sci.electronics.basics
[Top] [All Lists]

## Re: Resistors getting hot

 Subject: Re: Resistors getting hot John Larkin Wed, 19 Apr 2006 07:55:38 -0700 sci.electronics.basics
 ```On 19 Apr 2006 04:33:42 -0700, "MarkMc" wrote: >Hi I'm a bit of a novice, and I just tested the circuit below on a >breadboard, and I find that the RL and R2 resistors are getting >incredibly hot. They are 0.25W rated. I measured the current through >R2 to be 20mA, and voltage across R2 was 10v. > >Using P = IV, I make that 0.2W, and using P = I^2R, I get 0.19W, both >in spec as far as I can see. I wonder if I'm doing something else >wrong. > >RL will eventually be a relay, but on the breadboard, I've made this >470R and an LED. > > > > > > +-------+----+-------------- > | | | +12v > .-. .-.10k| > | | | | | > RL| | | | | > '-' ___ '-' |< > +-|___|-+ -| Q2 > 6.8k | 10k |\ > ___ |/ | > On/Off Sig o-|___|-+ -| Q1 | > .-. |> .-. > | | | | |R2 > 10k| | | | | > '-' | '-' > | | | > | | | > === === LED V -> > GND GND - > | > | > === > GND > > > > Q1 = BC107B (NPN) > Q2 = BC177B (PNP) > RL = 470R > R2 = 470R > > RL and R2=470 both 0.25W both get very hot >(created by AACircuit v1.28.5 beta 02/06/05 www.tech-chat.de) > > >Regards, >Mark Get rid of RL - it serves no function now - and it won't get hot. Resistors usually get hot, too hot to touch, at rated power. It's common to derate then maybe 2:1 to improve reliability and avoid discoloring PC boards longterm. John ```
 Current Thread Re: Resistors getting hot, (continued) Re: Resistors getting hot, MarkMc Re: Resistors getting hot, John Fields Re: Resistors getting hot, MarkMc Re: Resistors getting hot, James T. White Re: Resistors getting hot, Jason S Re: Resistors getting hot, MarkMc Re: Resistors getting hot, ehsjr Re: Resistors getting hot, MarkMc Re: Resistors getting hot, ehsjr Re: Resistors getting hot, MarkMc Re: Resistors getting hot, John Larkin <= Re: Resistors getting hot, John Fields Re: Resistors getting hot, Jonathan Kirwan