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Old 10th March 2008, 07:33 PM   #21
Tarzan is offline Tarzan  Belgium
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I use the following resistors for over 10 years now without any problems.

http://be.farnell.com/1998213/passiv...HMITE-C300K8R0

RESISTOR, WW 300W 10% 8R0
Series:280;
Resistance:8R;
Tolerance, resistance :10%;
Power rating:300W;
Resistor element type:Wirewound Vitreous Enamel;
Voltage, rating:1000V ac;
Temperature coefficient :400ppm/C; Cas;

Can be cooled if the input power is high.

I have another setup with 1 Ohm resistors that can be placed in serie and/or parallel depending on the ohmic value. (from 1 to 6 ohms / 800 Watts)
Okay; it's wire wound but the windings are so large I doubt if that can be of any concern.

I frequently check the value and it's within the 10% of the nominal value. So that's ok.

Cheers,
Tarzan
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Old 11th March 2008, 12:56 PM   #22
acid_k2 is offline acid_k2  Italy
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Quote:
Originally posted by TheMG



Why do you have that parallel diode in there? The way your schematic is right now, if you turn the pot until the wiper is towards the top in reference to the diagram, you're shorting out half the output wave right through the pot and diode!

You're right.
The parallel diode was part of a voltage doubler (with a condenser between trimmer and diodes) and was left in the circuit.
About the short circuit, the trimmer must be set at half (explained in the web page in italian language, sorry ), then trimmed for the right output on the vu-meter (before it smoke)
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Old 11th March 2008, 01:11 PM   #23
luka is online now luka  Slovenia
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Hi

All depends on what you need, I have 5 electric oven heaters each ~66R that were running on 230Vac,which would be about 4kw, if I would air cooled them they could handle a lot more and if I would use water, well just to say, you don't have that big power source at home
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Old 11th March 2008, 11:26 PM   #24
fotios is offline fotios  Greece
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Default Re: dummy load

Quote:
Originally posted by reddish75
hi guys whats the best way to build a dummy load?

thanks
chris
Hi Chris

This is a dummy load that i have invented by allone because my demands was the absolute absence of any inductance. Yes, indeed it is clear resistive by 99,9%. Also it is a double dummy load in the same package. Each one it is 8 and 2000W, thus by paralleling the outgoing cables we can obtain a 4 load also. Its length it is 0,5meters. By measurements, due to large heatsink sandwitch it touch the 50deg C as much when i check the output of an amplifier at 150W for half an hour continuously. Thus its ohmic value it shift at 8,05 after it pass this time. Very good stability. For further informations at your disposal.

Fotios

Click the image to open in full size.
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Old 12th March 2008, 08:16 PM   #25
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What about using an old 4 ohm speaker coil immersed in oil or water?
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Old 12th March 2008, 10:15 PM   #26
fotios is offline fotios  Greece
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I have a legitimate query. From as much posts i have read in this thread no one takes into account that for an accurate measurement of the output of an amplifier, the dummy load must be clear resistive. Capacitances or inductances are undesirable components in the load and the measurements that we take finally are unreliable. These all paralleled ceramic resistors which i see in the photos composed from a wirewound turned around a ceramic rod before cemented in their ceramic or marble or heatsink package. Thus they are also inductive loads. Also the method of using a speaker it is the worst case. According to my knoweledge all of them. Thus a better solution from the wirewound resistors it is the series/parallel connection of many metal film (they offered also in 3W power) resistors which don't present any inductance, dipped in a vessel filled up with oil.
I used this method before the load presented in the photo of my previous post.

Fotios
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Old 13th March 2008, 08:29 AM   #27
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by EWorkshop1708
What about using an old 4 ohm speaker coil immersed in oil or water?
it could be anywhere between 2r5 and 3r5.
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Old 13th March 2008, 08:30 AM   #28
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by fotios
I have a legitimate query. From as much posts i have read in this thread no one takes into account that for an accurate measurement of the output of an amplifier, the dummy load must be clear resistive. Capacitances or inductances are undesirable components in the load and the measurements that we take finally are unreliable. These all paralleled ceramic resistors which i see in the photos composed from a wirewound turned around a ceramic rod before cemented in their ceramic or marble or heatsink package. Thus they are also inductive loads. Also the method of using a speaker it is the worst case. According to my knoweledge all of them. Thus a better solution from the wirewound resistors it is the series/parallel connection of many metal film (they offered also in 3W power) resistors which don't present any inductance, dipped in a vessel filled up with oil.
I used this method before the load presented in the photo of my previous post.

Fotios
most metal film have a spiral cut into them making them slightly inductive.
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Old 13th March 2008, 09:38 AM   #29
fotios is offline fotios  Greece
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Quote:
Originally posted by AndrewT
most metal film have a spiral cut into them making them slightly inductive.
You are absolutelly right. From mistake i have write metal film. In reality i mean metal plate, the well known non inductive MPC (Metal Plate Cemented) resistors of Futaba for example. BTW, there are many new types of non inductive resistors in TO220 package of 20W as i remember. But they are some expensive because their tolerance of 1%.
I promise to make a full description step by step, in my web page, of my dummy load by constructing a new from the zero with explanations and photos.

Fotios
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Old 13th March 2008, 09:50 AM   #30
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Default Hi !

This is old thema, but no thema without me.

Regards zeoN_Rider
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