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Normal resistors for High Voltage application

Discussion in 'General Electronics Discussion' started by Apocaloptigon, Sep 17, 2018.

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  1. Apocaloptigon

    Apocaloptigon

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    Jun 23, 2018
    Hi,
    What would happen if i used a regular resistor for voltages up to 10kv? I plan on using two of them for a high voltage probe (voltage divider) and I don't want to go out and buy/make high voltage resistors. Wouldn't the current want to flow through the wire rather than arc through the resister internally?
     
  2. Bluejets

    Bluejets

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    Oct 5, 2014
    It's to do with the understanding of insulation resistance.
     
  3. kellys_eye

    kellys_eye

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    Resistors have a maximum voltages rating. If using them beyond the recommended limits it is common to make the total value from a chain of series resistors such that the voltage across any one of them does not exceed that maximum for the individual.

    Attention to board placement/ track layout is also essential to prevent arcing.
     
    Cannonball likes this.
  4. hevans1944

    hevans1944 Hop - AC8NS

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    Jun 21, 2012
    Current doesn't want to do anything. It just is or it isn't, either case being determined by well-known laws of physics. All resistors have a voltage rating whose absolute maximum value is determined by the breakdown resistance of their surrounding environment, usually air, but sometimes a better insulating gas, such as nitrogen or sulfur hexafluoride, is used. It is common practice to connect equal-valued resistors in series strings to increase the breakdown voltage of the string. Below I discuss an example of this practice.

    A small tandem particle accelerator I used to operate and maintain had several hundred ordinary two-watt carbon composition resistors soldered together in series and then spiral-wrapped around the accelerator columns. About every fifty resistors or so (I don't recall the exact number), the spiral string would connect to an accelerator plate on the column. There were some fifty or sixty of these plates, each with a central hole through which ions passed, stacked alternately with insulating glass rings in between, for each of two accelerator columns.

    The entrance plate was at ground potential but the plate on the opposite end, through which accelerated ion particles emerged, could be as much as one million seven hundred thousand volts positive with respect to ground. The interior of the accelerator columns was maintained at a high vacuum, but the exterior was housed in a pressure vessel filled with dry SF6 pressurized to 120 psig. Even so, there would be an occasional "tank arc," even when operating at less than maximum potential.

    Tank arcs were infrequent, after proper "conditioning," but every once in awhile a cosmic ray would create an ionized conduction path and cause a tank arc to happen. I first witnessed this phenomenon in the 1950s at the Oak Ridge Atomic Energy Museum that I visited as a pre-teenager while living with my grandparents, who lived in Morristown, TN. One of the museum exhibits was a large tank with glass sides and metal plates for the top and bottom. The plates constituted a parallel-plate capacitor that was energized with several thousand volts DC. About once an hour or so, this capacitor would discharge with a loud crack (sounded sort of like an M80 firework) that could be heard throughout the museum when a cosmic ray passed through the tank. It is impossible to shield against cosmic rays, but the ones that reach the surface of the earth are infrequent over small areas. The museum tank was perhaps two square meters in area and was discharged by a cosmic ray no more often than once an hour.

    So, to answer the question... go ahead and connect resistors in series to increase the voltage rating of the string. Common resistors have 1000 V DC or less ratings, so plan on using perhaps a dozen or so to safely reach 10 kV. Or look on eBay for "new old stock" high voltage resistors, sometimes available at reasonable prices. If making your own series strings (recommended), you are free to make up any resistance value you choose, a major advantage over purchasing ready-made high-voltage resistors which are generally available only in a limited range of values.

    I suggest that you also slide a close-fitting length of thick-walled Tygon tubing over the resistor string to decrease the chances of arc-over across individual resistors. Seal the ends with clear RTV cement to keep moisture out and it should give you years of trouble-free service.
     
    Cannonball, Hopup and OBW0549 like this.
  5. kellys_eye

    kellys_eye

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    High voltages usually also means low current (μA) and resistance values in the MΩ range so you'll be using series resistors anyway to reach those values.
     
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  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    One interesting thing which also happens if that the effective resistance of a resistor may drop at voltages above it's maximum rated voltage.

    Voltage ratings are often related to the power rating of the resistor. This is due to several factors, two of them being physical size and the length of the resistive element. The first allows for larger creepage distances, the latter works similarly to having more resistors in series.

    In addition to this, there are some special high voltage resistors manufactured. Sometimes these are in a glass envelope (often evacuated) and others employ substrates and coatings which have better insulating properties.
     
    hevans1944 likes this.
  7. WHONOES

    WHONOES

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    Another thing to consider is that body contaminants from handling the resistor can significantly reduce its breakdown voltage. So, if you construct you "high voltage resistor" from a number of others, make sure you clean it thoroughly in alcohol and then pop it into a warm oven for 1/2 hour or so to drive all the moisture out of your contraption then be careful how you handle it afterwards.
     
    hevans1944 likes this.
  8. Hopup

    Hopup

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    hevans1944 likes this.
  9. OBW0549

    OBW0549

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    Smoke. Fire. Explosion. Toxic fumes. The neighborhood burns down. Electrical shock, and possibly other things too horrible to contemplate.

    Bad move.

    As has already been noted, current doesn't "want" to do ANYTHING. Current flows according to the laws of physics, period.

    Also noted, is that all components have maximum ratings beyond which failure-- with sometimes catastrophic consequences-- is likely to occur. Do not exceed maximum ratings. EVER.

    The other respondents in this thread have given you good advice. Read 'n heed.
     
    hevans1944 likes this.
  10. kellys_eye

    kellys_eye

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    There's been a lot said about 'resistors' in just this thread. Who'd'a'known that resistors were so...... err, complicated?
     
  11. hevans1944

    hevans1944 Hop - AC8NS

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    Um... me knows that. Hard lessons learned over a long period of time, but I kept careful notes. I suspect there are others, too, on this forum who learned the same lessons and just haven't spoken up yet.

    I am working on a project that hopefully will initiate the newbie on the three major passive components: resistors, capacitors, and inductors. With that in mind, I could use all the help I can get before distilling it all down into a few pages, useful to the beginner without overwhelming them with history and theory. I think perhaps I might tend to be a little pedantic with my writing.

    This topic of resistor voltage tolerance is a good one to explore, right up there with the voltage sensitivity of multi-layer ceramic dielectric capacitors and the "memory" charge storage effects of capacitors with certain plastic dielectrics. Resistor Johnson noise is also a good topic that most beginners are unaware of until it bites 'em.
     
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  12. kellys_eye

    kellys_eye

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    That's a project I'd be interested to follow Hop.

    Keeping it as 'math-free' as possible prevents attention-loss and I commend your approach in that sense - I find the 'practical' approach to be most useful and easiest to absorb.

    I'm going to give that project some time and attention and offer comments where applicable though you look to have covered it very comprehensively already.

    It should be (when finished) a 'sticky' for all students that come to the forum.

    Brilliant.
     
  13. OBW0549

    OBW0549

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    "Thorough" sounds better and is more accurate, IMO. I reserve "pedantic" for those exhibiting a tendency toward out-of-control, pointless, compulsive, combative nit-picking.

    At one time I thought of doing much the same thing regarding operational amplifiers and comparators, but in the end decided against it because there's already so much information on the subject readily available on the interwebz. If the noobs don't avail themselves of it, they're not going to bother reading anything I write, either.
     
  14. Chemelec

    Chemelec

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    Jul 12, 2016
    Most 1/4 watt resistors are only rated at 250 Volt and 1/2 watt ones at 450 Volts.
    For High Voltage you should buy some High Voltage Types.
    On Source is Dale/Vishay They sells these HV types of Resistors.
     
    hevans1944 likes this.
  15. Cannonball

    Cannonball

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    What ever you do, BE CARFULL, you can make only 1 mistake.
     
  16. Chemelec

    Chemelec

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    Jul 12, 2016
    Here are 2 Dale/Vishay HV Resistors.
    The 2 Inch one is rated at 15 KV and the 3 Inch one is rated at 22.5 KV.
    Dale-Resistors-1.JPG
     
    hevans1944 likes this.
  17. kellys_eye

    kellys_eye

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    Note the resistance values..... 30MΩ and 50MΩ - quite typical for HV applications so as a 'normal' component user you'd be using a series of resistors to reach such values using 1MΩ, 2.2MΩ etc (or sometimes even smaller values like 470kΩ.

    At 250V per 1/4w resistor and using 10kV as the target you need 40 (yes, FORTY) resistors to make the divider chain..... this is where using specialist parts comes into it.
     
  18. Chemelec

    Chemelec

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    Those Two HV Resistors I posted were just 2 of many values I have.
    I have resistance values ranging from 100K and up to 30 Gig-Ohms.
    Also some over 6 inches long, rated at 45 KV.
     
  19. Chemelec

    Chemelec

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    Jul 12, 2016
    This is the Largest HV Resistor I have, 38 Gig-Ohm, 1% Tol with a 10,000:1 division ratio and rated at 100KV. It is Custom made in 4 Sections that Screw together. You Never Touch this with your Fingers as that will contaminate the surface. If I remember Correctly, it cost about $80.00.
    38-GigOhm.JPG
     
    Last edited: Sep 19, 2018
  20. Apocaloptigon

    Apocaloptigon

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    Jun 23, 2018
    (Sorry for the long response time)
    Thanks for the ton of information, i guess the moral of the story is don't- or
    I'll probably just go and buy(/make?) some high voltage resisters rather than deal with all that.
    That leads me to another question, are liquid resisters (saltwater in an insulated tube, basically) a good way to contain high voltage (assuming it's long enough)?
     
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