ebay power supply

[ecat]

I bought three of these 500W 50.5V power supplies for £12 way back in 2009.. as time has passed I mislaid the documentation.. A google search Found the missing PDF here. My Lazy way of solving this problem was to buy three 20 Amp bridge rectifiers to limit the output Voltage to Less than 50V therefore solving the slightly higher than 50v output.
My reason for this solution is messing with switch mode PSUs is quite dangerous. (a friend of mine was hit in the temple by a flying electrolytic case when delving into a switch mode psu. With such force it caused a tear in his retina.)

Your circuit hacking/reverse engineering provides a more elegant solution thank you.

Is their any reason why my "Lazy solution" of 2 x the diode volt drop was not considered as a solution in regulated mode?

Good question, I think at the time my list of solutions looked like:

Plan A
Modify existing circuit.

Plan B
There should always be space for a plan B even if it lacks a concrete definition ;)

Plan C
As the required voltage drop is small use one or more semi-conductor junctions (diodes or transistors)

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Plan A
The PSUs were quite cheap so it didn't really matter if I blew one up. In fact the 'spare parts' alone would be worth more than £12.

I had two PSUs to play with, potentially losing one was not a problem. In fact I did lose one, I left it in the office and forgot to bring it home when the company closed down - Grrrr

The whole idea of switch-mode PSUs is to efficiently convert one voltage to another. Modification is the correct solution.

After all that, any modification had to be as simple as possible, as safe as possible and not involve anything near the high voltage side.

Should only be attempted by those who are aware of and acknowledge and are are happy with the risks involved in opening a SMPS


Plan B
Very simple solution.

Does not involve opening the PSU

Safe to implement but in use this sense of safety is somewhat deceptive:

1) The forward voltage (Vf) drop of a semiconductor junction (diode) is not a fixed value. Buy rule of thumb we often assume something around 0.6v, 0.7v but look at the datasheet for the device you intend to use and you may find that Vf = 0.7V at 1A but up to Vf = 1.0V at 10A and as low as 0.4V at <0.1A. So two diodes in series may give you a 1.4V drop or they may give you a 2.0V drop or a 0.8V drop - over 50% variation depending on current draw. Whether or not this matters is entirely down to what you are trying to achieve, but I'm sure some bright spark is certain to attempt to use 10 to 15 diodes in series for 7V to 10V drop - well, at 50% or more variation this is when the magic smoke can escape.

2) Heat! If each semiconductor junction drops 1V at 10A draw then each and every semiconductor junction will dissipate 10W at 10A draw, for continuous use we now need heatsinks and possibly insulation.

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That covers most of the risk / reward assesment. Two diodes are great if you've done your homework and everything checks out, in my case I was happier making the mod :)