Electrochemistry: Liquid Power Resistor for 2000VAC
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From: markp on 13 Mar 2010 07:50 "D from BC" <myrealaddress(a)comic.com> wrote in message news:MPG.2604f963b6e4e1d19896fe(a)209.197.12.12... > In article <8011svF34dU1(a)mid.individual.net>, map.nospam(a)f2s.com says... >> >> "D from BC" <myrealaddress(a)comic.com> wrote in message >> news:MPG.26047ecfa9832fe9896fa(a)209.197.12.12... >> > In article <8004pkFd3U1(a)mid.individual.net>, map.nospam(a)f2s.com says... >> >> >> >> "D from BC" <myrealaddress(a)comic.com> wrote in message >> >> news:MPG.26041fc5df0edf429896f1(a)209.197.12.12... >> >> > I'm setting up a test circuit and I need a 100k resistor that can >> >> > handle >> >> > 2000VAC for a few seconds. >> >> > >> >> > I was going to run off to the local surplus shop but got the idea of >> >> > making a liquid resistor. Saves a trip. >> >> > >> >> > I added some salt to water and got 100kohm. Copper electrodes (all I >> >> > got >> >> > at the moment). >> >> > >> >> > Question is... Will this stay 100kohm +/- 10kohm up to 2000VAC? >> >> > My circuit only needs to run for 5 seconds. >> >> > >> >> > iows...Does salty water resistance vary with voltage? >> >> > >> >> >> >> EBG do a nice range. You could for example put 5 560k SSP52 in >> >> parallel: >> >> http://www.ppmpower.co.uk/downloads/products/PPM295.pdf >> >> >> >> Mark. >> > >> > I might track down where I can get one. >> > >> >> It seems from the spec that they can also be over-rated for 5 seconds at >> 5 >> times the power rating, which means you may well get away with just one >> 100k >> SSP52, or possibly two put 220k's in parallel for belt and braces. >> >> Mark. > > I checked Mouser using keyword SSP52. No results. > > I'll get resistors when I've build up a parts list worthy of making a > Digikey order. > > I might order these .. 4 X $2.00ea.. > http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=RSD- > 25KRCT-ND > > OR > > $13.00 for a single R. > http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name= > 850F100KE-ND > > In the meantime, I'm taking my chances with a liquid cooled 2W resistor > from the junkbox. > Either option seems to be OK, although in the single resistor version you're closer to the edge of over-driving it (i.e. there's more slack in the 4 resistor version voltage wise). Although you can think of the liquid as some kind of ideal heatsink the 2W resistor package is going to have a finite thermal resistance, so localised heating might damage it before the heat can flow out of the package into the liquid. Careful too with dielectric breakdown voltage. Mark.
From: markp on 13 Mar 2010 07:57 "markp" <map.nospam(a)f2s.com> wrote in message news:801fpfFm7dU1(a)mid.individual.net... > > "D from BC" <myrealaddress(a)comic.com> wrote in message > news:MPG.2604f963b6e4e1d19896fe(a)209.197.12.12... >> In article <8011svF34dU1(a)mid.individual.net>, map.nospam(a)f2s.com says... >>> >>> "D from BC" <myrealaddress(a)comic.com> wrote in message >>> news:MPG.26047ecfa9832fe9896fa(a)209.197.12.12... >>> > In article <8004pkFd3U1(a)mid.individual.net>, map.nospam(a)f2s.com >>> > says... >>> >> >>> >> "D from BC" <myrealaddress(a)comic.com> wrote in message >>> >> news:MPG.26041fc5df0edf429896f1(a)209.197.12.12... >>> >> > I'm setting up a test circuit and I need a 100k resistor that can >>> >> > handle >>> >> > 2000VAC for a few seconds. >>> >> > >>> >> > I was going to run off to the local surplus shop but got the idea >>> >> > of >>> >> > making a liquid resistor. Saves a trip. >>> >> > >>> >> > I added some salt to water and got 100kohm. Copper electrodes (all >>> >> > I >>> >> > got >>> >> > at the moment). >>> >> > >>> >> > Question is... Will this stay 100kohm +/- 10kohm up to 2000VAC? >>> >> > My circuit only needs to run for 5 seconds. >>> >> > >>> >> > iows...Does salty water resistance vary with voltage? >>> >> > >>> >> >>> >> EBG do a nice range. You could for example put 5 560k SSP52 in >>> >> parallel: >>> >> http://www.ppmpower.co.uk/downloads/products/PPM295.pdf >>> >> >>> >> Mark. >>> > >>> > I might track down where I can get one. >>> > >>> >>> It seems from the spec that they can also be over-rated for 5 seconds at >>> 5 >>> times the power rating, which means you may well get away with just one >>> 100k >>> SSP52, or possibly two put 220k's in parallel for belt and braces. >>> >>> Mark. >> >> I checked Mouser using keyword SSP52. No results. >> >> I'll get resistors when I've build up a parts list worthy of making a >> Digikey order. >> >> I might order these .. 4 X $2.00ea.. >> http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=RSD- >> 25KRCT-ND >> >> OR >> >> $13.00 for a single R. >> http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name= >> 850F100KE-ND >> >> In the meantime, I'm taking my chances with a liquid cooled 2W resistor >> from the junkbox. >> > > Either option seems to be OK, although in the single resistor version > you're closer to the edge of over-driving it (i.e. there's more slack in > the 4 resistor version voltage wise). Although you can think of the liquid > as some kind of ideal heatsink the 2W resistor package is going to have a > finite thermal resistance, so localised heating might damage it before the > heat can flow out of the package into the liquid. Careful too with > dielectric breakdown voltage. > > Mark. Another thing to watch is the type of resistor package you use. Most of the moulded types transfer their heat through the legs (unless they have an integral heatsink or provision to add one), not through the package itself which has a much higher thermal resistance, so make sure both legs are fully immersed in the liquid if it is this type. Mark.
From: Ecnerwal on 13 Mar 2010 10:31 In article <hneulk$pi3$1(a)news.eternal-september.org>, "Tim Williams" <tmoranwms(a)charter.net> wrote: > "D from BC" <myrealaddress(a)comic.com> wrote in message > news:MPG.26047cd7780c7eeb9896f9(a)209.197.12.12... > > I do have CuSO4. > > > > http://en.wikipedia.org/wiki/Electrolysis_of_water > > 'The following cations have lower electrode potential than H+ and are > > therefore suitable for use as electrolyte cations: Li+, Rb+, K+, Cs+, > > Ba2+, Sr2+, Ca2+, Na+, and Mg2+. Sodium and lithium are frequently used, > > as they form inexpensive, soluble salts.' > > > > Those are all group 1 and 2 elements.. > > So I suppose I should use CuSO4 to prevent H2 and O2 bubbles creating > > resistance error on the electrodes. > > If you use Cu electrodes in CuSO4, you'll get straight electroplating. A > few seconds run time won't make a difference. > > Tim We used copper sulfate with copper electrodes as resistors on Marx generators. Works. May not be completely stable, but that's not going to be a drastic change in a few seconds thing, more like over weeks or months the resistance will change. If the copper sulfate solution is fully saturated, the resistance will be less likely to change. Something like 2" diameter tubing, and the whole mess drowned in transformer oil. Voltage was DC and 10's of KV for charging. IIRC the big one erected to over a million volts, but none of the resistors would see all that. -- Cats, coffee, chocolate...vices to live by
From: D from BC on 13 Mar 2010 11:57 In article <weOdnViNweT29QbWnZ2dnUVZ_gGdnZ2d(a)posted.localnet>, robertbaer(a)localnet.com says... > > John Larkin wrote: > > On Fri, 12 Mar 2010 12:02:46 -0600, "Jon Slaughter" > > <Jon_Slaughter(a)Hotmail.com> wrote: > > > >> D from BC wrote: > >>> I'm setting up a test circuit and I need a 100k resistor that can > >>> handle 2000VAC for a few seconds. > >>> > >>> I was going to run off to the local surplus shop but got the idea of > >>> making a liquid resistor. Saves a trip. > >>> > >>> I added some salt to water and got 100kohm. Copper electrodes (all I > >>> got at the moment). > >>> > >>> Question is... Will this stay 100kohm +/- 10kohm up to 2000VAC? > >>> My circuit only needs to run for 5 seconds. > >>> > >>> iows...Does salty water resistance vary with voltage? > >> Why not experiment with it since you already have the setup? Try putting a > >> 100k res in series and measure the voltage. Sweep up to 200VAC and see if > >> the volage devates from 100VAC at the divider. > >> > >> I imagine if the liquid gets hot the resistance will change so you might try > >> an ice bath. You could then try with and without the ice bath and find the > >> deviation to get some temperature dependence relation. > >> > >> > > > > Copper will be electrolyzed into solution and the resistance will > > drop. And bubbles will form on the electrodes and resistance will > > increase! > > > > Neither happens fast at low currents. > > > > But the dissipation is only 40 watts for 5 seconds. A string of, say, > > ten or so 2-watt carbon film resistors would work fine. > > > > John > > > I thought the voltage rating of those resistors was 150V, not 200V - > so one would need 14 resistors in series; make them 5W and be OK for > "any" time period. If not a power limit, I'm guessing the 150V limit is to stay within R tolerance due to voltage changing the resistance. That or there's electrostatic damage. I'd have to look at some data sheets.
From: D from BC on 13 Mar 2010 11:57
Cool :) |