I'm working on an atmel on a breadboard, where I use a voltage regulator from 12VDC to 5VDC. The fact is that, I also program the chip while it's on the breadboard, do I need to put a diode in the main 5VDC line, when I program it using an external power ? Do the voltage regular may be damaged getting 5VDC on its output pin ?

Thanks !

  • \$\begingroup\$ It depends on the regulator you are using. What type is it? \$\endgroup\$ – AngeloQ Mar 5 '17 at 17:26
  • \$\begingroup\$ Please provide more information so we can help you. Your question title says "Arduino". But, which one? You speak of "programming the chip while it's on the breadboard", but you don't say whether you're programming it via serial or via USB (so: where's your external 5V coming from?). \$\endgroup\$ – Enric Blanco Mar 5 '17 at 17:31
  • \$\begingroup\$ I'm programming an ATMEL ATMEGA328P-PU using an Arduino Uno as ArduinoISP, so it's 5 VDC programmed using SPI as explained here arduino.cc/en/Tutorial/ArduinoISP \$\endgroup\$ – Leonardo Bernardini Mar 6 '17 at 17:06
  • \$\begingroup\$ And the regulator is an lm7805 \$\endgroup\$ – Leonardo Bernardini Mar 6 '17 at 17:33

If the output voltage of a linear regulator is more than 6-7V from the input, it may damage the pass element.

It's not uncommon to add a diode from the output to the input of a regulator to protect it in cases such as what you describe. Some regulators include such protection, but probably yours does not (e.g. something like a 7805). Take a look at Fig 27 of this datasheet for how that is wired.

You might also want a diode to protect the 5V programming source. You could use two diodes, one from the regulator and the other from the programming source, with both cathodes attached, feeding your Atmel. Then neither source will be damaged by the other.

EDIT: Added schematic of this. There would be a voltage drop from each diode so your circuit must be able to run on slightly less than 5V.


simulate this circuit – Schematic created using CircuitLab

  • \$\begingroup\$ If I'm not wrong putting them in serial implies a voltage drop so even when I want to protect the atmel inputs I should use a parallel connection right ? \$\endgroup\$ – Leonardo Bernardini Mar 6 '17 at 17:45
  • \$\begingroup\$ @LeonardoBernardini I'm not sure what you mean. Diodes will give you a voltage drop. I'll add a sketch of what I mean. \$\endgroup\$ – AngeloQ Mar 6 '17 at 18:55

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