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Couple questions on SMPS module and tuning voltage output

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fastline

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Basically, we need to configure a power supply for a machine that has +24, +8, +5, -5 volts. We have a supply here that has 2, 5, 5, 12, 24 volt modules in it. Bought it used. Everything works but the dual 12V module will start smoking after about 1min run time. Current into the whole supply is only .4A at 120V so with PF considered, probably 10 watts.

I quickly unplugged and inspected the module. There seems to be obvious heat around the resistor array shown in pic. The bands are even a little cooked. I think these are supposed to be blue and they are now gray. I am curious if this seems like an isolated resistor issue or (probably) if this is pointing to a bigger problem and possibly the output caps not doing their job? I rarely see power resistors go bad for no reason and they all seem roasty except for one.

Also, This will come as possibly in idiotic question but the OEM does not support these supplies and we REALLY need to get our 8V somehow. I was considering just installing a regulated on the 12V module to buck down to 8V or maybe add a transformer but kind of concerned about that. Can a reg fail shorted? This circuit seems pretty smart so I am not sure I can fool it. It might just shut down but comparing modules of different voltages, it is obvious there is more than a couple differences. Transformers, resistors, diodes, not to mention any chip programming there might be.

For anyone not familiar, this is a modular supply in which the front end is just a DC rectifier and supplies 330VDC to a common rail for these modules to soak up.
 

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I'm not sure what your asking, but there are a few things to be concerned about.

1. An SMPS may require a minimum load on certain specific outputs. Without that load, even a good PS will burn up.
2. Certain systems that the power supply delivers too may require a certain power supply sequencing.
3. There is a difference between the performance of a system using a +-5V power supply and an +-5 tracking power supply.
 
Just to get back to the repair end of this card, I am curious what might cause the pack of resistors shown in the pic to get SUPER hot and discolor? They actually start smoking after 1-2min of idling. There are 4 other cards like this in the supply and none of them smoke or have discoloration so I am confident is has nothing to do with minimum load and there is something wrong with the supply. I thought most of them had built in min load via resistors anyway?

I did remove all the resistors and tested. They are al good at 68 ohms each. Would really just like to figure it out for my own sanity.
 
IN looking at this circuit, it is obvious that this pack of resistors are parallel with one of the outputs probably to statically load the circuit. The card produces the required voltage and all the resistors out of circuit measure 66.8ohms. I cannot even read the bands on them anymore but if you do the math, that is only a shade over 2 watts!!! The reason I opened the circuit in the first place is the resistors are smoking and obviously hot. They, however seem to be deteriorated on the outside and possible the OD has burned away. I am curious if anyone has seen where a resistor will run hot long enough to actually reduce the size? Could it be that these resistors now do not have enough mass to dissipate the heat?
 
I don't think the problem is with your power resistors, it's unlikely. Using autorange and measuring resistance, what do you read on the 12V line wrt GND? It seems you have a short, not a dead short for those resistors to start smoking.
 
But the resistors are "directly" paralleled with the output. They will see a full 12V. Unless the resistance somehow dropped to cause an over current condition, I fail to see how the circuit would work any other way.
 
Are you saying the 12V line does not drop when the resistors start smoking? 12V remains stable? Its seeing too much current, that's why the smoke.
 
I'm not sure they are 'seeing too much current,' but they are dissipating just over 2 watts, a bit high for 2watt resistors. As long as there is 12V across your 68 ohms, there is 176mA flowing through them. 12V x 0.176A = 2.12W. If your 12V seems stable, then just scale them up to 4 or 5 watt versions.
 
The resistors may be there simply to provide a minimum load when there is nothing connected to the output terminals. They may also form part of a current sensing network. If the resistors are smoking, the smps is either outputting a higher voltage at it's rated current and not folding back like it's supposed to in order to regulate the output, or you are back feeding it from the machine from some other source. The over current has to come from somewhere!

I have seen a couple of wirewound resistors fail short, but it's extremely rare. Metal oxide and Carbon Film types never do.

I think I would be building a linear supply for the machine and doing away with the unreliability that smps brings to the table...easier to fault find too in the unlikely event of a failure :)

rgds
 
The machine has +24, +8, +5 and -5 volts. Total four supplies.
You have substituted with another supply which has 2, 5, 5, 12, 24 volt modules in it. Total five supplies.

But How have you connected for the -5V supply?

I assume, you have directly connected 24 and 5 V supplies on machine and used your 12V for the 8V supply on machine. It is not a good idea to substitute a 12V supply for an 8V. Apply caution. Machine could endure damages on its circuits where it uses 8V supply.
 
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