'scope smps update

Hi all,

Well, having established that the mains filter had fused and having cut it out of circuit, I was advised to reconnect the supply to the scope and whack up to full volts from the mains supply. I did this through the variac I'd recently acquired and that seems to have blown it, I'm sorry to say. Not the variac so much as my false assumption that a variac is simply a jumbo-sized rheostat that when turned up to maximum could only output no more than its input. Big mistake! I reckon I applied about

10-12% more than the smps's design voltage of 230V to it and now something else has gone phut. No smoke, but a distinct sizzling noise that lasted about 5 seconds before petering out. So the question now is, what type of component is least likely to be able to tolerate this marginal over-voltage? thanks, cd.
Reply to
Cursitor Doom
Loading thread data ...

Rectifiers and caps in the supply .

Reply to
Sjouke Burry

This is a Philips going back a few decades, so I doubt that would apply in this instance.

The chopper in this design is a power TO-3 cased BJT.

I'm just thinking maybe if they'd used some caps with a voltage rating of

250V then that would have been briefly exceeded, albeit by a small amount, by my error. There are some dodgy looking 'tropical fish' caps that showed clear signs of cracking apart even before this incident!
Reply to
Cursitor Doom

Many switch mode supplies do not like a slow ramp up on the input voltage. Some cheap designs require it to have voltage sitting there for proper inrush sequencing to take place for the feed back to get started.

Some of these cheap supplies will actually hold the switching mosfet in a DC state and over head the inductor, power tranny or both.

Most variacs have only a little more than input side, I don't think that little over percentage caused the problem.

Jamie

Reply to
M Philbrook

Is there a zener diode across any of the secondary supplies? Often these short when the supply loses regulation.

Mark Z.

Reply to
Mark Zacharias

Undervoltage on a SMPS causes high currents, so don't try to run it from a Variac at low voltage.

It's easy to see (black soot and melted metal) or sniff out (acrid smoky aroma) a failed component in this scenario. Pay attention to the pass transistor and the bridge rectifier, those frequently fail short (and a mains filter rarely fails at all). Also look for fusible resistors (they're marked like resistors, but act as fuses). If the pass transistor has failed (open or short), also check its base drive for damaged components.

Reply to
whit3rd

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.