Re: Any problem to have 2 sources powering the Raduino??

Gordon Gibby <ggibby@...>
 

Thanks Carl --- would it also make sense just to cut the red (+5V) line in whatever cable you are going to use to control or program a nano while in use?   In the application I foresee, the Raduino would be powered by a 7-12V supply, but computer control of the radio might be coming in over the micro-usb port of the nano (CAT emulation).


(I'm leaning toward having the power to the raduino board be constant and unchanging, because the application requires the radio to be usable with or without computer control, and tThe users of this installed system might be confused by the need to turn this or that power supply off.....)


Thanks for your thoughts,


gordon





From: BITX20@groups.io <BITX20@groups.io> on behalf of RCBoatGuy via Groups.Io <ijnfan-HamRadio@...>
Sent: Thursday, December 7, 2017 12:40 PM
To: BITX20@groups.io
Subject: Re: [BITX20] Any problem to have 2 sources powering the Raduino??
 
Small boards like the Nano are not designed to give every supply source adequate protection in order to a) keep costs down and b) keep the PCB small.  So sometimes you let the smoke out if you are not very careful with the supplies.

Thus, do to its small nature, the Nano is designed to use **one and only one** of 3 possible power sources.  Only 1 power source is actually protected by a diode, so you are only supposed to connect one supply source at a time. 

When plugged into the Raduino board, the Nano is powered externally by a regulator on the Raduino board, which also has no protection diode.  So if you put 12V on the Nano Vin supply pin, you are shorting the outputs of the Nano's regulator to the Raduino's regulator, which in turn are connected via a diode to the USB supply.  The diode on the USB supply cannot provide adequate protection from the other supplies unless diodes exist on the other supplies, so when connecting the Nano via USB to a computer, the other supplies should be disconnected.

So, if you connect 12V to the Nano Vin supply pin, you must first disconnect the Raduino's 5V supply connection to the Nano.  Even then, you should still disconnect the 12V supply when programming the Nano via the USB port (just add a switch to the 12V supply line going to the Nano).

73
Carl  K0MWC

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