Model 152 PSU dummy loads - Re: NeXT Cube - powers on briefly then off again

Paul Koning paulkoning at
Mon Oct 26 10:30:44 CDT 2015

> On Oct 26, 2015, at 11:21 AM, Tapley, Mark <mtapley at> wrote:
> On Oct 26, 2015, at 10:09 AM, Paul Koning <paulkoning at> wrote:
>>> I concur. Tap water may have contaminants (such as chlorine) that will cause electrical leakage.
>> Sure, but so what?  A dummy load is supposed to have electrical leakage.  All that water contamination would do is reduce the effective resistance of your load by a hair.
> 	OK, so a couple years back, I wanted to have some chemistry fun with the kids. So, I got out the jump cables, clamped them onto some aluminum foil, stuffed the foil into test tubes, filled the tubes with water, inverted both of them in the same basin and sprinkled in a little salt, cranked up the car, and sure enough … bubbles started evolving off the foil and collecting in the test tubes. 
> 	Just as expected, one tube was filling with gas twice as fast as the other. 
> 	Just as expected, when we held that tube over a candle, it went “WHEEP” and got hot (the flame was barely visible). 
> 	Um… the OP had a 12V supply, right? How *do* you keep from electrolyzing your coolant in this apparatus?

Some observations.  One is that I've used this scheme for ages, and the only bubbles seen were steam (after having the dummy load dissipate 100 watts for a couple of minutes).

Second: if you deliberately make the water conductive by adding salt, that's a different test setup.  Try this with plain water.  If your water is too cruddy, use deionized water; that's still a very cheap option.


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