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Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
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#1
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'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. |
#2
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'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 . |
#3
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'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! |
#4
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'scope smps update
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#5
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'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. |
#6
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'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. |
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