Electronics > Projects, Designs, and Technical Stuff
I have a short ( i think) and don't know where to start
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Hextejas:

--- Quote from: james_s on December 18, 2018, 06:09:39 pm ---When you say "wire up" do you mean when you put a load on them? Or when you just hook something up to them? If connecting wires blows the fuse then it sounds like you're missing the insulators between one or more power transistors and the case.

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Yes, wired up meant to put a load to both sides.
Question. How do i go about sizing a thermistor and would i need one for each side (rail) ?
SiliconWizard:
Could you try without C10? (Maybe you already did?)
nick_d:
I feel that something obvious must be getting missed here.

I suggest to get a low value high wattage resistor and put it in series with the incoming AC. If all works totally fine with a moderate load on both outputs then yeah maybe I buy the theory that it's inrush current to the many large caps that causes the issue. In that case, vary the resistance to see what you need to stop it misbehaving. And test the hell out of it -- that the supplies CAN float independently of each other and that it can drive almost the designed load on both channels independently once this possible startup issue is worked around with the small resistor to limit the inrush. You might also try a slow-blow fuse if it's really the inrush issue.

I bet that won't be the case. There's gotta be another issue.

cheers, Nick
fsr:
If connecting the load blows fuses, then maybe your load has a problem. What is your load, anyways?
Also, if you replaced a 18+18v transformer by a 35+35v, there can be problems. Over a resistive load, the current is the voltage divided by the resistance, and you doubled the voltage.
You can test the psu by connecting both grounds together. I would first connect the grounds at the transformer, with nothing else connected, just the transformer by itself, and chech that you have your 35+35v. Of course, before connecting the grounds, make sure that they're two independent secondaries, like you think. Measuring voltage over the wires of separate secondaries should give you no voltage at all. With the transformer unpowered you can also test resistance. It should be infinite. The transformer's datasheet should tell you this, anyways.
With the grounds connected at the transformer, you can connect the grounds at the output of your psu. Nothing should blow, or you still have a problem in your psu. Then you can test by using two power resistors as loads, from + to gnd and from gnd to -
Use resistors of enough power rating, and their resistance should be enough for the current to be below the limits of your psu.
I also think that using fast recovery diodes on a bridge rectifier is strange.
AngraMelo:

--- Quote from: sokoloff on December 17, 2018, 02:51:52 pm ---
--- Quote from: Hextejas on December 17, 2018, 02:43:53 pm ---This is strange. I am using a benchtop power supply ( ps ), a GPC3020,  that can put out 0-35v DC. I have it set to 20v and this is what happens.
It has 3 output connections. (---   GND   +++)
I connected the +++ to U1 and when i connected the --- to U2, the ps blinked and turned off the C.V. led and turned on the C.C. led.
I think that it means it switched from constant voltage to constant current. I don't know if that was good or bad and the implications. Anyhow, what measured 20Vdc before now measures a lot less.
I am thinking I need to do something to fix that.
Secondly, when I try the same thing with U4 +++ and U3 ----, the ps stays at CV (20v8 dc), but none of the output shows any voltage.
Let me see if I can find a 1N4148 or some such.

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A typical bench power supply will supply a set voltage or a set current, whichever is less. If you set it to 20V and a max of 100mA, it will provide 20V and 0mA when not connected to anything (and will be in "constant voltage mode"). If dead shorted, it will supply 100mA and way less than 1 volt. (It will be limited the voltage to whatever can push 100mA through the short. This is "constant current mode".)

In your case, it's telling you that you have a very low resistance path (maybe a dead short) on the U1/U2 side.

--- Quote from: Hextejas on December 17, 2018, 02:43:53 pm ---I am thinking I need to do something to fix that.
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Yes. Find the short.  :-DD

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So it seems your board has a short, if you are having problems with 20V I doubt it is a clearance issue. It might just be a dead short.
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