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REQ: How to make Resistive Dummy Load?

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happy123

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how to build a dummy load

I found from google on a circuit that consist of 60 * 1500 ohm resistors to make a dummy load.

I wondering is this load - resistive enough. I need this to tune a ATU, and therefore the load resistive nature is important.

Any comment is welcome. Thanks.
 

flatulent

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inductance

The reason many small power resistors are in parallel is the inductance. The inductance of a resistor goes up with the power rating. by using low power resistors the inductance per resistor is smaller. Then when they are placed in parallel the total inductance is even smaller.
 

webdog

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Please share with us what frequency you want to operate at and what power level???
Using parallell coupled standard type resistors do work up to aprox 30 MHz.

Not sure what impedance you are looking for, but having all 60 in parallell should give you 25 ohm! (1500/60=25)

It is important that you choose non-inductant resistors i.e. not wire-wounded types, or metal film.

The lower the frequency the easier it gets.

/WD
 

g579

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webdog,

I think you are wrong. Parallel resistors can make good matched loads up to 10 GHz if you properly design it and use small size resistors (eg. 0603).
g579
 

webdog

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g579
You do have a point... using small SMD resistors will work up well into the microwaves, but reading the orginal post, he intended to use 60 pcs!
Even using small SMD resistors will only help so much, the stray inductance and capacitance will prevent it from working at higher than UHF.

It would be interesting thou if someone did try make a try and see what is really possible with say 40-60 resistors! One should take into consideration what power dissapation you get too, it might be better to shoot for fewer but higher rated (i.e. larger 0805 or 1206) rather than a massive amount of smaller ones.

Cheers
/WD
 

VSWR

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What is the maximum frequency you will use? What is the maximum power level the dummy load shall be able to handle? Duration?

For frequencies up to 30 MHz I built a dummy load consisting of 10 pcs of 470 ohm 2 W carbon type resistors in parallel. I mounted the assembly in a one litre tin can filled with transformer oil.
The result was excellent: VSWR <1.1 @ 30 MHz.
The dummy load can easily handle 100 W for an hour or so due to the effective cooling of the oil.

http://www.qsl.net/iz7ath/web/02_brew/15_lab/03_dummy/pag01_eng.htm

http://www.qrpp-i.com/kit_60-Watt_Dummy_Load.htm
 

klug

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For frequencies up to 30 MHz I built a dummy load consisting of 10 pcs of 470 ohm 2 W carbon type resistors in parallel. I mounted the assembly in a one litre tin can filled with transformer oil.
If you need to make it quickly, you can use resistors sunken in a glass with clean water.
 

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