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Registered Member #3637
Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068
Okay, so here's an interesting one. I've got this interesting SMPS from a tape drive server we're taking apart. It outputs 5 volts at 3 amps, and -5 volts at 2.5 amps.
It's pretty neat, as it seems to be made completely of discretes save for one 6 pin dip, which I can only assume to be an optoisolator (it's covered in hot glue, so who knows?)
As far as using it, I was wondering if it'd be possible to modify it somehow to output a higher voltage, something more useful such as 12 volts. But there's no IC or anything to really change, so is that even possible? It also has a 5 volt regulator on the output (just a 7805).
Registered Member #133
Joined: Fri Feb 10 2006, 10:27PM
Location: Pensacola, Florida
Posts: 47
Without knowing more about how the +5 V at 3 A is being produced, the solution to produce +12 V could either be a simple modification or a more challenging one.
One thing to consider is that without any modifications, you may utilize both outputs to produce 10 V at up to 2.5 A. Use the +5 V node for your +10 V and the –5 V node as the return for your 10 V supply.
Depending on the power supply topology used, you may be able to adjust each 5 V output (both the +5 V and the -5 V) to each produce 6 V and then use the method described above to produce your +12 V. Your current availability may drop a bit depending on the capabilities of the components that were utilized in the design.
The 7805 is likely either 1) being used as another auxiliary +5 V supply at lower current, 2) being used along with a pass transistor stage to produce the +5 V at 3 A, or 3) being used to develop a house keeping supply that is used for control functions of the supply.
Registered Member #3637
Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068
that's odd. When I probed the outputs, none seemed to have -5 volts...just +5's. (I didn't get a 10 volt reading no matter what combination of pins I tried)
Registered Member #133
Joined: Fri Feb 10 2006, 10:27PM
Location: Pensacola, Florida
Posts: 47
The power supply may require some loading in order to regulate correctly. Try placing 10 Ohm loads across each of the 5 V supplies (+5 V to return, and -5 V to return) and measure the -5 V output again. This will place 500 mA loads on each output. If power resistors are not readily available that will handle the 2.5 W dissipation, then you could try some lighter loading (higher resistance).
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How to modify a SMPS for higher voltage output?...
