|
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! |
|
Thread Tools |
18th Sep 2016, 8:46 pm | #1 |
Retired Dormant Member
Join Date: Apr 2003
Location: Salt Spring Island, BC, Canada
Posts: 368
|
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.
John. |
19th Sep 2016, 1:05 pm | #2 |
Octode
Join Date: May 2011
Location: Lancashire, UK.
Posts: 1,349
|
Re: Quick power load
I've done that before now
Its also worth using a toneburst test signal which is much easier on things. |
19th Sep 2016, 1:15 pm | #3 |
Dekatron
Join Date: Jun 2016
Location: London, UK.
Posts: 3,687
|
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 |
19th Sep 2016, 1:39 pm | #4 |
Nonode
Join Date: Oct 2008
Location: Warsaw, Poland and Cambridge, UK
Posts: 2,669
|
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
__________________
What's going on in the workshop? http://martin-jones.com/ |
19th Sep 2016, 1:47 pm | #5 |
Dekatron
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
|
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. |
19th Sep 2016, 2:39 pm | #6 | |
Guest
Posts: n/a
|
Re: Quick power load
Quote:
|
|
19th Sep 2016, 2:49 pm | #7 | ||
Nonode
Join Date: Oct 2008
Location: Warsaw, Poland and Cambridge, UK
Posts: 2,669
|
Re: Quick power load
Quote:
Chris
__________________
What's going on in the workshop? http://martin-jones.com/ |
||
25th Sep 2016, 8:43 pm | #8 |
Hexode
Join Date: May 2015
Location: Northampton, Northants, UK.
Posts: 380
|
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?
|
25th Sep 2016, 9:17 pm | #9 |
Octode
Join Date: Oct 2003
Location: Newbury, Berkshire, UK.
Posts: 1,770
|
Re: Quick power load
The resistance of the water should be significantly higher than the resistors, so won't appreciably effect things.
__________________
Chris |
26th Sep 2016, 5:49 am | #10 |
Dekatron
Join Date: Jul 2009
Location: Sleaford, Lincs. UK.
Posts: 7,638
|
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.
__________________
Curiosity hasn't killed this cat...so far. Last edited by Diabolical Artificer; 26th Sep 2016 at 5:54 am. |
26th Sep 2016, 12:24 pm | #11 | |
Rest in Peace
Join Date: Jun 2006
Location: Chard, South Somerset, UK.
Posts: 7,457
|
Re: Quick power load
Quote:
Al. |
|
26th Sep 2016, 12:39 pm | #12 |
Moderator
Join Date: Jan 2003
Location: Ipswich, Suffolk, IP4, UK.
Posts: 21,192
|
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.
__________________
Graham. Forum Moderator Reach for your meter before you reach for your soldering iron. |
26th Sep 2016, 1:27 pm | #13 | |
Dekatron
Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
Posts: 7,061
|
Re: Quick power load
Quote:
However, as pointed out, the shunting effect on an 8Ω load will be negligible. |
|
26th Sep 2016, 6:33 pm | #14 |
Octode
Join Date: May 2011
Location: Lancashire, UK.
Posts: 1,349
|
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.
|
26th Sep 2016, 6:57 pm | #15 |
Rest in Peace
Join Date: Apr 2008
Location: London, UK.
Posts: 2,508
|
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.
|
26th Sep 2016, 10:09 pm | #16 | ||
Rest in Peace
Join Date: Jun 2006
Location: Chard, South Somerset, UK.
Posts: 7,457
|
Re: Quick power load
Quote:
Quote:
Al. |
||
27th Sep 2016, 4:11 pm | #17 |
Dekatron
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 13,953
|
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..... |
27th Sep 2016, 5:40 pm | #18 |
Dekatron
Join Date: Jun 2015
Location: Biggin Hill, London, UK.
Posts: 5,190
|
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? |
27th Sep 2016, 6:34 pm | #19 |
Nonode
Join Date: Jun 2007
Location: Coulsdon, London, UK.
Posts: 2,152
|
Re: Quick power load
It depends on the linearity of its resistance at different voltages, temperatures, etc.
|
27th Sep 2016, 6:39 pm | #20 |
Rest in Peace
Join Date: Apr 2008
Location: London, UK.
Posts: 2,508
|
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. |