EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: somlioy on December 27, 2012, 05:02:53 pm
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Do I need some sort of protection on the output of a L7805 when powering the circuit from the ICSP header? Likely from a STK500 or AVR Dragon.
If I leave pin 2 (power) unconnected on the ICSP header, and power the circuit from the L7805, will the programmer still work?
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I can only answer for the PIC ICD programmer, but it is likely similar for others. The PIC ICD will not program when the chip is externally powered if it does not sense a voltage on Vdd. I think it has to do with the programmer being able to set correct levels on PGC and PGD.
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Alright. How I understand this is that the AVR Dragon dosent supply 5v to my circuit because it's only powered from USB, but the STK500 does, because it has an external supply.
So how does the L7805 handle 5v input on the output, when the input is disconnected?
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7805s will not like having 5v on the output with 0v on the input. Some regulators are built to handle this but most are not.
The solution is simple. Connect a diode between input and output so that the diode will supply ~5v to the input when external power is connected. In normal operation, the diode is reverse-biased and has no effect on the circuit.
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Great, thanks for clarification.
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Small nit-pick: if you're using a STK500 or AVR Dragon, then you're using an Atmel AVR. Atmel calls their programming interface ISP (not ICSP), short for In-System Programming.