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Hints, Tips and Solutions (Do NOT post requests for help here) If you have any useful general hints and tips for vintage technology repair and restoration, please share them here. PLEASE DO NOT POST REQUESTS FOR HELP HERE!

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Old 18th Sep 2016, 8:46 pm   #1
JHGibson
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Default Quick power load

I wanted to examine my 100 watt stereo amplifier under full drive conditions. I did not have any 8 ohm 50 watts power resistors and to make one would require large finned heatsinks, so instead I took two bare 8 ohm power resistors and dropped each into a separate bowl of water.
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Old 19th Sep 2016, 1:05 pm   #2
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Default Re: Quick power load

I've done that before now

Its also worth using a toneburst test signal which is much easier on things.
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Old 19th Sep 2016, 1:15 pm   #3
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Default Re: Quick power load

Watch out for the ceramic encapsulated ones. The cheap ones are porous and crack easily at which point they go bang the moment you power them up outside of water. Found this one out the hard way.

Some nichrome wire curled up in a bowl of water would do the job as well. Plus it leaves the water clean enough to stick a teabag in
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Old 19th Sep 2016, 1:39 pm   #4
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Default Re: Quick power load

Good trick. I've used it for load testing too. It carries on working well even after the water's started boiling, because the water's latent heat of vapourisation is large so it absorbs lots of heat. The office gets a bit steamy though.

I found out that the popular metal-clad power resistors work well with this technique. Trying to run them over their ratings in air, even on a heatsink, doesn't work well - they tend to explode, which is both annoying and dangerous. But dunking them in water they're quite happy with enormous overloads.

Attached is a photo of a two-gallon bucket of water boiling merrily during a 3kW load test earlier this year. I wouldn't have made tea with the resulting soup, though.

Chris
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Old 19th Sep 2016, 1:47 pm   #5
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Default Re: Quick power load

Here's the ones I used when I worked for Sony out in BC, Canada for load testing Sony amplifiers at full pelt, by the time I left the plywood was a bit charred due to a few nutty experiments I did, the nature of which I can't bring to mind at the moment.

Lawrence.
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Old 19th Sep 2016, 2:39 pm   #6
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Default Re: Quick power load

Quote:
I found out that the popular metal-clad power resistors work well with this technique.
Don't let them cool down in the water, they suck it in and when you re-apply power again they self dismantle. I found this out while heating water for a cup of coffee from my motor cycle battery during a power cut at work, I was called away for a chat and turned it off only to return and turn on again. The cup survived!
 
Old 19th Sep 2016, 2:49 pm   #7
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Default Re: Quick power load

Quote:
Originally Posted by merlinmaxwell View Post
Quote:
I found out that the popular metal-clad power resistors work well with this technique.
Don't let them cool down in the water, they suck it in and when you re-apply power again they self dismantle.
Oh! Thank you for the tip. Note taken...

Chris
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Old 25th Sep 2016, 8:43 pm   #8
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Default Re: Quick power load

Great tip, I never would have thought of this. Except, I think I'm missing something. Won't the water short out the resistors? Or am I just being thick?
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Old 25th Sep 2016, 9:17 pm   #9
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Default Re: Quick power load

The resistance of the water should be significantly higher than the resistors, so won't appreciably effect things.
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Old 26th Sep 2016, 5:49 am   #10
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Default Re: Quick power load

ETI used 50w R's in a bucket of water to test the amplifier's for their hifi review articles circa mid 70's. I thought the water would short the R's too. This dummy load met it's demise while testing the Phase Linear 700.

I was told, by a College tutor that it's the impurities in water that causes the problem's with electrical devices. Apparently you can drop a mobile phone into pure water without any ill effect's. Having been told repeatedly since a kid not to put electricity anywhere near water and having seen countless victims killed by a radio/electric fire in the bath on crime drama's sticking a dummy load in water is counter intuitive, no matter if the physic's says otherwise.

You can get up to 300w 8 ohm r's off ebay for not very much, Chinese made of coarse. If one wants the mutts nuts of the dummy load resistors, look out for Dale/Vishay non inductive R's, there not cheap though.

Andy.
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Old 26th Sep 2016, 12:24 pm   #11
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Quote:
Originally Posted by Dr Wobble View Post
I was told by a college tutor that it's the impurities in water that causes the problems with electrical devices. Apparently you can drop a mobile phone into pure water without any ill effects.
Quite so: pure water is an insulator. It's the contaminants in water which lower its resistance: small amounts of impurities can cause a disproportional reduction in its resistance. Really pure water (i.e. composed of hydrogen and oxygen and nothing else) is actually quite difficult to produce. For a start, you have to be 100% sure that the container holding it and any mechanisms used to transport it are absolutely free of any contaminants. That is not easy to achieve. And then there is the supplementary problem of subsequently determining just how pure your '100 % pure' water actually is.

Al.
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Old 26th Sep 2016, 12:39 pm   #12
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Default Re: Quick power load

ISTR that when I was at school they had some kind of apparatus for making distilled water. To check it's purity the electrical conductivity of the distilled water was measured using a built in meter.
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Old 26th Sep 2016, 1:27 pm   #13
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Default Re: Quick power load

Quote:
Originally Posted by Skywave View Post
pure water is an insulator. It's the contaminants in water which lower its resistance: small amounts of impurities can cause a disproportional reduction in its resistance.
There are some ions in pure water though - I seem to recall the figure for the product of the concentration of H+ ions and OH- ions as 10^-14. So small, but not zero. So there will be some conduction in pure water, more than pure hydrocarbons.

However, as pointed out, the shunting effect on an 8Ω load will be negligible.
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Old 26th Sep 2016, 6:33 pm   #14
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Default Re: Quick power load

Electrolysis is a big problem for water damaged equipment where there is any voltage present on the board. You get 'fretting' around PCB traces and component leads.
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Old 26th Sep 2016, 6:57 pm   #15
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Default Re: Quick power load

Very much so, especially when DC is applied. In a small volume of water I imagine the charge carrier concentration can rapidly run away even if the original water purity is good. I once used some water-cooled resistors to ballast a 35mm projector soundhead exciter lamp requiring 4V 0.75A. The regulated power supply did not arrive at the venue so I took the battery off the van and made up a suitable value of ballast with a some 1/4 watters I had in the toolbox. The power rating was not high enough so I suspended the resistors in a mug of water. By the end of the show, one end was stripped bright and the water was turning green with dissolved metal ions.
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Old 26th Sep 2016, 10:09 pm   #16
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Quote:
Originally Posted by Skywave View Post
Pure water is an insulator. It's the contaminants in water which lower its resistance: small amounts of impurities can cause a disproportional reduction in its resistance.
Quote:
Originally Posted by kalee20 View Post
There are some ions in pure water though - I seem to recall the figure for the product of the concentration of H+ ions and OH- ions as 10^-14. Small, but not zero. So there will be some conduction in pure water.
Indeed: point accepted: thank you.

Al.
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Old 27th Sep 2016, 4:11 pm   #17
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Default Re: Quick power load

I once used a toaster as an impromptu 50-ish Ohm load when testing a RF amplifier. Given the duty-cycle of a typical SSB transmission the toaster didn't get that hot even when driven with 500 Watts PEP, so it never 'popped up'. I chose a toaster rather than anything with a coiled heater-element because the toaster has its element zigzagged on flat mica plates rather than formed into a nastily-inductive coil.


It was kinda odd to see a length of RG8 coax wired to a toaster though.....
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Old 27th Sep 2016, 5:40 pm   #18
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Default Re: Quick power load

I once dimmed an AC lamp (110V on an isolating transformer in case you are worried) using a resistor consisting of 2 metal electrodes in a beaker of salt water (just common table salt, NaCl). The fact that it was AC meant that electrolysis was not a problem.

I am told similar dimmers were used for mains lamps at one time, possibly in theatres.

I wonder about using a similar setup as a dummy load at audio frequencies. Is there a good reason not to?
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Old 27th Sep 2016, 6:34 pm   #19
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Default Re: Quick power load

It depends on the linearity of its resistance at different voltages, temperatures, etc.
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Old 27th Sep 2016, 6:39 pm   #20
Lucien Nunes
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Default Re: Quick power load

Liquid dimmers were used in the early days of theatre lighting, with electrodes rasied and lowered on the end of wire ropes operated from the board. I have jury-rigged liquid dimmers for 230V too, as per the suggestion in Pitchford & Coombes 'Projectionist's Handbook'. It is useful to equip them with a bypass switch so as not to rely on metal-metal contact in the tank to achieve full-on.

In tank form, with three ganged isolated electrodes rotated on a shaft like the moving vanes of a tuning capacitor, they were popular as rotor-resistance starters for large slipring induction motors. Some of these are still in use, along with the 'automatic' variety where bubble formation on the surface of permanently immersed plates alters the resistance to offer a positive voltage coefficient of resistance and hence current-limiting behaviour. NaOH is the preferred electrolyte.
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