Re: How hot is too hot? #ubitx

Jerry Gaffke

My Raduino with display draws 0.100 Amps.
Voltage drop across your 47 ohm resistor would be 47ohms * 0.1amps = 4.7 volts.
Assuming a 12v supply, that leaves 12-4.7 = 7.3 volts at the LM7805,
which gives it the required headroom of 2 volts over the 5 volt output of the LM7805.
Power consumed by the resistor would be 0.1amps * 4.7volts = 0.47 Watt.

How hot is your 1/4 Watt resistor?
I'd suggest at least a 1/2 Watt resistor if sticking with 47 ohms.
Or a 25 ohm 1/4 Watt (better, 1/2 Watt) resistor, then the LM7805 will stay in regulation
down to a power supply of  5.0v+7.0v + 25ohms*0.1amps  = 9.5 volts.

The LM7805 can get hot enough to sizzle spit and still not be breathing hard:

But the IRF510's while also rated for a 150 degrees C junction temp, should never
have tabs so hot that you can't keep your finger on them:


On Wed, Jun 13, 2018 at 08:54 pm, Skip Davis wrote:
Bo I found the same thing what I did was to add a 47 ohm resistor in the 12vdc into the regulator. I added it on the Raduino board and now I can hold my finger on the regulator without burning my finger. I just cut the trace from pin 15&16 on the Raduino and wired in a 1/4 watt resistor.

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