Heat dissip. series vs. parallel
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From: Tim Wescott on 1 Apr 2010 13:38 Martin Brown wrote: > Kevin Roberts wrote: >> 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. > > 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. > 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
From: Winston on 1 Apr 2010 15:23 On 4/1/2010 3:54 AM, Kevin Roberts wrote: > 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. 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
From: Dave Platt on 1 Apr 2010 15:44 In article <4bb478cc.1076781(a)news.tpg.com.au>, Kevin Roberts <kevinroberts(a)astrotec.com> wrote: >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. 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 <dplatt(a)radagast.org> AE6EO Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior I do _not_ wish to receive unsolicited commercial email, and I will boycott any company which has the gall to send me such ads!
From: Sjouke Burry on 1 Apr 2010 21:04
Kevin Roberts wrote: > 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 A series chain is more likely to fail, one failing resistor takes out the whole chain. |