Building a variable power supply

[spleenharvester]

Hey all.

I do (well try to do!) board level repair. I need a power supply for the "burning method" used to identify faulty caps (where you have current on max, and gradually increase voltage to find out what gets hot).

I have an FSP NB90 http://www.fspgroup.co.uk/u.k/02_newproducts/NB90.htm which is missing all the tips so would be the ideal candidate for some chopping up I think. Same specs as the original supply used in the laptop.

Pretty simple build, 230v two-prong in, 19v 4.74A two-pin out.

Is there any way to whack a potentiometer on the + lead to get the voltage to vary?

The laptop basically shorts the supply when it's plugged in, my Dell PA-10's overload protection kicks in. But if voltage is low initially it shouldn't be dangerous, should it?

Cheers

[Elysarian]

Re: Building a variable power supply

A pot wouldn't work in this case as it would limit the current as well as the voltage.

This is why (for testing mains-powered equipment) we use a Variac - a transformer with a variable AC output.

I guess you could build a linear PSU with a variac as the input but it would be quite expensive and the output voltage would be unregulated...

[momaka]

Re: Building a variable power supply

The simplest method I can think of (and that I have tried, but only briefly) is to build a PWM circuit based on the 555 timer and use that to drive some large MOSFETs. You'll also need fairly large heat sinks too, though.

[mariushm]

Re: Building a variable power supply

This design outputs up to 18v 2 A



With bigger heatsink than what this design allowed (and maybe using two TIP3055 with the proper balance resistors) , you can take out the relay and send 19v all the time, therefore you won't need a 12v output.

So you can take out everything up to the 3300uF/35v cap and replace it with the leads from your 19v 4A adapter.



You do need +/-8v (I guess you could replace it with 9v using 7905 and 7915 ics) and +/-5v for the opamps, but a +/-12v 0.25-0.5A transformer is cheap, under 10$.

The rest of the parts are jellybean components, tip3055 is probably the most expensive part at around 2-3$

VR3 and VR4 lets you adjust voltage and current limit (short the output and adjust pot to raise current limit)

You can measure current by measuring the voltage drop across that 0.1ohm resistor R57 (which should be probably rated for 10-20w)