dummy load

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I use the following resistors for over 10 years now without any problems.

http://be.farnell.com/1998213/passive-components/product.us0?sku=OHMITE-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
 
TheMG said:



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 :att'n:(explained in the web page in italian language, sorry:rolleyes: ), then trimmed for the right output on the vu-meter (before it smoke):D
 
reddish75 said:
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 8Ù and 2000W, thus by paralleling the outgoing cables we can obtain a 4Ù 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 8,05Ù after it pass this time. Very good stability. For further informations at your disposal.

Fotios

An externally hosted image should be here but it was not working when we last tested it.
 
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
 
fotios said:
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.
 
AndrewT said:
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
 
Hi !

This is old thema, but no thema without me.

Regards zeoN_Rider
 

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fotios said:
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.

Fotios


Hi fotios,

I agree, for accurate measurements on amplifiers (power, THD, ... ) you have to use a resistive load.

But (IMMO)
for test an amplifier "on the field" a resistive/inductive/capacitive dummy load is the best (and cheaper) solution. The coil of a real speaker is not a pure resistive (= optimal) load, so the same amp could have different response, or could run badly in real-life conditions.
 
www.hifisonix.com
Joined 2003
Paid Member
My post did not com e out correctly. A typical electric kettle element is about 22 Ohms (so about 2-2.2kW) on 220VAC mains.

if you wire these in parallel you get about 7.5 Ohms. If these are immersed in Water, you have a 6kW 7.5 Ohm load.

Kettle elements are cheap - get them from your local hardware store for about $15 each. $45 for a 6kW load is quite good in my view
 
acid_k2 said:
But (IMMO)
for test an amplifier "on the field" a resistive/inductive/capacitive dummy load is the best (and cheaper) solution. The coil of a real speaker is not a pure resistive (= optimal) load, so the same amp could have different response, or could run badly in real-life conditions.

Hi acid_k2

Of course we must also make these "real life" tests. To do this, and concretelly to find that the output filters of the amplifier works well i make also the following tests:
1) For estimating if the output inductor has good damping in capacitive loading i place in parallel with the resistive dummy load a capacitive load which is a polypropylene cap of 2ìF a standard value to simulate a electrostatic speaker.
2) For estimating if the zobel network has good filtering in inductive loading i place an inductive load in parallel with the resistive load which is a coil of 1mH a mid value to simulate all speakers from 5" to 18".
I make always the 3 tests separatelly to get a clear view on my scope by injecting in the input in sequence 4 square waves; 100Hz, 1KHz, 10KHz, 100KHz.
Also i must point out that the second test with the inductive load it is not so interest, on the contrary the first test with the capacitor it is indispensable.

Regards
Fotios
 
If you are a testing lab that needs to make measurements to a defined standard NI resistors are neccessary. For a DIYer or someone soak testing amplifiers after repair the inductance is irrelevant as the L of most low value resistors is quite low and will not pose a practical issue. As has already been pointed out, ANY speaker is far from a purely resistive load.
 
You could try using an element from an electric dryer. They are around 10 ohms, used at 220v, this is around 4800w at full load. To run at this power level you would need some fans to cool them but it will work. Just cut the element down to the length necessary for 8 ohms, and you could also put a tap at the 4 ohm point, or anywhere else you need.
I got this idea from AudioExpress Magazine a while back, if I remember correctly.

Peace,

Dave
 
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