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Topic: Can 5V from USB burn LM7805 Voltage regulator? (Read 2 times) previous topic - next topic

bratan

Dec 02, 2012, 05:59 pm Last Edit: Dec 02, 2012, 06:02 pm by bratan Reason: 1
I made a 7805 based 5V power supply for my ATmega chip. Connected LED via resistor and hooked it up to 12 VDC power supply. LED lit fine, voltage was around 5V on the out pin and everything seemed ok. I disconnected 12V supply.
Then I connected FTDI USB header to program the chip. LED lit up again (which surprised me at first, but then I realized that it provides power from USB so I don't even need to connect 12V supply to the jack), and I burned Bootloader to the chip just fine, but few minutes later I noticed that LED went off. I touched LM7805 and it was super hot, and apparently burned out.  Nothing else was damaged.
This took me by surprise, I didn't even used my 12VDC external power supply in the setup.  What did I do wrong?  Do I need put a diode between LM7805 and +5V line?
Here's a basic schematic (doesn't include ATMega, and other components)
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Grumpy_Mike

Feeding a voltage into the wrong end of the regulator is not recommended, however I have never come across a case of this burning out the regulator.

Was the 12V supply left connected up but not powerd up. That could cause a short on the input side and produce what you saw. If this is the case then a diode would prevent this from happening.

dhenry

Quote
What did I do wrong?  Do I need put a diode between LM7805 and +5V line?


Typically a reverse-connected diode between 7805's input and output.

bratan

#3
Dec 02, 2012, 06:32 pm Last Edit: Dec 02, 2012, 06:42 pm by bratan Reason: 1

Feeding a voltage into the wrong end of the regulator is not recommended, however I have never come across a case of this burning out the regulator.

Was the 12V supply left connected up but not powerd up. That could cause a short on the input side and produce what you saw. If this is the case then a diode would prevent this from happening.

Nope 12V wasn't connected at all :(
I must've wired somewhere wrong somewhere... Can't tell now...

Quote
What did I do wrong?  Do I need put a diode between LM7805 and +5V line?


Typically a reverse-connected diode between 7805's input and output.



Thanks! You mean like this?

Would it drop my output voltage noticeably?

Also I was wondering, if I'm using my own power supply, I don't really have to connect FTDI's VCC to the circuit at all right? Just the ground and RX/TX?
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retrolefty

Thanks! Would it drop my output voltage noticeably?

No, it just forces any 'reverse current' around the regulator thus protecting it.

Also I was wondering, if I'm using my own power supply, I don't really have to connect FTDI's VCC to the circuit at all right? Just the ground and RX/TX?

That is correct and probably your best choice. However I would wire two 1k ohm series resistors between the Rx and TX pins from the FTDI pins to pins Rx and Tx pins on the AVR chip so that if your supply is powered up but the USB cable is not plugged into the PC that some reverse current won't try and back into the unpowered FTDI chip.

Lefty



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