Bench power supply short?

[Pagusi]

Hey,
I often test things with my bench power supply, but yesterday, as I tested some sparks with a coil I shorted something and the fuze didn't blow. Now I have a supply which is locked to 30 Volts but can deliver the standard 3 amps.
My question is, if you could tell me the parts that could be broken (maybe to fix). It's not digital, it is analog.
Thanks
Paul

[Pagusi]

Could be the case, if it's not, do you have another idea?

[Electronics Wizardy]

If there was a spark and its stuck at max voltage it seems like it might have blown the output transistor. What model is this? Id test that part first.

[Pagusi]

6 minutes ago, CUDAcores89 said:

Its possiblr the trimpot used to adjust the voltage is broken. Maybe thats why you cant adjust the voltage

thanks

[Pagusi]

[Electronics Wizardy]

2 minutes ago, Pagusi said:

Can you take it apart?

 

Check to see if anything looks damaged.

[Pagusi]

I plan to do it in the next days

[bob345]

I doubt its a blown potentiometer. The pot doesn't directly control your output voltage, it will control the main output transistor which will regulate the voltage. More than likely the output transistor will be shorted source to drain which would cause the behavior you are seeing by just passing the mail power rail voltage straight through with no additional regulation. Should be an easy fix as long as you can get a hold of the correct or comparable part.

[Unimportant]

5 hours ago, bob345 said:

I doubt its a blown potentiometer. The pot doesn't directly control your output voltage, it will control the main output transistor which will regulate the voltage. More than likely the output transistor will be shorted source to drain which would cause the behavior you are seeing by just passing the mail power rail voltage straight through with no additional regulation. Should be an easy fix as long as you can get a hold of the correct or comparable part.

@Pagusi This should be easy to test. Turn down the current limit dial and short the output. If it still limits the current then the output transistors are almost certainly fine and the problem lies somewhere in the voltage control loop. If it does not limit current and blows it's fuse then the output transistors are likely shorted.

[Pagusi]

I don't check how the current dial works yet. I have tested it before, but it didn't make a difference even if I rotated the dial all way. Can anyone explain this. (I tested it before the accident

[Unimportant]

20 hours ago, Pagusi said:

I don't check how the current dial works yet. I have tested it before, but it didn't make a difference even if I rotated the dial all way. Can anyone explain this. (I tested it before the accident

The current dial allows you to set a current limit. When the load tries to pull more current then the set limit, the power supply will lower the output voltage as much as is required to keep the current at the limit. Since it only comes into action when the load tries to pull too much current you will not see any effect with little or no load. Normally one shorts the power supply's output in order to set the current limit.

 

So, with your broken power supply. Turn the current limit dial all the way down to the lowest setting and then short the supply's output terminals with a (thick, decent) piece of wire. If it still limits the current (voltage needle will go to nearly zero) then the output transistors are likely OK and the problem lies in the voltage control loop. If it does not limit the current and the fuse pops then likely the output transistors are shorted out.

[Pagusi]

i opened it and there were no signs of burning or overheating. After that I tried the thing were I plugged in a wire to short it and voil la, the voltage goes to nearly nothing :D.

the transisters look fine too. I have no idea where to look next

[bob345]

6 hours ago, Pagusi said:

i opened it and there were no signs of burning or overheating. After that I tried the thing were I plugged in a wire to short it and voil la, the voltage goes to nearly nothing :D.

the transisters look fine too. I have no idea where to look next

how about current? if you turn the current limit knob can you see a change on the meter? If everything but voltage control is working it you should be able to adjust the current up and down with the output shorted

[corrado33]

The current knob won't really show a difference unless you are currently powering something.

 

Say, for example, you're have a 1W 1 Ohm resistor across the output of the power supply. If you put in 2 V you're going to be running 2 Amps and you'll burn out the resistor (because it'd be 2W). But what you can do is turn the current knob down and limit the current to 1 A and then the resistor wouldn't blow (but it'd still get super freaking hot.)

 

A way to test it is to turn the current knobs all of the way counterclockwise, make sure "current limiting" is on, if that's a switch somewhere. Then short the output with a piece of wire or something. If you turn the power supply on, it should show 0 A. Regardless of the voltage you try to set. Now if you turn the current limit knobs right a bit, it'll allow a bit of current to go through, and the voltage will go up a bit.