This 13.8V 7/9A DC (EPS-79) power supply has been sitting idle in my workshop for quite some time. It seemed to be working and I wondered if it wouldn't be able to change the voltage output to something more useful for me. I went looking for a schematic on the net and I was lucky to come across an, what looks like, original schematic (http://www.radiomanual.info/schemi/ACC_powersupply/Euro-CB_EPS-79N_sch.pdf). I also found another one which was very similar. By combining both of them and doing some verifications on the actual power supply I was able to update the schematics so that they resemble the power supply I have (corrected schematic lower on the page).
Can I modify the output voltage from 13.8V to 12V?
I was thinking that if I a would succeed into modifying it into a 12V power supply I would probably be using it more often.
When I saw the schematic it was obvious to me that it would be possible to change the output voltage to 12V by changing zener diode Z1 to an appropriate value.
Maybe it would even be possible to have multiple voltages when I would put in different zeners? I desoldered Z1 and R5 and made a connection to a breadboard to do some testing. As expected, when I put in different zeners, the output changed accordingly.
Maybe I could give it different discrete output voltages?
If I would put in a rotary switch and a variety of zeners, I would be able to change the output voltage to a few pre-selected levels.
Why not try to modify it into continuous variable output voltage?
I was growing more ambitious and was wondering if it would not be possible to make a continuous variable output voltage. What if I would put a potentiometer in parallel with the zener and would feed the wiper contact through R5 to Q3?
I had seen a similar schematic with a potentiometer parallel over a zener on the web previously. (https://www.hobby-electronics.info/nl/elektronicacursus/simpele-regelbare-voeding-schema) In this schematic the zener is placed before the regulator and in my power supply it is placed after the regulator... I was curious if it would work and decided to give it a go. A 24 kohm potentiometer which I had available seemed to give the best results and I got a continuous variable output voltage! To increase the maximum output voltage I decided to put 2 zeners in series and I got and output voltage in the range 0.7 - 23V. Great, not?
What about the stability once under load?
If I would put a load on the power supply, maybe the output voltage would collapse? I did a few tests depending on the loads I had available. The results are in the table. So depending on the load, the output voltage dropped somewhere between 0.3 and 0.6V. Not bad at all for such a rudimentary power supply.
Building the new components into the housing.
I decided to keep the power supply in this setup. I soldered the potentiometer on a small print on which I also installed the zeners and R5. This small print would than, through to potentiometer, be fixated within the housing. Via my previously installed test wires I then connected this small print to the main print.
Adding an internal current and voltage meter.
I was so satisfied with the result that I decided to upgrade the power supply with an internal voltage and current meter. It would greatly increase its functionality. When looking for a cheap solution I came across a plethora of this kind of meters: https://www.amazon.de/-/en/Greluma-Voltmeter-Ammeter-Multimeter-Detector/dp/B09P8J32GN?th=1. I decided to try one.
Basically no overload protection!
According to the text on the housing of the power supply it should be equipped with a built in electronic protection ('protection electronique'). And indeed RV1, RV2 and Q4 should offer some protection... Unfortunately it didn't feel like it was really doing anything when I tested the power supply shortly under various (excessive) loads.
Considering also that this protection was developed for a 13.8V supply and now it was turned into a 0.7 to 23V supply, this protection probably wasn't going to be of much help anyway for most of the working range of this power supply.
I took a few measures to allow for some protection:
First of all, I tried to make the available protection as sensitive as possible: I turned RV2 as much as possible towards the output voltage (position 2 on the schematic) without impairing the regular functioning of the power supply (it is at about 75% of its' range) and I made RV1 as small as possible. I put it at about 25% of its range (close to position 4 on the schematic). Both actions will insure that Q4 starts conducting as fast as possible when the current rises, thus lowering the voltage at the base of Q2 which in turn will reduce the output voltage.
Secondly I added a fast 10A fuse to the output to protect it against short circuits.
Thirdly I calculated how much current the power supply could deliver at each voltage level without going above its rated internal power dissipation. In its rated operation point, the supply would deliver 13.8V - 7A (9A peak). The redressed 18V from the transformer gives about 25V DC at the collector of the power transistors. In its rated working point the power dissipation in the power transistors is thus: (25V-13.8V) x 7A = 78.4W. For peak current this would be 100.8W.
In a spreadsheet I could now calculate the maximum allowed output current for each output voltage without going above its rated internal power dissipation.
A quick check of the datasheet of the power transistors showed they can handle up to 15A and 115W each. The rectifier diodes only have a rated current of 3A though. I also have no information about the cooling element nor about the rated current of the transformer. So overall it seemed prudent to limit the output current to 7A (9A peak) for voltages above 14V as per the original rated characteristics.
I glued a summary of this table on the power supply. It is now up to the user (me) to assure that the power supply remains within these limits.
A basic power supply for a few euros!
While spending only a few euros (for the AV-meter) I turned an item that was useless to me into something which certainly will come in handy on a few occasions in the future. I greatly enjoyed the possibility this project gave me to refresh my knowledge of electronics and to try again some basic reasoning on electronic schematics.
Although this is far from a professional lab power supply and having no current limiter is certainly a drawback for using it on the more sensitive projects, I am certain that for basic power needs it will do just fine.
Although it is nothing special, I am happy that I am able to add this device to my workshop.
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