Heat dissip. series vs. parallel

Each resistor is dissipating 3.6 watts - the only thing I can think of is maybe if you used smaller resistors the space wouldn't be so confined? :)

I need to fit 20 Ohms worth of power resistor as a dummy load in a confined space. In practice, is there any difference in terms of heat disspation between using two 10R in series or two 40R in parallel?

Voltage is 12V giving about 7W dissipation. I would be using 25W rated resistors just to be sure.

Thank you,

Kevin Roberts

Two 40R in parallel will cost more - not an E12 value (39R is). A failure in either one of a series pair will go open circuit.

20R is 20R for power dissipation but physical layout could matter in a confined space. Heatsinking might be helpful to keep it cool.

Regards, Martin Brown

You're stuffing 50 watts of dissipation capability in there where you need only about 15. I guess if you have room and cost doesn't matter, it's a case of "why not use a bigger wattage". Series or parallel doesn't matter. Depending on layout, you could get some advantage by physically locating the "hot spots" (where the resistors are installed) to minimize heating of nearby components. That's about the only reason to use multiple resistors in this case, where your budget and space allows selection of a 25W resistor to dissipate ~ 7 watts. Maybe 4 5W 5 ohm resistors would serve you better?

Ed

Smaller, lower dissipation resistors with more airflow might work better than cramming the space full of higher dissipation resistors -- but everything depends on the details.

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com

In addition to the other responses and at the risk of appearing nit-picky, please consider how you are going to get rid of those 7 watts. The 25 W rating on your resistors is in 'free air' that has an unlimited ability to absorb heat rise convecting off the body of your dummy load. Enclose those resistors tightly enough and you have to derate them, perhaps by a lot.

--Winston

Assuming that the resistors are otherwise identical, there would be no difference at all in long-term (steady-state) heat flow from each resistor to the ambient environment. Each resistor will be dissipating the same amount of heat.

The parallel arrangement might be slightly more reliable. If one resistor were to fail open, you'd still have half of a load present, rather than having the whole load go open-circuit. Depending on the details of your circuit, this might be safer.

--
Dave Platt                                    AE6EO
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A series chain is more likely to fail, one failing resistor takes out the whole chain.