Bleeder resistor question

SgtWookie

Joined Jul 17, 2007
22,230
Nice job. One somewhat glaring omission is a heat sink on the 7805 regulator. Since you're showing 15V in and you should get 5v out, more than 2/3 of the total circuit power will be dissipated in the regulator as heat.

The 7805 has a Thermal Resistance Junction-Air of 65°C/Watt; and it goes into shutdown at 125°C. So if your room temp is 25°C/77°F, that's an absolute maximum of ~1.5Watts power that it can dissipate before it will shut down; that's for a 750mW (150mA) load. Your actual mileage will be a bit worse, as the 7805 has a standby current flow of 5mA to 6mA, and you have somewhere between 5mA and 9mA flowing through your green LED. Let's just say 127mA output load will be your max before your regulator overheats.

Put a heat sink on that regulator tab.
 

Thread Starter

tracecom

Joined Apr 16, 2010
3,944
Sgt,

Thanks for your input, and you are correct, as usual. Believe it or not, when I was laying out the PCB, I thought about you, and left room for a heat sink.

Yeah, I know it's not big enough, but it's better than nothing. And yes, I have thermal paste on the tab under the sink.

So good to have you post on my thread.
 

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