Electronics > Beginners

power supply newbie question

(1/3) > >>

lomka:
Hi guys,

i needed to power up something using 5V, -5V, 12V.

I had this psu:



that has 2 adjustable and one (5V) fixed outputs that should be isolated from each other.

So i thought i'm gonna connect together minus poles on those two adjustable ones (set to 12V and 5V) and plus pole on 5V (giving me -5V on minus pole) and use that as ground. It actually seemed to work (at least dmm was showing correct voltages)
But then i went to check whether the psu is not overshooting on power on so i connected my rigol scope. I started with -5V (psu3) connecting scope's ground to psu3's plus (that was connected to those minus poles of psu1 and psu2) and measuring psu3's minus pole. It looked fine.

So then i connected psu1 plus to scope (ground still the same), powered up the psu and bang, circuit breaker tripped.

Fortunately both psu and scope have survived it but i'd like to know what did i mess up (i think it has something to do with the fact that probe is grounded to earth) so can anyone more knowledgeable (almost anyone here) explain it to me?

Thanks

disasm:
I'm no expert, but having built a power supply recently, the negative poles are most likely already internally wired together. When you connected the plus pole to the ground, you created a short circuit. A short circuit draws an infinite amount of current, so the breaker should pop given that there is no protection in the power supply. Most likely though, the power supply has a regulator circuit consisting of an LM-317 IC or similar. What happens is when the IC is overloaded with current, it overheats. A transistor will sense this and shut down the circuit.

Sam

lomka:

--- Quote from: disasm on March 01, 2011, 03:00:25 pm ---I'm no expert, but having built a power supply recently, the negative poles are most likely already internally wired together.
--- End quote ---
Actually they don't seem to be, i've measured it. And it was working fine (with negative-negative-positive connected together) and dmm was showing correct voltages.
It went wrong only after i've connected the scope.

Neilm:
That picture shows that there are options to connect the two supplies in series so there should not have been a problem. It looks as if all the supplies are current limited so if there were a short on them they would be OK. If it measures fine with a DMM and only went wrong with the scope then the problem is probably a ground loop.

You mention that a trip went. Was this an RCD? If so what happened is you had enough current drawn from the earth (scope -ve on most oscilloscopes) that it causd the RCD to trip. Generally, these will trip on 30mA. Check that you have not connected the negative terminal to ground.

Yours

Neil

lomka:

--- Quote from: Neilm on March 01, 2011, 06:41:57 pm ---You mention that a trip went. Was this an RCD? If so what happened is you had enough current drawn from the earth (scope -ve on most oscilloscopes) that it causd the RCD to trip.
--- End quote ---
Nope, it was a 6A or 10A circuit breaker (not RCD)


--- Quote ---Check that you have not connected the negative terminal to ground.
--- End quote ---
I guess i did, scope probe's ground clip is connected to earth (and i connected that to my ground i created by connecting those -.-.+ terminals on PSU)

Navigation

[0] Message Index

[#] Next page

There was an error while thanking
Thanking...
Go to full version