power supply newbie question - Page 1
Forum section: electronics / beginners

author: lomka
author_rank: Newbie
timestamp: March 01, 2011, 02:38:32 pm
content: 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

author: disasm
author_rank: Newbie
timestamp: March 01, 2011, 03:00:25 pm
content: 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

author: lomka
author_rank: Newbie
timestamp: March 01, 2011, 03:53:06 pm
content: Quote from: disasm on March 01, 2011, 03:00:25 pmActually 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.

author: Neilm
author_rank: Super Contributor
timestamp: March 01, 2011, 06:41:57 pm
content: 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.YoursNeil

author: lomka
author_rank: Newbie
timestamp: March 01, 2011, 07:24:55 pm
content: Quote from: Neilm on March 01, 2011, 06:41:57 pmNope, it was a 6A or 10A circuit breaker (not RCD)QuoteI 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)

author: lomka
author_rank: Newbie
timestamp: March 01, 2011, 08:53:44 pm
content: One more thing, there seems to be ~44VAC between earth (either scope probe's ground or middle (green) terminal on psu) and both negative and positive terminalsIs that normal/ok?

author: Psi
author_rank: Super Contributor
timestamp: March 01, 2011, 09:23:28 pm
content: Quote from: lomka on March 01, 2011, 08:53:44 pm44VAC sounds like either you're measuring between earth and a floating ground or your powersupply is damaged and is outputting unregulated 44Vac.Have you checked all the powersupply voltages with the multimeter after the issue? There are 7 measurements to make.-SIDE1-GND <-> + (should be half what display says)- <-> GND (should be half what display says)- <-> + (should be exactly what display says)-SIDE2-GND <-> + (should be half what display says)- <-> GND (should be half what display says)- <-> + (should be exactly what display says)-SIDE3 FIXED -+ <-> - (5v)Are you sure the powersupply green terminals are mains earth and that none of the fixed output 5V terminals are grounded?The only way i can see for you to have poped a breaker would be if you connected the scope probe ground to something which had voltage with respect to mains ground. But it would have had to be pretty high voltage/current to draw enough to pop the breaker. I dont see how any powersupply voltage (5v/12v) could draw enough current from the 230v side to pop a breaker. Its a bit of a mystery.Maybe a good idea to check the mains plugs on the powersupply/scope are wired correctly (if they've been changed or are using a detachable cable)

author: lomka
author_rank: Newbie
timestamp: March 01, 2011, 09:48:30 pm
content: Quote from: Psi on March 01, 2011, 09:23:28 pmI was measuring between GND and +, GND and -There's 44VAC on both regulated outputs and 32VAC between GND and 5V terminalsQuoteVoltages between + and - terminals are ok, there was no DC between GND and +/- even before the incident, i just haven't measured AC beforeQuoteYesI'm just not sure what's the use for themQuoteThey both have detachable IEC cords, i'll check them if they're wired the same way (but there's 0 ohms and 0 VAC between PSU's GND and scope's BNC shield)

author: Psi
author_rank: Super Contributor
timestamp: March 02, 2011, 12:56:22 am
content: Quote from: lomka on March 01, 2011, 09:48:30 pmhm.. ok, i just assumed it was duel polarity on each of the two sections.

author: lomka
author_rank: Newbie
timestamp: March 02, 2011, 12:25:41 pm
content: So I got this idea to connect negative pole to the GND terminal (so now all of my 3 power sources have common ground which is tied to earth)Nothing have blown up, connecting all 3 voltages to scope doesn't make the circuit breaker go mad.Any theoretical/practical problem with this setup?Also, what i can't understand with my basic knowledge is that though there seems to be 44VAC between +- terminals and GND terminal, I can't measure and current flowing.

author: Psi
author_rank: Super Contributor
timestamp: March 02, 2011, 12:59:57 pm
content: Quote from: lomka on March 02, 2011, 12:25:41 pmThe + and - terminals are electrically isolated from the mains which means they are free to float, they could be at any voltage with respect to mains earth. Obviously they have to be at some voltage with respect to mains earth so they pickup any leakages in things and settle at some arbitrary voltage, in this case 44VAC, but it could easily be anything between the voltages involved (zero and 230vac).Because its just leakage the current is extremely small. Thats why you cant seem to measure it. Its even possible that the voltage between your gnd and +/- is much higher than 44vac, it could be 230vac but when you connect the DMM to measure it the DMM itself requires more current to measure the voltage than there is available, so the voltage is pulled down to 44vac.As the voltage is pulled down the DMM draws less current until a balance is reached (44VAC in this case) so the meter reads 44VAC.

author: Zero999
author_rank: Super Contributor
timestamp: March 03, 2011, 07:26:16 pm
content: There will be some capacitance between the primary and secondary side which will leak a tiny current. If you know the input impedance of your meter you can calculate the leakage current and the impedance of the primary to secondary insulation.