who does not follow good safety practices. I like to use an isolation transformer so that the voltage while still potentially lethal does not have a circuit to ground. I am careful and respectful of the danger. It is always a good idea to get help from an experienced person if you haven't been trained to work with mains voltages.

The board when powered up had plus and minus 12 volt supplies, a five volt supply , and a 35 volt unregulated supply. I had an application for the 12 volt and 5 volt outputs but not the 35 volt one. Most of the components on the left end of the board proved to be associated with other boards and were not needed. I decided to remove the un-needed parts and modify the right third of the board to accommodate my need for a +/- 12 Volt , 5 Volt supply.

Here is what the board looked like with un-needed components removed:

After a few minutes with a saw it was trimmed down to a functional size:

The ground, low voltage switch line, and +12 volt connections were already on the gray terminal strip in the upper right corner. I would need to locate and move the -12 volt, 5 volt outputs and in addition I wanted to add a few extra ground connections. The removal of the left side of the board had fortunately left  9 connection pins empty and unused. I decided that for the fun of it I would also add monitor LEDs for each of the output voltages. These would be installed in the lower left of the board where I had removed a header that had the three output voltages originally.

I used a dental drill and cut several of the un-used traces and I modified the traces in the lower left to work with the LEDs. Here are a couple pictures of these modifications:

Now all that was left was to hook it up and test it. My little breadboard meters will not read negative voltage but if I hook them up with revers polarity I can trick them. In this next picture the middle 12 volt reading is really a negative 12 volts with respect to the ground. Sorry the camera does such a poor job with the LEDs They are really quite clear to the eyes.

As part of the test process I also hooked up the outputs to an electronic load. The main 12 volt regulator has a small heat sink and seemed to be fine with delivering 1.5 amps continuously. The negative 12 volt and 5 volt regulators however were just attached to the board and it was apparent that 1 amp draw from them could not be sustained for more than a few seconds. They did however perform and stabilize with a draw of 500 mA. This could probably be doubled if small heat sinks were to be added. Fortunately the application that I have in mind for this power supply, a signal generator kit, will be more than happy with the present current capabilities. Here is the finished board complete with labels:

If you look close you can see that I sealed the cuts in the circuit board traces. I went to the local dollar store and bought different colors of nail polish that I like to use for applications like this or just to patch up scratches and other blemishes. In this case it is a slightly lighter color of green. Just in case I change my mind on how to use this power supply or any of my board hacks I always produce a paper file, like a simple data sheet, that I can use to refresh my mind with the circuit in case it sits on the shelf longer than my memory lasts. Now days that isn't very long sometimes. The paperwork will just be the notes I took while analyzing the circuit and any pictures that were printed. These will be kept with the circuit board until it is incorporated into a device and then the paperwork will become a part of a larger set of notes on the host device.

John