Electrochemistry: Liquid Power Resistor for 2000VAC
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From: VWWall on 12 Mar 2010 13:23 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? > No, but the resistance at the electrode/liquid interface may vary even over a few seconds. Put it in series with a known resistor and put a scope HV probe on it. Check the divider ratio with various voltages over the 5 second time. -- Virg Wall
From: D from BC on 12 Mar 2010 13:26 In article <gr0lp5l453651l6vritsfpdqlcft89lc87(a)4ax.com>, jjlarkin(a)highNOTlandTHIStechnologyPART.com says... > > 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 Yup. I don't have any but odds are better that the surplus store will have 2W power resistors as opposed to a single power resistor. It'll probably be better if I run out and get the resistor/s. groan :( ..
From: Tim Williams on 12 Mar 2010 13:41 "John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in message news:gr0lp5l453651l6vritsfpdqlcft89lc87(a)4ax.com... >>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! Not at AC faster than the diffusion rate of the liquid. Even a pokey 60Hz modulation is notorious for having terrible electrolysis capacity. Tim -- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms
From: D from BC on 12 Mar 2010 13:53 In article <hndvkc$ejp$1(a)news.eternal-september.org>, Jon_Slaughter(a)Hotmail.com says... > > D from BC wrote: > > I'm setting up a test circuit and I need a 100k resistor that can > > handle 2000VAC for a few seconds. > > > > I was going to run off to the local surplus shop but got the idea of > > making a liquid resistor. Saves a trip. > > > > I added some salt to water and got 100kohm. Copper electrodes (all I > > got at the moment). > > > > Question is... Will this stay 100kohm +/- 10kohm up to 2000VAC? > > My circuit only needs to run for 5 seconds. > > > > iows...Does salty water resistance vary with voltage? > > Why not experiment with it since you already have the setup? Try putting a > 100k res in series and measure the voltage. Sweep up to 200VAC and see if > the volage devates from 100VAC at the divider. > > I imagine if the liquid gets hot the resistance will change so you might try > an ice bath. You could then try with and without the ice bath and find the > deviation to get some temperature dependence relation. I might. I do have a isolated variable transformer and a single 2W 100k resistor. And I could try graphite electrodes. I found a pencil. I'm just wobbling between curiosity /laziness vs practicality. To set up an experiment or... forget about that and run off to the surplus store and get some real resistors. mmm
From: Jon Slaughter on 12 Mar 2010 14:09
D from BC wrote: > In article <hndvkc$ejp$1(a)news.eternal-september.org>, > Jon_Slaughter(a)Hotmail.com says... >> >> 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. > > I might. I do have a isolated variable transformer and a single 2W > 100k resistor. > And I could try graphite electrodes. I found a pencil. > > I'm just wobbling between curiosity /laziness vs practicality. > To set up an experiment or... forget about that and run off to the > surplus store and get some real resistors. mmm If it's just for an experiment then why not put the resistor in an ice bath? I'm not sure how well this will help out but it should easily allow a 1/4W resistor to last 5 seconds? I imagine the resistor may last 5 seconds without the bath. The main issue is, of course, that the resistance may change drastically but you could try it and see. Use the 2W 100k resistor in an ice bath and you should be fine. You could put 200 liquid containers in series or parallel if you really wanted to have fun ;) |