On 19 Apr 2006 04:33:42 -0700, "MarkMc" <mmcnospam@xxxxxxxxxxx> 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
>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 -
> 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)
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.