HP 3456A - measuring with brief overvoltages up to 2 kV - Page 1
Forum section: electronics / testgear

author: SharpEars
author_rank: Unknown
timestamp: November 21, 2015, 03:50:32 am
content: Does anyone know if the HP 3456A can handle brief overvoltages on its 1000 V setting (i.e., auto-ranging disabled). By brief, I am talking maybe <5 seconds and up to 2 kV. I can't find anything in the manual and since I only have a non-searchable PDF of it I am wondering if I am overlooking something. The manual states that it has a 1 kV max across its input terminals, but it can actually measure up to 1.1 kV (although I am not sure if doing so is safe for the unit) before displaying an overload condition. I could not find any info about brief overvoltages up to 2 kV in the manual and whether the unit is protected from such. Usually, this sort of information is explicitly stated along with timer periods deemed safe, but I can't seem to find it.So, if anyone has any experience with this sort of test (either because it is explicitly stated in the manual or if you have performed physical testing similar to this without damaging the unit), please let me know.

author: MadTux
author_rank: Frequent Contributor
timestamp: November 21, 2015, 05:39:20 am
content: The 3456A will go in overvoltage above 1200V IIRC. Since it has internal spark gaps, it will depend on the impedance of your source. If it is high impedance, say above 1MOhm, the spark gaps will probably protect the relays/FETs from overvoltage as the spark gaps can easily dissipate that much power. I would guess that they trigger before any relay/FET sparks over. But I wouldn't try it with low impedance source, because that most certainly ends with a badly toasted 3456A, which would be sad, since those are really nice DVMs.The right way to measure HV with 3456A would be to solder an external voltage divider from cheap 10MOhm resistors. Then you can basically hook any voltage to your 3456A. If it is low freq, no fancy capactitive compensation is required.

author: Messtechniker
author_rank: Frequent Contributor
timestamp: November 21, 2015, 06:24:26 am
content: Quote from: MadTux on November 21, 2015, 05:39:20 amCarefully now! Don't forget the voltage rating of cheap resistors of 500 V or so.Even that I would not like to stretch to the limit.Yours - Messtechniker

author: MadTux
author_rank: Frequent Contributor
timestamp: November 21, 2015, 06:39:35 am
content: resistors: more than oneSolder a chain of like 10 (4kV) or 50 (20kV) or how many you like to get like 100M or 500M. then use a resistor value like 100k or 500k to get 1000:1 ratio. Then you are in 10Gohm input resistance zone up to 12kV. Use higher ratios and longer chains for even more voltage.If precision is required, you might add a trimmer pot and a small capacitor as low pass across your last resistor, over which you are measuring the voltage. Then use a Fluke box like 332D or 335D to calibrate that thing at 1000V. Use trimmer pot to adjust it, or use awesome math function with correction factor to directly calculate your measured voltage

author: Messtechniker
author_rank: Frequent Contributor
timestamp: November 21, 2015, 07:56:07 am
content: Must correct:Cheap 1/4 W carbon and metal film resistors in the 10 MEG range have a voltage rating of 250 V only.In any case have a good look at the spec sheet for the type of resistor you will be using.Yours - Messtechniker

author: Zero999
author_rank: Super Contributor
timestamp: November 21, 2015, 08:23:04 am
content: According to the datasheet, the input impedance is 10M on the 100V and 100V settings, so a single 10M resistor rated to 1kV is all that's needed but it might be cheaper to simply connect 10 cheap 1M metal film resistors in series.http://www.rlscientific.com/documenti/HP3456A.pdf

author: Gyro
author_rank: Super Contributor
timestamp: November 21, 2015, 10:19:59 am
content: Err, wouldn't you be better using NINE 10M resistors to make a 10:1 divide?

author: Zero999
author_rank: Super Contributor
timestamp: November 21, 2015, 06:17:54 pm
content: Quote from: Gyro on November 21, 2015, 10:19:59 amWell if he needs to measure up to 2kV then he only needs to divide by two.

author: MarkL
author_rank: Unknown
timestamp: November 21, 2015, 07:07:08 pm
content: Quote from: Hero999 on November 21, 2015, 08:23:04 amActually, it's not a good idea to depend on the meter's input impedance as part of the divider.If you turn the meter off and back on, or forget to manually set the range, it will end up on the most sensitive range with 10^10 impedance. Then you get your 2kV across the input.There's plenty of ways to build the divider, but keep the meter shunted with a known external resistance and incorporate the shunt value and meter input impedance (if significant) into your calculations.And if you don't want to build a chain of lower voltage resistors, you can take a look at high voltage resistors such as the Ohmite TFS or the Vishay ROX series.EDIT: Hopefully this goes without saying, but since this is a line-powered meter, be sure to also pay attention to the 1kV max isolation voltage from the input terminals to ground.

author: SharpEars
author_rank: Unknown
timestamp: November 21, 2015, 07:27:50 pm
content: The thread moved on a tangent. I can certainly build a voltage divider, the question was about the base unmodified unit's ability to handle brief 2 kV "spikes" from a low-impedance power supply capable of a maximum of 50 mA of output current.If there are spark gaps at the input, as long as they can survive the max 50 mA load (which will not be fully delivered anyways because of the high impedance of the 3456A's input of 10 MOhm), I think I am OK.

author: Zero999
author_rank: Super Contributor
timestamp: November 21, 2015, 08:48:45 pm
content: Quote from: MarkL on November 21, 2015, 07:07:08 pmDoes that actually matter? The 10M resistor will limit the current to 200µA which shouldn't do any damage.

author: MarkL
author_rank: Unknown
timestamp: November 21, 2015, 08:50:50 pm
content: Quote from: SharpEars on November 21, 2015, 07:27:50 pmMy answer to your question is "don't do it". I have a 3456A. I wouldn't purposely exceed the maximum ratings and rely on the 30+ year-old protection circuitry to recover with absolutely no side effects to 6.5 digits. It might, and given HP equipment probably will, but why risk it?So, given that implied answer, it wasn't a tangent. I think we were all trying to give you suggestions to not put your meter at risk when you can easily avoid it.The input protection circuitry is described in the service manual in section 8-26.According to the diagram there, there's an RC network involved, so it's going to depend on what you mean by "brief spikes".And in an voltage overload, the protection circuitry looks like it's providing a lower impedance path to ground to shunt away the excess voltage. So, don't assume the 10Mohm input impedance will be maintained throughout a "spike". It could end up drawing significant current.It's your meter. Let us know what happens.

author: cncjerry
author_rank: Unknown
timestamp: November 21, 2015, 09:15:12 pm
content: I have a couple of 3456a meters, haven't tested it at that voltage, but it would be a bad move. I predict if you try it, and the amount of time has little to do with over voltage, your future nickname will be 'Sparky'.

author: MarkL
author_rank: Unknown
timestamp: November 21, 2015, 09:18:31 pm
content: Quote from: Hero999 on November 21, 2015, 08:48:45 pmNo matter how low a current, if high voltage breaks down an undesirable path, it could provide a new leakage path under normal operating conditions.Likely to happen? Probably not. But you might never know the integrity of your meter was damaged until it's too late.

author: Monkeh
author_rank: Super Contributor
timestamp: November 21, 2015, 09:28:17 pm
content: Quote from: SharpEars on November 21, 2015, 07:27:50 pm'no'.It says 1kV max, it means it. The spark gaps typicallyare notafter the input impedance, they are directly across the input.