1. Jan 7, 2014

### electricfreak

Hey guys. I need a resistive dummy load that can handle 2.5 W, but I need it to be much smaller than a 2.5 W resistor. I have looked for different ideas, but I need it to be simple like a resistor and smaller than a 2.5 W resistor.

Any ideas?

2. Jan 7, 2014

### maajdl

A "2.5 W resistor" doesn't make a lot of sense.
A resistor is described by its resistance.
Eventually, its capacity to sustain so heat dissipation (like 2.5 W) is a practical information.
I am quite sure that some very tiny resistor can dissipate 2.5 W and more.
I am thinking for example to the filament of a old-fashion flashlight.

3. Jan 7, 2014

### the_emi_guy

Huh?

The "tiny" resistors we use (0603, 0402) are only rated in the 1/10W to 1/20W range. Even in the old days 1/4 watt resistors were most common. A 3W resistor is a big resistor.

OP: how small do you need? Any air flow available? How stable does the resistance need to be? Any object nearby you can heatsink it to?

Last edited: Jan 7, 2014
4. Jan 7, 2014

### davenn

and added to the emi guy's questions

what is the peak power likely to be supplied to the dummy load
what is the average power likely to be supplied to the dummy load
is it continuous ?
is it pulsed ?
if pulsed ... what is the duty cycle, on time/ off time ratio
That will decide what wattage rating the resistor would need to be

but some other basic info from you mite also help....

is this audio or RF ?

cheers
Dave

5. Jan 7, 2014

### Baluncore

What resistance, 50R ?

If you use cheap lossy coax, (such as RG58), it will absorb much of the energy before it gets to the dummy load, which can be placed where there is more space and cooling available.

If you distribute the power into several small resistors you will dissipate heat more efficiently.

Two 100R, 1W in parallel gives you 50R, 2W.
Three 150R, 1W in parallel gives you 50R, 3W.

Four 100R, 0.25W in parallel gives you 25R, 1W.
Two 4 x 25R, 0.25W in series gives you 50R, 2W.

Six 150R, 0.25W in parallel gives you 25R, 1.5W.
Two 6 x 25R, 1.5W in series gives you 50R, 3W.

6. Jan 7, 2014

### davenn

who knows LOL
hopefully he will come back and answer all the questions

we don't even know if its audio RF or something else ????

Dave

7. Jan 8, 2014

### meBigGuy

The power rating of a resistor is based on its maximum permissible temperature and its thermal conductivity to the ambient air. The only way to make a smaller resistor withstand higher power dissipation is to improve its thermal conductivity or lower the ambient temperature.

For example, if the physical situation is such that you can mount a 1/2 watt resistor to a metal surface of some type with thermally conductive epoxy (creating a heat sink), maybe you can keep the temperature rise low enough.

I don't know about the thermal conductivity from the core of the resistor to the surface, and what type of resistor is best from that viewpoint.

8. Jan 8, 2014

### Baluncore

Everyone has to start somewhere.

In the air cooled situation it is better to generate heat on the surface of a leaf than at the centre of a cylinder or sphere.

9. Jan 8, 2014

### RocketSci5KN

The only way to make a 'smaller' 2.5W rated resistor is to dramatically increase either the airflow across it (forced airflow), or some form of liquid cooling, direct or through a metal interface. Heatpipes would also transfer the heat elsewhere, but I doubt that's what you're after. The only other option is to find some resistive material that can operate at very high temps, say 200C - 500C, which would greatly increase its ability for the 2.5W to be transferred to the air via convection. Of course, safety issues are a problem here... The wattage ratings on standard resistors are largely based on their maximum permissible operating temps.