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R1 running hot

PostPosted: Wed Jun 17, 2015 4:29 pm
by k1qn
I noticed that the big 22 ohm resistor, R1, is running hot to the touch. I know it's a 2 watt resistor, and I'm not leaving any skin on it, but just how hot is it supposed to run?

Re: R1 running hot

PostPosted: Wed Jun 17, 2015 4:45 pm
by W4MMP
k1qn wrote:I noticed that the big 22 ohm resistor, R1, is running hot to the touch. I know it's a 2 watt resistor, and I'm not leaving any skin on it, but just how hot is it supposed to run?


Hi,
VERY hot. So hot that I can not touch it but for a very brief moment. Mine run about 42°C with a somewhat inaccurate IR thermometer. The 3V and 5V regulators run even hotter.

Re: R1 running hot

PostPosted: Wed Jun 17, 2015 6:21 pm
by k1qn
The voltage drop is 4.82 volts, indicating a current of about 220ma, which is well within spec. Also indicates a dissipation of about 1 watt which is well within the 2 watt limit.

Suggestion: Instead of making a direct current measurement, measure the voltage drop across R1. (I'm an old EE Lab Instructor.)

Re: R1 running hot

PostPosted: Wed Jun 17, 2015 6:33 pm
by W4MMP
W4MMP wrote:
k1qn wrote:I noticed that the big 22 ohm resistor, R1, is running hot to the touch. I know it's a 2 watt resistor, and I'm not leaving any skin on it, but just how hot is it supposed to run?


Hi,
VERY hot. So hot that I can not touch it but for a very brief moment. Mine run about 42°C with a somewhat inaccurate IR thermometer. The 3V and 5V regulators run even hotter.


Correction: It is the two 5V regulators that run hot. The 3V does not.