High voltage probe and high voltage resistors - Page 1
Forum section: electronics / general

author: xzswq21
author_rank: Frequent Contributor
timestamp: July 31, 2021, 04:56:56 am
content: Hello, I want to make a high voltage probe (20kV), there are several different types of high voltage resistors with different Materials: Metal film, ceramic and etcThe working frequency is 30KHz to 100KHz. Which material should I choose?Thanks

author: bob91343
author_rank: Super Contributor
timestamp: July 31, 2021, 08:52:56 pm
content: Primarily, the material choice is concerned with temperature coefficient of resistance.Having said that, the majority of resistors are helically built and thus have parasitic inductance. There are many techniques for constructing resistors to minimize parasitics. Your best bet is to contact one or two manufacturers and ask for recommendations depending on your requirements.

author: David Hess
author_rank: Super Contributor
timestamp: July 31, 2021, 08:58:32 pm
content: At that frequency anything can be used except wirewound, but pay particular attention to the maximum voltage rating which is what distinguishes high voltage resistors from common resistors.

author: 2N3055
author_rank: Super Contributor
timestamp: July 31, 2021, 09:01:53 pm
content: Define high voltage: 5kV, 20kV 50kV?

author: xzswq21
author_rank: Frequent Contributor
timestamp: July 31, 2021, 09:03:01 pm
content: Quote from: 2N3055 on July 31, 2021, 09:01:53 pm20kV to 30kV

author: tggzzz
author_rank: Super Contributor
timestamp: July 31, 2021, 09:11:32 pm
content: http://hvstuff.com/high-voltage-resistors

author: TimFox
author_rank: Super Contributor
timestamp: July 31, 2021, 09:12:56 pm
content: And what resistance do you need?As stated above, almost all high-voltage resistors will be helices, and hence inductive. The non-inductive construction techniques will reduce the voltage capability by reducing spacing between nodes at different voltages.However, the resistance is usually much higher than the inductive reactance (and hard to measure).For high resistance values, the best temperature co-efficients are probably metal-oxide (cermet). Metal film can have better tempco, but the resistivity is lower and the resulting resistor will be physically larger.A good manufacturer of precision high-voltage resistors (in the 20 to 30 kV range) is Caddockhttp://www.caddock.com/Online_catalog/high_voltage/high_voltage.htmland they are available through distribution. They have a large range, some of them are good for high frequencies, as well.If you really need high voltage at high frequency, you may be better off with a capacitor divider, so long as you don't need to measure DC. Choose the resistive load on the capacitor tap to obtain the required low-frequency cutoff.

author: xzswq21
author_rank: Frequent Contributor
timestamp: July 31, 2021, 09:35:43 pm
content: Quote from: TimFox on July 31, 2021, 09:12:56 pmDo you have any information about Thick Film resistors? Ohmite manufacturer produces high voltage thick film resistors such as maxi-mox series:https://www.mouser.com/datasheet/2/303/res_maximox-253273.pdfI have chosen two 500Mega Ohm MOX-3 resistors but the freight charge is 150$ !One of my problem is the capacitor type, as you know high voltage capacitors are Y5P and Y6P, can I use these or not?

author: TimFox
author_rank: Super Contributor
timestamp: July 31, 2021, 10:01:31 pm
content: Thick-film is another term for metal oxide or cermet. A thick film is applied like through a silk screen, being an ink comprising resistive stuff and insulating glass. When fired, it binds to the ceramic substrate and the the binder in the ink burns off. The antonym is "thin film", which means a metal alloy deposited from vacuum.I wouldneveruse X, Y (such as Y5P), or Z dielectric ceramic capacitors in a measurement system, since they lose capacitance drastically with applied voltage. For ceramic, you need NP0 or C0G (basically synonymous). However, there are special capacitors for high voltage, such as reconstituted mica or high-voltage polypropylene film.I have used the Ohmite maxi-mox resistors, which have reasonable performance, but I prefer the Caddocks. I have used them up to 40 kV, with a more complicated structure (with corona guards) up to 100 kV.Note that all of these non-metallic resistors have a voltage co-efficient of resistance that is substantial at high voltage. I was able to get matched co-efficients from Caddock to make a voltage divider, and they sell pre-assembled voltage dividers that are specified. (I keep reminding people that Ohm's Law is an approximation that is often useful, but not guaranteed for a given resistor-shaped object.)Your Ohmite data sheet does not specify the voltage co-efficient, some Caddock parts are so specified.Ohmite makes a "Supermox" series with typical specifications on VCR.https://www.ohmite.com/assets/docs/res_supermox.pdfA 30 kV rated Supermox can have a "typical" VCR of 0.1 to 0.15 ppm/V which sounds good, but when you multiply by 30 kV that is a shift of 0.3 to 0.45% in the resistance.You didn't give a country flag: to where would the freight charge get that high from Texas?

author: David Hess
author_rank: Super Contributor
timestamp: July 31, 2021, 10:10:40 pm
content: Quote from: 2N3055 on July 31, 2021, 09:01:53 pm1.6 to 10kV for metal film resistors and higher for thick film resistors.Quote from: TimFox on July 31, 2021, 09:12:56 pmThe high voltage thick film resistors I have seen all use non-inductive serpentine construction.

author: TimFox
author_rank: Super Contributor
timestamp: July 31, 2021, 10:16:27 pm
content: I believe that some Caddock resistors are serpentine.

author: NiHaoMike
author_rank: Super Contributor
timestamp: July 31, 2021, 11:30:06 pm
content: Given that a high voltage probe is going to use resistors well into the megaohms, inductance is unlikely to cause any problems. Capacitance is going to be significant and that will have to be compensated for.Here's a video on DIY high voltage probes:

author: xzswq21
author_rank: Frequent Contributor
timestamp: August 01, 2021, 05:29:52 am
content: Quote from: TimFox on July 31, 2021, 10:01:31 pmBefore I come here, I had googled about high voltage capacitors, for low voltage filters I always use C0G capacitors but for high voltage capacitor I couldn't find anything in Digikey or Mouser except Y5P, N4700, Z5T, . I found several large size and unusual and very expensive capacitors. Can I use 1kV C0G capacitors in series? In theory there is a problem, for example, the voltage drops by 1kV per capacitor but there is 20kV and 19kV on the first capacitor!20kV -||- 19kV -||- 18kV -||- 17kV -||- 16kV -||- ....I think I can't use 1kV capacitorsI wanted to buy from Digikey.com, the shipping cost for other components is 30$ but for the Ohmite resistors is 150$. It's better for me to find the the components from other distributors.

author: PartialDischarge
author_rank: Super Contributor
timestamp: August 01, 2021, 06:47:00 am
content: The inductance in hv helix resistors is not a problem due to the extremely low currents handled. The real reason for serpentine construction is to avoid magnetic field coupling from external sources into the resistor, this magnetic field is not filtered by most shield cans, specially at low freqs.Capacitors used for hv are tricky, as their rating should be set according to partial discharges, not Ac or dc rating.Capacitors in series do not distribute the voltage linearly unless compensated by resistors. This is a well studied problem in hv ceramic/glass insulators for distribution power lines

author: Gyro
author_rank: Super Contributor
timestamp: August 01, 2021, 10:10:53 am
content: Here's a 10kV peak R-C attenuator I built for my Gay MTA Memory (peak capture) Voltmeter. I included as much detail as possible in the post, including how I chose parts and calculated the capacitances for the input impedance of the Voltmeter and connecting cable strays...https://www.eevblog.com/forum/testgear/high-voltage-x10-attenuator-for-my-gay-mta-memory-voltmeter/An important notethat this is not intended or suitable for high energy circuits, or to handle 10KV DC continuously (resistor wattage ratings). But it may help you in the construction of a higher voltage probe (more stages).Due to the nature of the termination and the input characteristics of the meter, I wasn't able to directly look at the square wave response on a scope - I was more interested in the overall frequency response of the probe and meter combination, so I had to check the roll-off with a signal generator. It made it to 5MHz though.It might be of some help anyway.

author: xzswq21
author_rank: Frequent Contributor
timestamp: August 01, 2021, 11:29:46 am
content: Quote from: Gyro on August 01, 2021, 10:10:53 amYou have used SL temperature coefficient capacitors, Do you recommend me SL capacitors? Even 1MHz passband flatness is good

author: Gyro
author_rank: Super Contributor
timestamp: August 01, 2021, 12:36:23 pm
content: Quote from: xzswq21 on August 01, 2021, 11:29:46 amIt's not so much a recommendation, as what is available. Ideally you would use C0G or NP0, but these are not commonly available, that I could find, in 6kV rating (possibly overkill but I wanted a good safety margin with 3 capacitors in series on 10kV - the MTA original probe stated 15kV pk).Murata information indicates that SL is a class 1 dielectric and 5% tolerance, even though probably not as good as [Edit: higher Dissipation Factor than], NP0 and C0G (also class 1). So seemed good enough for the purpose - certainly much better than the Yxx and Zxx dielectrics found in other high voltage ceramics.Here's a link to the Murata High voltage capacitor and dielectric data...https://www.murata.com/-/media/webrenewal/support/library/catalog/products/capacitor/ceramiccapacitor/c85e.ashx?la=en-us&cvid=20210203042629000000EDIT: and detailed spec sheet...https://search.murata.co.jp/Ceramy/image/img/A01X/G101/ENG/DEC_E.pdf

author: Gyro
author_rank: Super Contributor
timestamp: August 01, 2021, 01:06:45 pm
content: The TE Connectivity HB series thick film resistors are also a good choice. They're low inductance and have good voltage coefficient, I don't know what overall resistance you are aiming for in your divider (and split across how many resistors) but they achieve around 1ppm/V up to 10Meg, and 2ppm/V up to 100Meg. Not cheap, but not ridiculously expensive either.https://uk.farnell.com/w/search/prl/results?st=hb%20resistor

author: joeqsmith
author_rank: Super Contributor
timestamp: August 01, 2021, 01:22:51 pm
content: I designed and constructed a few different high voltage probes. Normally I use wire or copper foil to form the compensation network. This video goes over the design of my 40kV 20MHz wideband probe. I broke the ceramic insulator a few years back and ended up changing the design a bit to improve the performance.

author: TimFox
author_rank: Super Contributor
timestamp: August 01, 2021, 01:37:53 pm
content: Quote from: xzswq21 on August 01, 2021, 05:29:52 amOf the three dielectrics you mentioned, "N4700" has a high temperature co-efficient (-4700 ppm/K), but its voltage dependence should be reasonable. See others' comments about DC distribution across series capacitors: you must use resistors to equalize the DC voltages.

author: xzswq21
author_rank: Frequent Contributor
timestamp: August 01, 2021, 07:07:13 pm
content: Quote from: TimFox on August 01, 2021, 01:37:53 pmYes , I want to use two 500MegOhms or four 250MegOhms resistors.

author: Gyro
author_rank: Super Contributor
timestamp: August 01, 2021, 07:23:07 pm
content: It sounds as if you should build it out of a series - parallel combination of 500Meg resistors in that case. You don't have your country flag set, but Farnell have a 25% price break at 10pcs. I would think that other distributors would have something similar.Edit: Sorry, I missed the "or".

author: TimFox
author_rank: Super Contributor
timestamp: August 01, 2021, 07:25:22 pm
content: The simplest approach is to make a series string of several resistors (more is better) and put equal appropriate capacitors across each one. Again, resistors have a noticeable voltage co-efficient at high voltage, but ceramic capacitors can have huge voltage dependence—read the data sheet for individual types. When last I worked with 100 kV, good high-voltage capacitors were easier to find (used in CRT supplies, etc.).

author: xzswq21
author_rank: Frequent Contributor
timestamp: August 01, 2021, 07:35:32 pm
content: Quote from: TimFox on August 01, 2021, 07:25:22 pmI found several resistors as following:MOX94021007FTE: (MOX940)MOX96021007FTE:(MOX960)SM108035006FE:

author: TimFox
author_rank: Super Contributor
timestamp: August 01, 2021, 07:47:36 pm
content: Choosing resistors with a specification (even “typ”) on VCR is a good idea. Note that the spec is “ppm/V”, so the ppm (10-6) must be multiplied by the voltage across the resistor to get the fractional deviation (this can be derived from a quadratic function of current vs. voltage, rather than linear).

author: joeqsmith
author_rank: Super Contributor
timestamp: August 01, 2021, 08:29:52 pm
content: Shown is a 20kV differential probe in apple jelly jars. Wonder what ever happened to this channel.

author: TimFox
author_rank: Super Contributor
timestamp: August 01, 2021, 08:44:08 pm
content: The first high voltage divider I built (in grad school) was carefully constructed in an oil-filled glass housing with the HV connections on top (not hermetically sealed, with a substantial air gap above the transformer-oil level), for 100 kV.That became a good demonstration of dielectric forces: the oil flowed upwards to leave the region of non-uniform field and leaked out the top.

author: xzswq21
author_rank: Frequent Contributor
timestamp: August 01, 2021, 08:50:01 pm
content: Quote from: TimFox on August 01, 2021, 07:47:36 pmWhat do you think about this capacitor? But the price is 40$ and the diameter is 21mmhttps://www.mouser.com/ProductDetail/Vishay-Cera-Mite/715C40KTT10?qs=TiOZkKH1s2TbBg3om3yqqQ%3D%3Dhttps://www.mouser.com/datasheet/2/427/715c_kt-1762164.pdf

author: TimFox
author_rank: Super Contributor
timestamp: August 01, 2021, 09:41:15 pm
content: For obvious reasons, that’s called a “doorknob” capacitor.The typical curve of capacitance vs. applied voltage (% of rated max) is shown below the table.Does this meet your requirements?N4700 is probably the “worst” of the Class-1 dielectrics. The dielectric materials get “worse” as their dielectricconstantincreases, but high voltage capacitors need a thick dielectric for insulation, based on their dielectricstrength.HVStuff.com has some interesting high-voltage polystyrene capacitors. They cater to the Tesla-coil enthusiasts.http://hvstuff.com/high-voltage-capacitors/film/polystyreneThey also have HV ceramic capacitors, but don't specify the dielectric (some have Q about 250, which is probably N4700 again). Their prices are reasonable. The polystyrene capacitors should be good, but their values are a bit high.MaxGain Systems have some interesting HV NP0 ceramic capacitors:https://mgs4u.com/product/npo-rated-doorknob-capacitors-5-pf-5-kv/, used, at quite good prices.

author: xzswq21
author_rank: Frequent Contributor
timestamp: August 03, 2021, 05:03:21 am
content: Consider I have a 1000:1 voltage divider and the input voltage is 20kV, if I Want to use ten 100MegOHM in series, the differential voltage across the first resistor would be only 2kV but the absolute voltages across the resistor would be 20kV and 18kV! I think I should use a resistor with the voltage rating of 20kV. What do you think? I think there is the same philosophy for the capacitors. Thanks

author: bob91343
author_rank: Super Contributor
timestamp: August 03, 2021, 05:36:24 am
content: It depends on the mechanical configuration. If properly designed, the absolute voltage won't matter.

author: xzswq21
author_rank: Frequent Contributor
timestamp: August 03, 2021, 05:44:11 am
content: Quote from: bob91343 on August 03, 2021, 05:36:24 amas your statement I can use 100MegOHM resistors with the voltage rating of 3kV in series. OK? and the same philosophy for the capacitors.