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Old 4th August 2007, 03:33 PM   #21
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Default Hi Luka!

First I am working with amplifiers over 30 years .This dummy load is not expensive for me , because i think very pro.I fired up 2 amps. into 4ohm, about 1h or more, never demaged any amp.This is good thing for cold winter nights
Red thing is PVC plate (insulator), & with jumpers I, manage 4, 8, 2ohm.


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Old 4th August 2007, 03:50 PM   #22
luka is offline luka  Slovenia
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Hi

This is what I have. It is 5x about 66ohme, 800w each power resistors that will be in wind tunnel, air pushed by 120mm/230Vac fan, one in pic is same size, but 12Vdc. I will be able to wire them any way I will need them to be. I need top and bottom plate one with hole for fan and connectors all on one side.

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Old 6th August 2007, 04:06 AM   #23
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Old 6th August 2007, 05:14 AM   #24
mzzj is offline mzzj  Finland
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Quote:
Originally posted by luka


Hi mzzj

I was thinking about resistors at first. But one 5w resistor costs 0.38€ here and you need a lot of them

OK, I get my resistors much cheaper from local surplus sales.
More likely something like 5euros/1000pcs.
Selection of resistance values is usually limited but there is always some sort of series/parallei combination that works for me.

If you are tight on budget look for toasters, washing machines, oven elements and so on whatever you can find in scrap piles.
Sauna stove elements are especially easy to find(at least here in Finland) and can take some serious loading.
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Old 8th August 2007, 03:12 PM   #25
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altough a bit late,

I used 2 * 2 halogen light bulbs 12 v 50W in series setup to load-test power supplies for my class A amp. (which should deliver + and - 27 V @ a 3 A)

Fairly cheap and basically a . No need for a fan, only a pair of shades is needed .

Easy to extend to higher voltages and (by paralleling) to higher currents.
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Old 8th August 2007, 04:35 PM   #26
Danko is offline Danko  Hungary
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A note about using bulbs: The cold resistance of the filament is about 1/10 of the warm resistance of the filament. So, if you connect a cold bulb to the PSU, the bulb draws about 10 times more current, while it's not fully warm.
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Old 9th August 2007, 03:51 PM   #27
AndrewT is online now AndrewT  Scotland
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Quote:
Originally posted by djQUAN
I figured if the amp would survive bench testing on 3.7ohm loads then it will do 4ohm easy. besides, resistance goes up when they start heating up.
no.
an amp rated to operate into a 4ohm speaker is much more highly stressed than a 2r0 test load.
Even 3r7 is not as severe as 8ohm reactive to 60degree phase angle.

I usually check the 10degree phase angle load at HALF the lowest value of operating speaker load.
i.e 4ohm 10degree and 8ohm 60 degree for an 8ohm capable amplifier.
The 4ohm test can then be run safely for the two or at most three seconds it takes to gather the test data before switching the input/output voltages back to -20db. And then letting it cool.
This allows setting up at 1% power levels and flick the switch to 0db to check output voltage or check scope display and switch back to -20db to make further adjustments. One can see clip developing as the smoothing caps discharge in half second or so.
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Old 1st September 2007, 04:11 AM   #28
draynes is offline draynes  United States
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Here's a "transistance" electronic load that's infinitely variable. It can be jazzed up with voltage and current meters, a cooling fan, etc.

http://www.qsl.net/vu2upx/images/psu6.jpg
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File Type: jpg psu6.jpg (24.0 KB, 470 views)
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Old 1st September 2007, 08:12 PM   #29
anatech is offline anatech  Canada
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Hi draynes,
I don't think those electronic loads would appreciate an AC source like an amplifier.

HP made some really nice loads, other brands as well. Nicely metered for voltage and current. DC loads only though!

-Chris
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Old 1st September 2007, 08:56 PM   #30
draynes is offline draynes  United States
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Okay, I thought the subject was power supply dummy loads. Duh.

Anyway, I built my own audio dummy load with a large number of 24 ohm 30 watt resistors I got from a surplus dealer. I series/paralleled dozens of them with a rotary switch to give me 4, 8, and 16 ohm loads for each channel, and added a bridge rectifier and scaling resistors for power meters on each channel. It's not pretty, but it works.

In an earlier life I used to work on 75-100 watt amps on an almost daily basis - electronic sirens - and got real workout replacing burned and melted parts. Curse the invention of the 30 amp fuse!
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