Electronics > Projects, Designs, and Technical Stuff

Switching Power Supply - Can not get it to work - sigh - FIXED!!!!!

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daveyk:

--- Quote from: daveyk on April 19, 2019, 03:51:50 pm ---
--- Quote from: xavier60 on April 19, 2019, 07:51:14 am ---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?

--- End quote ---

You want to see another interesting schematic of self oscillation with NO feedback as far as I can tell; I don't think it's regulated against load.  Here is the schematic of the 325 volt supply attached. Oh, and part 42 is a TLC555 timer! Simple switcher.

--- End quote ---

I've replaced all the film capacitors, SCR, FET, Resistors, etc, but I can not get it to fire up.  I did pull the 100 ohm pot and made sure it was good.  I put $250-$300 (with shipping, and getting the exact parts) in to this power supply and it doesn't oscillate.

The initial problem was a shorted FET and SCR and those 1.5 ohm resistors.  I tripple and more checked the correct insertion of the SCR.  I can not get this bitch to oscillate.  There is no shorts on the secondary supplies as far as I can tell.

I'm stumped.   I guess it's time to call it as un-repairable.  The customer has a bunch of parts units that I can get power supplies out of.  He is going to have to send me one now.

Dave

daveyk:
Okay, dummy me, I got it working.  I had 1N914B, 184, in backwards.  If the very tight space, it was hard to tell.  I kept buzzing out the circuit and FINALLY found that.

Does anyone have theories on how pot 186 is adjusted?

Anywho, as is it's oscillating and the power supply outputs are correct!!

Dave




--- Quote from: daveyk on April 29, 2019, 04:29:24 pm ---
--- Quote from: daveyk on April 19, 2019, 03:51:50 pm ---
--- Quote from: xavier60 on April 19, 2019, 07:51:14 am ---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?

--- End quote ---

You want to see another interesting schematic of self oscillation with NO feedback as far as I can tell; I don't think it's regulated against load.  Here is the schematic of the 325 volt supply attached. Oh, and part 42 is a TLC555 timer! Simple switcher.

--- End quote ---

I've replaced all the film capacitors, SCR, FET, Resistors, etc, but I can not get it to fire up.  I did pull the 100 ohm pot and made sure it was good.  I put $250-$300 (with shipping, and getting the exact parts) in to this power supply and it doesn't oscillate.

The initial problem was a shorted FET and SCR and those 1.5 ohm resistors.  I tripple and more checked the correct insertion of the SCR.  I can not get this bitch to oscillate.  There is no shorts on the secondary supplies as far as I can tell.

I'm stumped.   I guess it's time to call it as un-repairable.  The customer has a bunch of parts units that I can get power supplies out of.  He is going to have to send me one now.

Dave

--- End quote ---

daveyk:

--- Quote from: duak on April 20, 2019, 03:29:16 am ---I wouldn't think that the value or tempco are particularly important, unless the resistor is used to compensate for something like temperature.

--- End quote ---

The board didn't appear to have an other re-work.  That 6 band 33K, 1%, 50PPM tempco resistor has been replaced with one of the exact value.  You may be right that the value doesn't mean much.  The BOM calls for an 18K.  Go figure.  Probably an update since they weren't using the BUZ80 anymore and the SCR was a different type.  Everything else was exactly as called for in the BOM.   From what I can tell, any SCR/Thyristor may have worked as long as the voltage spec was high enough.  I dunno.

Any idea how that pot (100 ohms) 186 was supposed to be adjusted?  I left it as it was.  It is paralleled by two 1.5 ohm resistors and then the tap has a 470 ohm resistor going to ground.  The adjustment was probably fairly minor.

I'm mentally burned right now.  I think I am shutting the shop down early today and go out and pressure wash - lol.  I had a very hard time getting it to work until I realized, I installed 1N914B diode 184 in backwards.  I think it would have just mimicked the SCR turned on all the time. 

When I try to figure out how this power supply is supposed to work, with the secondary feedback through the opto-isolator, I get a migraine.  When I went to Electronics school in the late 1970's, switching supplies were not taught, and if they would have, they would have just skimmed over the basics.   I have been watching switching power supply repairs on YouTube.  Some actually try to teach the theory, some just show the repair.  None come close to matching this monstrosity.  Keep in mind this is just one board out of five that could have problems.  The other power supply boards have the same kind of excellent schematics (sarcasm).

I don't have all the caps needed to recap all five boards.  I only found one swelled capacitor and leaking capacitor and it's been replaced.  For the future, I need to get a supply of capacitors in-stock.  That isn't easy.  I haven't found a good 105 degree electrolitic "kit" for purchase that covers most of what someone would need.  I am going to go through the BOM and order 10 of every value I see and stock them.

Dave

xavier60:
The Pot adjusts the power supply's current limit.
While the MOSFET is on, primary current ramps up. This causes a corresponding ramp up of the voltage drop across the 1.5Ω shunt resistors. The Pot divides this voltage and applies it to the SCR's gate which turns off the MOSFET.
When regulation voltage is reached, the opto-coupler adds current to the SCR's gate circuit causing it to fire earlier causing the MOSFET to be turned off at a lower primary current.
If you decide to adjust the Pot, make certain that is has no dead spots and stay away from the bottom end. This will totally disable the current limiting.

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