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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 27th Sep 2016, 11:22 pm   #21
IanBland
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Default Re: Quick power load

Salt water dimmers in theatre were colloquially known as "p*ss pots", according to the older chaps I worked with. By the time I started in the business, it was all thyristor dimmer racks controlled by computer boards (then hideously expensive custom hardware), but big rheostats with clutches had only just been phased out (we still had a Strand Grandmaster up on a perch that hadn't been dismantled), there were still a few motorised transformers hanging around in TV studios and there were many people working who remembered the old technology. I had a post-war catalogue that featured such products as a "portable" control board (six circuits, a four man lift with stout handles on each end) and the new S-Type lighting battens with the modern technology of spun aluminium reflectors (instead of the back painted white).

The motorised luminaire had just arrived (the Vari-Lite) and was an object of wonder. The technology was so secret that only authorised technicians were allowed to dissect them, in a special tabernacle. There was great secrecy about how they changed colour so rapidly- the secret turned out to be they used a spring wound by a motor to flick the colour wheels rapidly around.

The Grandmaster was the top of the line rheostat board found in most West End theatres, Opera Houses, etc. It had four drive shafts ("presets") onto which you locked the rheostats with clutch handles. The four presets mapped originally onto the four colours in a standard batten- white, red, amber and blue (I think), in fact our plugboxes at the Duke Of Yorks were (until I had them rewired) labelled with the four standard colours (which confused the heck out of people who thought they were old phase colours or something).

Back then at least we looked down on cheapo dimmers that used triacs instead of proper thyristors.
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Old 28th Sep 2016, 3:37 pm   #22
dave cox
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Default Re: Quick power load

An alternative to water cooled is fan cooled. Its not necessary to have a huge heatsink as ceramic resistors can be operated at high temperature so reducing the surface area requirements.

My own load was made from a bag of ~ 100 X 7W/15ohm WW resistors I found these for less than £10. I used 64 of them, wired as 8 groups of 8 in parallel, to form 8 X 1.875ohm sections. This is all built on 2 length of tag-board with the resistors spaced out. Even a 60WPC stereo amp will get this hot enough to see a rising air column! With an 8" fan I can run a 300W / 4ohm single channel amp up to clipping.

dc
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Old 28th Sep 2016, 10:59 pm   #23
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Default Re: Quick power load

What about just using lamps? Other than the low resistance when they're cold?
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Old 30th Sep 2016, 2:04 pm   #24
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Default Re: Quick power load

Lamps have been the 'traditional' dummy-load: indeed, Randall and Boot used lamps as the load when they got their first Cavity Magnetron producing 400-odd Watts of microwave power at 10cm.

Problem is, lamps have wildly-varying resistence as their temperature varies [5:1 is typical...] they're also inductive (which causes issues at RF).

While the temperature-effect can be useful - for example when used as a current-limiter in a battery charger - it can be a real pain in other environments such as audio- or RF-amplifier testing: the last thing you really want when trying to identify a source of amp non-linearity is for your nice low-resistance load to suddenly turn into a high-resistance one on signal peaks.
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Old 1st Oct 2016, 11:26 am   #25
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Default Re: Quick power load

I have a large array of immersion heater elements wired in parallel, mounted on a formica-plywood base. They rarely get that hot. I use them for big audio amplifiers.
For testing large 24V power supplies we used to have an array of 24V, GLS 100W light bulbs in holders drilled into a length of 50mm trunking. Used to get very bright and they were switched in banks with toggle switches. If all turned on together it could trip a switchmode power supply so I used to build up the load gradually while monitoring for any voltage sag.
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Old 4th Nov 2016, 4:43 pm   #26
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Default Re: Quick power load

Enamelled copper wire wound around a wooden frame, windings spaced by about 1cm.

About 50m of 0.71mmsq ecw for my 2 ohm dummy load (I have four in case I need 8 ohms).

This regularly has >2kW blasting through it, and it hardly gets warm. If you need to monitor the amplifiers output, just hang the scope probe in the coil.
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Old 8th Nov 2016, 11:19 pm   #27
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Default Re: Quick power load

Many years ago I remember seeing a dummy load for testing standby generators on a radar station that looked like a large metal central heating oil storage tank filled with liquid. I am not sure whether it had resistors and water inside or plates of some description and a conductive compound. Either way I am sure it would have made some impressive boiling sounds when in use. Probably wouldn't be allowed nowadays.
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Old 9th Nov 2016, 5:56 pm   #28
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Question Re: Quick power load

Quote:
Originally Posted by kevinaston1 View Post
Enamelled copper wire wound around a wooden frame . . .
This regularly has >2kW blasting through it and it hardly gets warm.
I'm a bit puzzled by that: perhaps I'm missing something obvious! Or perhaps there's a bit more to it
Two kW+ is a lot of heat, so I'm surprised that it "hardly gets warm". And isn't there a risk of the wooden frame burning? Perhaps your using some sort of external cooling arrangement, like a fan?
I'm intrigued!

Al.
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Old 14th Nov 2016, 1:57 pm   #29
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Default Re: Quick power load

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'...I am not sure whether it had resistors and water inside or plates of some description and a conductive compound. Either way I am sure it would have made some impressive boiling sounds when in use. Probably wouldn't be allowed nowadays.'
Yes, they're allowed. We used a 500kW 'liquid load' at Rampisham and Skelton. It consisted of a pumped column of a solution of deionised water and sodium carbonate, circulated from a heated storage tank and passed through a heat-exchanger with a glycol secondary circuit to external cooling fans.

The match to the transmitter was adjusted by either diluting with D/I water or adding sodium carbonate to the solution to arrive at 50 Ohms. Because the column match varied with temperature, a mismatch occurred after load had been on a bit, but not critically. A flowmeter and temperature sensor were linked to a Marconi-designed processor (based on a BBC micro) where the dissipated power was displayed on a CRT, along with the VSWR. Two thermometers were provided for the traditional means of power calculation.
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Old 14th Nov 2016, 2:12 pm   #30
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Default Re: Quick power load

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Originally Posted by Biggles View Post
Probably wouldn't be allowed nowadays.
Liquid-loads are still used for Neutral-Earth resistors at substations, and liquid (electrolytic) starters for motors are still manufactured.

The ones we used to use at British Steel in the '70s for testing large series-wound 250V d.c. and 500V d.c. traction motors probably wouldn't be allowed, being an open tank with electrodes dipped in it!
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Old 6th Dec 2016, 6:56 pm   #31
John Caswell
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Default Re: Quick power load

No reason why you cannot use oil either, a la Heathkit 50R transmitter dummy loads.

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Old 6th Dec 2016, 8:18 pm   #32
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Default Re: Quick power load

True - oil is effective as a heat remover. But water with something dissolved in it works both as a coolant and as an electrolyte, so you don't need resistors, just plates. Only useful for AC though.

Transformer oil is quite expensive, though motor oil would probably be OK at low voltages. Oil is a good insulator, and has obvious merit after a breakdown - you just empty it out and pour in fresh, no messing about unwinding and rewinding! And circulating the oil removes heat.
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Old 6th Dec 2016, 9:38 pm   #33
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Default Re: Quick power load

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Originally Posted by kalee20 View Post
Only useful for AC though.
Why's that? We used to use a liquid-filled resistor to limit current to 250V d.c. series-wound traction motors when testing after we'd repaired them.
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