I've known for a while that there is a usable voltage (around 4.3V) on the Vin pin when powering the Uno or Duemilanove solely from USB.
I'm trying to understand why there would be a voltage at Vin when connecting these boards to only USB.
After studying the schematics, the only explanation I've come up with is that the voltage regulator on the Uno/Duemilanove (MC33269) allows "backflow" when Vo > Vi.
The voltage at Vin measures around 0.7V less than Vcc, so this seems to make sense. I have also noticed that the Vin voltage drops by a couple more tenths under small ( < 10mA ) loads, which also makes sense under my working theory.
The reason I care about this is because I want to drive off-board loads, Crydom D2425 SSR's for instance, with Vin. I prefer not to use Vcc so I can avoid loading up the 5V regulator. I would like the same circuitry to work whether on USB power or external power.
My question is whether or not using Vin as described above is OK to do with the Uno/Duemilanove boards on USB power?
Jim
After studying the schematics, the only explanation I've come up with is that the voltage regulator on the Uno/Duemilanove (MC33269) allows "backflow" when Vo > Vi
Correct. In general, linear regulators are not expecting Vo > Vi and can actually be damaged if significant current flows backwards.
My question is whether or not using Vin as described above is OK to do with the Uno/Duemilanove boards on USB power?
I would not do that. There is a significant risk of damaging your regulator. You could always put a diode from 5V to Vin to "go around" the regulator (Schottky diode is recommended).
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The Gadget Shield: accelerometer, RGB LED, IR transmit/receive, speaker, microphone, light sensor, potentiometer, pushbuttons
I would not do that. There is a significant risk of damaging your regulator. You could always put a diode from 5V to Vin to "go around" the regulator (Schottky diode is recommended).
That is a fact. I've seen manufacture example applications schematics using such a diode around the regulator to protect the device in situations where there is lots of capacitance on the regulator output/load side that could cause damage to the regulator when powering down the input side and allowing reverse current flow through the regulator.
RuggedCircuits:
You could always put a diode from 5V to Vin to "go around" the regulator (Schottky diode is recommended).
Done. Added MBR0530LT to schematic. Thanks.
Jim