Electronics > Projects, Designs, and Technical Stuff
Switching Power Supply - Can not get it to work - sigh - FIXED!!!!!
daveyk:
Please look at the picture of the resistor, attached. Tell me what its value is please.
If I go by an on-line resistor value calculator at All About Circuits, it tells me 33K, 3%, 50ppm Temco.
If I put the colors in the other way, I get: 212 ohms, 3%, 15ppm tempco.
The BOM shows an 18K Ohm in the location. That can't be correct. It reads 30 ohms. Everything else in the circuit around it is burned up, so I can not trust that reading although the resistor looks okay (there are two 1.5 ohm metals film resistors that look perfect but are open.
I have uploaded a copy of the circuit. Those numbers are NOT component values, but a reference number to the BOM. The resistor in question is component 188.
200 shorted
160 & 162 burned open (1.5 ohmes 1% each)
194 open (SCR)
184 shorted (Fairchild shows to use a 1N914B now, was a 1N4446)
Input voltage to this circuit is 325-350volts DC. This is the last stage PCB of a 5 board power supply. I need to check 158 and 172.
I have ordered all the parts so I hope the transformer is okay. That damn 188 resistor is a mystery to me. Maybe it is a 33 ohm; I dunno. Originally SCR 194 was supposed to be a 2N5061 (or equiv) by the BOM. New it had a BRY39 installed with its anode gate lead cutoff. Maybe 188 was changed for that part? Maybe 188 is just burned up? Usually a resistor doesn't short or go down in value when it burns up or fails.
Help and ideas appreciated.
Dave
pigrew:
The BUZ80 (Q200) has a max Vgs of +/-20V. It needs 5 to 10 V to turn on according to the datasheet. The gate is pulled up by R164+R166 (820k+470k=1.3M).
Using voltage division from 325VDC, R188 would only yield 50 mV if it were 212ohm, which is far too little. 18k would yield 4.43 V, which is nearly enough to turn on the FET, but it wouldn't surprise me if they were having issues that the FET is too close to its threshold voltage, causing it to overheat. 33k would yield 8V, which is a good voltage for turning it on, but may suffer slightly from too much charge being needed to turn on and off the FET. I think that 33k is likely the true value.
I think that the resistor failed by going short. They can do that. You may want to check other resistors in the circuit, too, and probably replace R164 and R166 (the high voltage ones).
EDIT: The BUK456-800B has an even higher Vt, so would need a higher value resistor to fully turn on.
-Nathan
daveyk:
Thank you for the response. I’m not at my computer in the shop and responding on my iPhone.
The +325 feeding this circuit is thru a 1.5amp pigtail fuse. So hopefully if the buzz80 shorted spontaneously the fuse popped (and it sure was flashed black) before the transformer could fry. Hopefully it’s okay and not the cause.
200 was actually a BUK456-800B, so that’s what I ordered, not the BUZ80.
I have orders in with EBAY, Mouser and Digi-Key. Mouser was the only one with 1.5 ohm 1% resistors. I prefer Digi-Key as their shipping charges are a lot less than Mousers, but I’m did what I could to source the six+ parts, ~$55-$60.
I had this instrument last week and it went through the 13+ hour calibration. Shut it off and shipped it to them. They hooked it up, turned it on and it only came on for <minute and started flashing on the crt and shut down. They thought it took a bad hit in shipping (114 pounds shipping weight). I think it is a coincidence.
The 325bolt rail is not well regulated. If the line voltage goes up it goes up to about 350 volts. Hopefully their line voltage doesn’t have nasty harmonics on it. I’ve seen than before in factories when not running off of solo transformers.
Anywho, I should know around next Wednesday if it works again.
Dave
Sent from my iPhone using Tapatalk
duak:
I was surprised that a metal film resistor could be damaged and have its value lowered by 3 orders of magnitude, but apparently it does happen in 5% of cases: https://www.navsea.navy.mil/Portals/103/Documents/NSWC_Crane/SD-18/ResistorsFailure.pdf I understand the mechanism is metal migration, probably from the end electrodes. It would seem that just the right amount of energy in a certain period would be needed. Too much and it would be more obviously damaged.
I've never seen a resistor with that many bands so I found this table: https://static4.arrow.com/-/media/arrow/images/miscellaneous/h/how-to-read-resistor-color-codes.jpg?la=en&hash=21018796E46BD04CD6E01630B12B645BB011FBA2
xavier60:
Those self oscillating choppers are scary, miss a damaged part and it can destroy itself again.
It's possible that the opto has been damaged also. I see no primary side protection for open regulation loop.
Are the rail zenners ok?
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