High voltage divider- teardown, repair - Page 1
Forum section: electronics / general

author: doktor pyta
author_rank: Frequent Contributor
timestamp: December 14, 2020, 02:33:55 pm
content: Some photos of HV dividers.They take advantage of precision wirewound resistors.First quite common Fluke Model 80E.

author: doktor pyta
author_rank: Frequent Contributor
timestamp: December 14, 2020, 02:38:33 pm
content: Julie Research Labs (JRL) model KV-10R.It uses hermetically sealed resistors mounted on PTFE stand-offs.However no adjustment pot is provided.

author: doktor pyta
author_rank: Frequent Contributor
timestamp: December 14, 2020, 02:42:30 pm
content: Fluke model 80D.Oil filled, temperature regulated 30kV divider from 1965.I will place some pics and repair info.

author: doktor pyta
author_rank: Frequent Contributor
timestamp: December 14, 2020, 02:43:10 pm
content: Fluke 80D repair description.During initial checking I found that at the room temperature device generates weird DC voltages ~5...10mV across output and input terminals of the divider.I eliminated input bias current of the meter.You should know, that the manual mentions using 'silicone fluid' as dielectric medium.The fluid I saw inside didn't look like silicone oil which is colorless transparent in most cases.It looked and smelled like a transformer oil. Some old, worn out transformer oil.The electricians know, that after many years in warm environment the mineral transformer oil reacts with oxygen and gets acidic.This causes decreasing of breakdown voltage, resistance, causes metal parts of the transformer to slowly corrode.I measured resistance between tank and divider's input for comparision:Then I evacuated old oil and cleaned the tank.The resistance meter showed:The weird voltages disappeared.I found a Polish manufacturer of silicone oils „Silikony Polskie” Sp. z o.o.http://www.silikonypolskie.pl/en/produkty/They have 50-years experience in making silicone oils, silicone rubbers etc.9 liters of low viscosity silicone oil Polsil OM-10 (10cst) cost approx 120EUR.They also have 1 liter bottles which I previously used to resurrect my TEK P6015 probe which was not good enough to hold freon.After long cleaning of the internals, I poured new oil.and measured resistance as before:Much better. Something like 555 times.The problems with generating electrochemical potentials disappeared.*Some potentials (say, ~200uV on 10V F.S. output) are still generated due to thermal EMF and temperature gradient between top and bottom of the tank.They are close to zero when the device is at the room temperature.

author: PartialDischarge
author_rank: Super Contributor
timestamp: December 14, 2020, 03:03:58 pm
content: What are the metallic shields connected to?

author: Gyro
author_rank: Super Contributor
timestamp: December 14, 2020, 03:09:01 pm
content: Ouch, Fluke really couldn't manage more than 10M input resistance on the 80E? That's a 10W load on the circuit under test at 10kV.Curious too that the Julie Research doesn't bother to mention the input resistance on the front panel, I would have thought you would want that information very much to hand.EDIT: Even worse on the 80D - 30W dissipation at 30kV!

author: doktor pyta
author_rank: Frequent Contributor
timestamp: December 14, 2020, 03:11:24 pm
content: Metal 'shields' are connected to separate (not precise) resistor divider taps for guarding purpose.

author: doktor pyta
author_rank: Frequent Contributor
timestamp: December 14, 2020, 03:14:00 pm
content: @GyroJRL has also 10Meg input resistance. Presumably to maintain compatibility with Fluke 80E. Even size, connector type are the same.

author: Gyro
author_rank: Super Contributor
timestamp: December 14, 2020, 03:22:55 pm
content: Thanks, that makes sense.It's just a bit puzzling, it wasn't hard to get 10M precision wire-wound resistors (I've got one upstairs), so making up a 100M stack (or rather more) wouldn't have been at all hard. I'm sure the temperature coefficients would have been much easier to deal with at a total dissipation of 1W or less.Presumably these things were used at DC rather than any particularly high frequencies with too much parasitic capacitance, even then, it could be compensated.

author: doktor pyta
author_rank: Frequent Contributor
timestamp: December 14, 2020, 04:10:22 pm
content: Keep in mind both Fluke and JRL were experts in custom resistor manufacturing.They must have had good reasons to make decision about such resistance values.

author: HighVoltage
author_rank: Super Contributor
timestamp: December 14, 2020, 05:10:02 pm
content: I bought this Keithley High Voltage Divider Model 6601A a few years ago in the condition shown.

author: HighVoltage
author_rank: Super Contributor
timestamp: December 14, 2020, 05:17:34 pm
content: ... and then I rebuild it partially with:- High Quality SHV connector- LowThermal binding posts- Vishay 10k resistor- New potentiometerAfter the rebuild it became a very stable divider, changed from 100:1 originally to 1000:1The DMM has to be set to High-Z to be hooked up to the output.

author: Gyro
author_rank: Super Contributor
timestamp: December 14, 2020, 05:56:31 pm
content: Quote from: doktor pyta on December 14, 2020, 04:10:22 pmSure, I agree that they must have had their reasons. I'm just not sure I understand the full picture.I wonder how it relates to the 4M input resistance of the Fluke 752A divider (plus another 4M in parallel for the lower precision guard divider) for a Max input of 1100V, so 300mW dissipation in the divide network. Being a Hamon divider, that one is self-calibrating though, maybe allowing them to use less long-term stable resistors?PS. Sorry if a bit OT, really just curiosity about the magnitude of the loading / dissipation.PPS. The 80D internals are a thing of beauty. It's always nice to see what resides in an oil bath!

author: TimFox
author_rank: Super Contributor
timestamp: December 16, 2020, 03:51:11 am
content: Quote from: HighVoltage on December 14, 2020, 05:10:02 pmIs that an “HN” input connector?

author: doktor pyta
author_rank: Frequent Contributor
timestamp: December 16, 2020, 11:15:39 pm
content: Repair description has been updated + interesting bonus from the manual: