Electrochemistry: Liquid Power Resistor for 2000VAC
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From: Phil Hobbs on 14 Mar 2010 18:08 On 3/14/2010 1:06 PM, Ecnerwal wrote: > In article<MPG.2605e3e6e5d856be989707(a)209.197.12.12>, > D from BC<myrealaddress(a)comic.com> wrote: > >> iic resistivity of pure water is 18.2Mohm cm^2/cm at 25C > > Ultrapure water is a great insulator. It's also "not found in the wild" > and takes constant deionization filtration to maintain in an ultrapure > state. If you go putting copper in it, it won't be staying ultrapure. > One of our pulse machines did use it (with stainless steel inner and > outer conductors) as the insulator for a coaxial line about 5 feet in > diameter (outer - inner was about 18 inches as I recall.) Transformer > oil was used for most applications requiring high-voltage and access to > parts - presumably the water gave a better capacitance for the > transmission line in that case (I worked on them, I didn't design them). > > From the practical point of view, the string of carbon (or wire-wound) > power resistors (or multiple strings in parallel if need be for power > handling) is simpler to implement, has no leaking fluid potential, and > is often cheaper. Strings of 2-watt resistors inside a vinyl tube (no > doubt not helping with power handling, but making them safer as far as > inadvertent shorting) were a common way to get a high-voltage resistor > without breaking out the big bucks (research budgets are not generally > lavish) for all-in-one piece high voltage resistors. Better cooling can > be had by wiring them into perfboard instead. > 18.2 Mohm-cm is the right value for ultrapure water at 20 C (or thereabouts). However it's a fairly steeply declining function of temperature, so with enough dissipation, it's likely to run away. Using a solution of some salt that disassociates fully would stabilize the conductivity vs temperature. (It still wouldn't be that stable, since the viscosity of water drops steeply with temperature too.) Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal ElectroOptical Innovations 55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net
From: JosephKK on 14 Mar 2010 22:02 On Sat, 13 Mar 2010 18:04:11 -0800, D from BC <myrealaddress(a)comic.com> wrote: >In article <eleop5lf9ee8auf5ohvn51aoj61vgiongl(a)4ax.com>, >OneBigLever(a)InfiniteSeries.Org says... >> >> On Sun, 14 Mar 2010 10:36:15 +1100, Grant <omg(a)grrr.id.au> wrote: >> >> >I dunno, 2kV + open water still sounds like Bang! to me ;) >> >> >> Would not do anything with pure water. > > >iic resistivity of pure water is 18.2Mohm cm^2/cm at 25C ^^^^^^^^^^ It won't stay pure when you place the resistor pack within, let alone when you apply voltage. > >If I got this right.. >Given a water wire with a 1cm^2 cross section and a length of 1cm then >the resistance of the water is 18.2Mohm. > >The bare resistor leads from a 100k resistor in this amount of water has >an error about > >100k//18.2Meg = 99454 > >Error 100k - 99454/100k * 100% = 0.5%
From: JosephKK on 14 Mar 2010 22:24 On Sat, 13 Mar 2010 15:35:21 -0800, D from BC <myrealaddress(a)comic.com> wrote: >lol.. >Too laborous but 0.25W resistors are cheap. > >If I were to do that, I might just glue them all to a heat sink instead >of using a fan. That would give you insulation breakdown problems.
From: D from BC on 14 Mar 2010 22:30 In article <kh6rp5duimobc2ffaie0neceki8qqlai3s(a)4ax.com>, quiettechblue(a)yahoo.com says... > > On Sat, 13 Mar 2010 15:35:21 -0800, D from BC <myrealaddress(a)comic.com> wrote: > > >lol.. > >Too laborous but 0.25W resistors are cheap. > > > >If I were to do that, I might just glue them all to a heat sink instead > >of using a fan. > > That would give you insulation breakdown problems. True.. Two parameters in conflict, thermal coupling and electrical isolation. I probably thought of using nonconductive thermal epoxy.
From: pimpom on 15 Mar 2010 01:44
D from BC wrote: > In article <kh6rp5duimobc2ffaie0neceki8qqlai3s(a)4ax.com>, > quiettechblue(a)yahoo.com says... >> >> On Sat, 13 Mar 2010 15:35:21 -0800, D from BC >> <myrealaddress(a)comic.com> wrote: >> >>> lol.. >>> Too laborous but 0.25W resistors are cheap. >>> >>> If I were to do that, I might just glue them all to a heat >>> sink >>> instead of using a fan. >> >> That would give you insulation breakdown problems. > > > True.. > Two parameters in conflict, thermal coupling and electrical > isolation. > I probably thought of using nonconductive thermal epoxy. Did you see my suggestion to fix the resistors with epoxy to a plastic box and then put some water in the box? |