'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.

'scope smps update
 

On 05.04.15 14:17, Cursitor Doom wrote:
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.

Rectifiers and caps in the supply .

'scope smps update
 

On Sun, 05 Apr 2015 16:21:22 -0400, M Philbrook wrote:

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.

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

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

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

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

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!

'scope smps update
 

In article , says...

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.

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

'scope smps update
 

"Cursitor Doom" wrote in message
...
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.


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

Mark Z.

'scope smps update
 

On Sunday, April 5, 2015 at 5:18:23 AM UTC-7, Cursitor Doom wrote:
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 ...I did this through the
variac ...and now
something else has gone phut. No smoke, but a distinct sizzling noise
that lasted about 5 seconds before petering out.

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.