HP 6525A High Voltage DC Power Supply blows GFCI outlet - Page 1
Forum section: electronics / testgear

author: SharpEars
author_rank: Unknown
timestamp: November 19, 2015, 02:46:33 pm
content: I just purchased a used 4 kV HP 6525A power supply and if I plug it into a GFCI outlet, it trips the interrupt circuit after about a second of being powered on. I took an ohmmeter and measured the resistance from both the hot and neutral terminals of the AC plug to ground to see if there was maybe a current path, but could not detect any such path.So, the question is why would this high voltage supply cause the GFCI to trigger and disable the outlet? Since there is only the neutral path and ground for the current to flow to, I can't imagine it going any other path and I am assuming there is no path to ground, since I measured the resistance from hot/neutral to ground to be "infinite." I don't know if this is a case of false tripping and how would I get around it if it is? The fact that it takes over a second of power on time to trip means that there is no immediate path from hot to ground.Plugging the supply into a non-GFCI outlet shows it to work as designed, but of course I do not know if there is any current leaking to ground in this case.Thanks for any tips.

author: krivx
author_rank: Frequent Contributor
timestamp: November 19, 2015, 02:56:07 pm
content: You're measuring live and neutral to ground when the unit is off? The fault may only present itself with the unit powered up, e.g. a shorted semiconductor behind a soft-start relay.

author: free_electron
author_rank: Super Contributor
timestamp: November 19, 2015, 03:06:06 pm
content: leaky filter caps

author: N2IXK
author_rank: Frequent Contributor
timestamp: November 19, 2015, 03:46:22 pm
content: Check the line bypass capacitors for excessive leakage.Many pieces of older gear used capacitors from one or both sides of the AC line to chassis ground, to give a low impedance path for high frequency noise. Unfortunately, some designers used caps large enough to pass enough current at 50/60 Hz that they will trip a modern GFCI. The maximum allowable trip current for a GFCI in the US is 5 mA, with some tripping well below that.You can replace them with lower value X or Y rated film or ceramic caps to retain the RF bypass capability while reducing the nuisance tripping problem.

author: SharpEars
author_rank: Unknown
timestamp: November 19, 2015, 04:11:56 pm
content: Quote from: N2IXK on November 19, 2015, 03:46:22 pmAre these caps even necessary at all with modern AC home circuits that are relatively noise free?

author: N2IXK
author_rank: Frequent Contributor
timestamp: November 19, 2015, 05:42:52 pm
content: Quote from: SharpEars on November 19, 2015, 04:11:56 pmYes.They are equally useful for preventing RF energy from being conducted OUT of a device INTO the powerline....

author: Monkeh
author_rank: Super Contributor
timestamp: November 19, 2015, 05:50:30 pm
content: Quote from: SharpEars on November 19, 2015, 04:11:56 pmModern circuits are substantially more noisy than they used to be!

author: SharpEars
author_rank: Unknown
timestamp: November 19, 2015, 06:04:42 pm
content: I am trying to figure out where this bypass capacitor that we are talking about "lives" in the circuit. I don't see it in the block diagram, at the very least or in the description of operation for the unit.Here is a link to the service manual:HP 6525A Operating & Service Manual

author: robert_
author_rank: Regular Contributor
timestamp: November 19, 2015, 06:05:23 pm
content: Also, you might have a case of failing MP capacitors somewhere line-ground.If they already leak several mA, then, if run with no GFI protection, these will blow up soon enough (and might set the board on fire) and tell you where to look/smell...Can you measure the isolation resistance with at least 250V DC? If its anywhere below a few hundred M, then there is something very wrong. If its good, measure the capacitance from (L/N parallel) to PE. You are running on 230V?

author: SharpEars
author_rank: Unknown
timestamp: November 19, 2015, 06:06:35 pm
content: Quote from: robert_ on November 19, 2015, 06:05:23 pmI am running at 110 V (which means 125 V in actual practice).

author: Monkeh
author_rank: Super Contributor
timestamp: November 19, 2015, 06:10:17 pm
content: Quote from: SharpEars on November 19, 2015, 06:04:42 pmJust follow the AC input. If you hit a rectifier and you still haven't found a capacitor, you don't have line filter caps. In which case you have some other cap breaking down. There are a number of paper caps in this unit, they are immediately suspect.

author: SharpEars
author_rank: Unknown
timestamp: November 19, 2015, 06:12:59 pm
content: Quote from: Monkeh on November 19, 2015, 06:10:17 pmReplacing those "ginormous" paper caps is going to be a big deal though, because some of them are of the high capacitance, 4 kV variety. Everything in this thing from the caps to the transformers is super-duper-oversize.

author: robert_
author_rank: Regular Contributor
timestamp: November 19, 2015, 06:13:22 pm
content: If there is trouble at 120V, then something is wrong. Normally, eqipment (according to IEC60950) has a leakage current <3.5mA at 230. If its above, there will be some marking and an extra PE terminal, etc.My bets are on faulty MP capacitors (RIFA PME usually), probably right in the IEC inlet/Line filter combo. HP has often used these parts, and they often fail when old enough (like, 20 years).

author: SharpEars
author_rank: Unknown
timestamp: November 19, 2015, 06:14:48 pm
content: Quote from: robert_ on November 19, 2015, 06:13:22 pmThe problem is that I don't see a line filter and there is no IEC inlet. We're talking circa 1971 equipment here. The power cable runs straight into the unit, it is not detachable and it seems like it goes directly to transformers (factoring in the fuses and power switch of course). I quote from the service manual in the link I posted several messages back:The 115Vac +- 10%, single phase, 48 to 440Hz input is applied to auxilliary power transformer T2 and to the line-voltage bridge and storage capacitor.

author: Monkeh
author_rank: Super Contributor
timestamp: November 19, 2015, 06:17:29 pm
content: Quote from: SharpEars on November 19, 2015, 06:12:59 pmSo test instead of blind replacement.Quote from: robert_ on November 19, 2015, 06:13:22 pmThis equipment predates IEC60950. It predates IEC 950, too.

author: SharpEars
author_rank: Unknown
timestamp: November 19, 2015, 06:19:35 pm
content: Quote from: Monkeh on November 19, 2015, 06:17:29 pmI do have a way of testing capacitance, but I do not have an ESR tester to test for leakage.

author: Monkeh
author_rank: Super Contributor
timestamp: November 19, 2015, 06:20:55 pm
content: Quote from: SharpEars on November 19, 2015, 06:19:35 pm.. apply voltage. Check current.Have you changed the power cable?

author: SharpEars
author_rank: Unknown
timestamp: November 19, 2015, 06:26:24 pm
content: Quote from: Monkeh on November 19, 2015, 06:20:55 pmIf I measured the power cable not to have conduction (line to ground and neutral to ground) then why replace it?

author: Monkeh
author_rank: Super Contributor
timestamp: November 19, 2015, 06:27:30 pm
content: Quote from: SharpEars on November 19, 2015, 06:26:24 pmBecause you measured at the open circuit voltage of a DMM, almost without doubt. This doesn't work.

author: SharpEars
author_rank: Unknown
timestamp: November 19, 2015, 06:31:31 pm
content: Quote from: Monkeh on November 19, 2015, 06:27:30 pmSo you're saying that the old cord is conducting due to dielectric breakdown in the plastic used to insulate the individual power wires caused by the plastic's age age?

author: Monkeh
author_rank: Super Contributor
timestamp: November 19, 2015, 06:33:01 pm
content: Quote from: SharpEars on November 19, 2015, 06:31:31 pmI'm asking if you've tried changing it, because it could be.It's one of those simple, obvious things to check.

author: SharpEars
author_rank: Unknown
timestamp: November 19, 2015, 06:50:51 pm
content: Quote from: Monkeh on November 19, 2015, 06:33:01 pmI haven't tried yet, because as already pointed out it is connected directly (i.e., soldered) to the circuit. There is no IEC connector to just swap out the cord. I am trying to figure out if there are simple things I can test in-circuit, before I start desoldering "this, that and the other" in an effort to figure out where the current leak to ground is happening. The caps are old, but the caps are also very expensive, because everything here is high voltage, high capacitance or both. I mean when was the last time you guys saw a 4 uF / 4 kV or 3,300 uF / 200 V capacitors (outside of an air-conditioner's condenser for the latter)?

author: Cubdriver
author_rank: Unknown
timestamp: November 19, 2015, 07:31:56 pm
content: Could you rig up a test setup that would allow you to put an ammeter in series with the ground lead and connect it to a non-GFCI protected source, then measure the actual fault current that's present? Knowing the magnitude - whether it's just barely over the limit for the ground fault interrupter or blatantly high - might help to give an idea what to look for.-Pat

author: SharpEars
author_rank: Unknown
timestamp: November 19, 2015, 07:57:55 pm
content: Quote from: Cubdriver on November 19, 2015, 07:31:56 pmSound like a good idea to me. It would also help isolate the part responsible if I start removing them individually from the circuit. I'll try to think of a way to make this possible.

author: Cubdriver
author_rank: Unknown
timestamp: November 19, 2015, 08:29:34 pm
content: Quote from: SharpEars on November 19, 2015, 07:57:55 pmPerhaps a handy box with a sacrificial extension cord run through it? Put a pair of banana jacks on it, cut the extension cord in half, strip a few inches from each cut end and feed them into the box. Inside the box, splice black to black and white to white to carry the power through it, and connect the earth ground leads to the banana jacks. The meter can then plug into the jacks to complete the ground circuit and measure the current.Another possibility would be to take an extension cord, strip some of the outer jacket from a section a few inches long, cut the ground wire and install banana plugs on the cut ends. Those plugs could then plug directly into the current jacks on the meter.A third would be to open up the unit, disconnect the ground wire where it connects to the chassis and put the current meter in line there. The downside to this last way is that it doesn't eliminate the cord from the equation as it is before the measurement point. In light of that, the first two methods would be better troubleshooting-wise.Be very careful when doing this, as when the meter is not in the circuit the equipment will not be tied to earth ground, and if there's a great deal of leakage the chassis may be nearly at line potential.-Pat

author: timb
author_rank: Super Contributor
timestamp: November 19, 2015, 08:41:08 pm
content: Desoldering "this, that and everything else" is part of troubleshooting old equipment. Get used to it! A good quality vacuum desoldering gun and a quality soldering iron plus desoldering braid will be your bestest friends! A lot of time, testing in-circuit won't tell you the whole story.As an example, I recently picked up a non-working Tek PG502 250MHz pulse generator off eBay for an absolute steal. The seller said it wasn't working, so I figured it was one of the fuses or something simple like that. It wasn't. After probing around the power section of the board, I realized the 5V regulator IC wasn't getting VCC (which is 12V sourced by a Zener on the 33.5V rail). Now, tracing back, I wasn't getting any voltage at the Zener. Directly in front of the Zener was a 2.2k resistor. I was getting 33V on one side of the resistor, but not the other, weird, right?So here's the relevant bit: I tested that resistor insitu with my DMM and it read 2.4k, a bit high but useable. So, this lead me to think the Zener was a dead short. So I unsolder it and power the unit on again, but I'm still only getting voltage on one side of the resistor. So I unsolder the resistor and it comes off the board in two separate pieces!You see, the resistor was sitting right beside a large power transistor that I guess was producing significant heat (as that area of the board is actually discolored) which caused it to eventually split in half. This is the same resistor that still showed the correct resistance with a DMM. After replacing it (with a higher wattage version for good measure) the unit powered right up and works perfectly!So, swapping components out or even removing them all together is a mainstay of troubleshooting gear.Just make sure you clean the flux residue off any high voltage components you swap to prevent arcing.Sent from my Tablet

author: klunkerbus
author_rank: Unknown
timestamp: November 19, 2015, 09:47:13 pm
content: Just to cover the possibilities, consider that the GFCI itself could at least be part of the problem. I think I've replaced two in our house over the years as older ones seemed to grow more and more prone to tripping. Do you have another GFCI outlet you could try for an easy comparison?