Full wave bridge rectifier waveform - Page 1
Forum section: electronics / repair

author: freak
author_rank: Contributor
timestamp: December 03, 2023, 12:45:45 am
content: I have a 120V floor buffer that uses a GBPC5006 full wave bridge rectifier to power it's motor. Its rated draw is 15A. The unit runs but under a normal load the current jumps up over 25A and blows the breaker. I pulled the rectifier and it checks ok with my meter. The waveform doesn't look like a textbook example but I'm not an expert. The image is with the rectifier alone powered through a isolation transformer. Is this waveform normal or does it indicate a problem?

author: Stray Electron
author_rank: Super Contributor
timestamp: December 03, 2023, 02:25:25 am
content: FIRST, I wouldn't use an oscilloscope on that kind of circuit. The motor inductance will cause voltage spikes and they could kill your scope.Second, no those aren't normal looking waveforms. It looks like you may have at least one shorted diode in the bridge. Which probably also explains the excess current draw.

author: DavidAlfa
author_rank: Super Contributor
timestamp: December 03, 2023, 02:44:40 am
content: That waveform definitely looks strange.Disconnect the motor and turn it on, does it consume any power?Looks like one half wave is partially filtered, while the other it's not, does it have a filtering capacitor at the output?

author: fmashockie
author_rank: Frequent Contributor
timestamp: December 03, 2023, 03:32:35 pm
content: That definitely looks strange. If one of the diodes in the rectifier failed open, you'd have a half wave rectifier. Which clearly isn't what you have on the scope. Now if one of diodes is shorted, you are going to draw a massive amount of current. But you said you got the above waveform by applying voltage across the BR with an isolation transformer. Does your isolation transformer have a fuse (most do)? If it does, it would have likely blown if one of the diodes was shorted in this rectifier. If we use the photo below as an example, the positive going side of the AC sinewave being rectified by D1 and D4 would not be affected, but the negative side would. You'd effectively be shorting the secondary winding of the transformer. So either the transformer would eventually fail open or diode D3 would fail open due to the overcurrent.

author: PA0PBZ
author_rank: Super Contributor
timestamp: December 03, 2023, 03:49:19 pm
content: Quote from: freak on December 03, 2023, 12:45:45 amDid you put a small load on the rectifier? If not you will not see a normal waveform due to capacitance, leakage and such.

author: freak
author_rank: Contributor
timestamp: December 03, 2023, 04:28:05 pm
content: Quote from: Stray Electron on December 03, 2023, 02:25:25 amThis is the rectifier alone on the bench. Nothing else connected.

author: freak
author_rank: Contributor
timestamp: December 03, 2023, 04:29:59 pm
content: Quote from: PA0PBZ on December 03, 2023, 03:49:19 pmNo there is no load on the rectifier in my example.

author: freak
author_rank: Contributor
timestamp: December 03, 2023, 04:31:27 pm
content: Quote from: fmashockie on December 03, 2023, 03:32:35 pmYes the isolation transformer is fused.

author: freak
author_rank: Contributor
timestamp: December 03, 2023, 04:33:57 pm
content: Quote from: DavidAlfa on December 03, 2023, 02:44:40 amIf i disconnect the motor there is nothing other than the rectifier. There are no capacitors.

author: freak
author_rank: Contributor
timestamp: December 03, 2023, 04:36:44 pm
content: Quote from: PA0PBZ on December 03, 2023, 03:49:19 pmI would like to re-test it with a load. Seems like an incandescent light bulb would be suitable?

author: bdunham7
author_rank: Super Contributor
timestamp: December 03, 2023, 04:45:32 pm
content: Quote from: freak on December 03, 2023, 04:36:44 pmAn incandescent bulb, even a small one, would be good. LED/CFL/etc won't be good!

author: freak
author_rank: Contributor
timestamp: December 03, 2023, 04:47:24 pm
content: Quote from: bdunham7 on December 03, 2023, 04:45:32 pmYeah I just clarified that. I'll re-test it in a bit.

author: DavidAlfa
author_rank: Super Contributor
timestamp: December 03, 2023, 04:56:34 pm
content: Yeah, or a heater, resistive load are the easiest to test.

author: freak
author_rank: Contributor
timestamp: December 03, 2023, 06:56:19 pm
content: Well here is how it looks with a 100W load on it. So that looks good I guess I'll have to think about the motor being the cause of the problem. Unless of course that rectifier has issues with a larger load.

author: bdunham7
author_rank: Super Contributor
timestamp: December 03, 2023, 07:03:47 pm
content: That type of rectifier is unlikely to have intermittent or variable problems when connected to mains--it will either work or it will pop entirely and permanently.

author: fmashockie
author_rank: Frequent Contributor
timestamp: December 03, 2023, 07:15:32 pm
content: Nice work! Check for a short between windings on the motor. What is the make/model of the floor buffer?

author: freak
author_rank: Contributor
timestamp: December 04, 2023, 03:48:03 am
content: It's a Tornado Glazer 1500.

author: fmashockie
author_rank: Frequent Contributor
timestamp: December 04, 2023, 03:07:32 pm
content: I'd be checking that Triac first actually. Easy to test with a DMM. If it went short, it could be causing your overcurrent issues.

author: freak
author_rank: Contributor
timestamp: December 05, 2023, 12:21:09 am
content: Quote from: bdunham7 on December 03, 2023, 07:03:47 pmI pulled 10 amps through it today on the bench and you were right. The waveform looked normal and nothing strange happened.

author: freak
author_rank: Contributor
timestamp: December 05, 2023, 03:27:25 am
content: Quote from: fmashockie on December 04, 2023, 03:07:32 pmIf the triac shorted wouldn't it be just like having it on high speed? Instead of chopping the waveform the full sine wave passes?

author: Swake
author_rank: Frequent Contributor
timestamp: December 05, 2023, 07:15:53 am
content: The breaker could have failed too. These things are not eternal unfortunately. Or there might be another unexpected load on the same circuit. Issues in the wiring or plugs can also be a cause. Check for loose or corroded connections.

author: fmashockie
author_rank: Frequent Contributor
timestamp: December 05, 2023, 04:41:56 pm
content: Swake is right. Fortunately, that is fairly easy to test - just plug the buffer machine into a different circuit. You could take it a step further and hook it up to a power analyzer or try a clamp meter on the incoming mains hot wire to check the amperage.But they are right in checking for loose connections. I usually go off the deep end and start checking for more 'complex' sources of fault, when really 9/10 its usually something stupid like a loose connection or cold joint. You'd think I would learn by now

author: freak
author_rank: Contributor
timestamp: December 05, 2023, 05:52:04 pm
content: Quote from: fmashockie on December 05, 2023, 04:41:56 pmThis unit exhibits the same problem in multiple buildings. The first thing I did was check the amp draw. When freewheeling it draws about 10 amps. Place it on the floor and amps goes over 25 and trips the breaker.

author: mikerj
author_rank: Super Contributor
timestamp: December 05, 2023, 06:09:00 pm
content: Sounds like either a mechanical issue or possibly shorted turns in the motor, 10 Amps with no load is really excessive. Can you easily spin the motor by hand when it's not powered? Are you seeing an unusual amount of sparking from the brushes when it's running?

author: fmashockie
author_rank: Frequent Contributor
timestamp: December 06, 2023, 03:50:31 pm
content: I would guess something mechanical in nature as well. Now that you provided the symptoms (I assumed it was immediately tripping the breaker when you powered it up). A bad pulley, belt, or bearing.

author: freak
author_rank: Contributor
timestamp: December 06, 2023, 10:34:04 pm
content: Quote from: fmashockie on December 06, 2023, 03:50:31 pmIf you read my original post I said "The unit runs but under a normal load the current jumps up over 25A and blows the breaker."

author: DavidAlfa
author_rank: Super Contributor
timestamp: December 06, 2023, 11:24:01 pm
content: But yeah, 10Amps is a lot for a small motor with no load!Sounds like a damaged coil or something!

author: fmashockie
author_rank: Frequent Contributor
timestamp: December 07, 2023, 03:22:14 am
content: Quote from: freak on December 06, 2023, 10:34:04 pmyep i see that now. my bad. good luck with it!