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udrc-2 and gpio pins for fan power

ynotssor@...
 

Greetings,

I have a udrc-2 on RPi3, powered by 12V to the udrc-2. Through which physical gpio pin is the RPi3 powered please, pin 2 or 4 according to "gpio readall"? Would drawing some small current from one of these affect either the udrc-2 or the pi?

The question has to do with using a 5V pin for power supply for a small 5V fan that would use switched power from the pin to the fan according to cpu temperature.

Stuart Longland VK4MSL
 

On Thu, Jun 15, 2017 at 03:04:48PM -0700, ynotssor via Groups.Io wrote:
I have a udrc-2 on RPi3, powered by 12V to the udrc-2. Through which
physical gpio pin is the RPi3 powered please, pin 2 or 4 according to
"gpio readall"? Would drawing some small current from one of these
affect either the udrc-2 or the pi?

The question has to do with using a 5V pin for power supply for a small
5V fan that would use switched power from the pin to the fan according
to cpu temperature.
I wouldn't be drawing power for a fan directly off a 5V pin. The
neighbours on that 5V bus may not appreciate the back-EMF the motor is
liable to generate even if the current draw is small.

I would suggest going a 12V fan, using a MOSFET gate to one of the spare
GPIO pins to control the fan. Then, the only thing that is likely to
get killed by back-EMF spikes is the MOSFET.

You can look on the schematics to find a spare pin that can be used for
GPIO fan control, perhaps PWM as well which means you can then control
the speed in software.
--
Stuart Longland (aka Redhatter, VK4MSL)

I haven't lost my mind...
...it's backed up on a tape somewhere.

 

First, for a dedicated radio server, I know of no apps that draw enough current to need a heatsink or fan.

But since you asked ...

On the 40 pin Header +5V is on pins 2 & 4 it is also available on the 26 pin expansion header. It’s easy to connect +5V to pin 4 and GND to pin 6 on the expansion header.

All of this is documented in the DataSheet

The UDRC-II regulator is designed for 3A continuous. The Pi takes much less than an amp and the rest is available to devices connected to USB as well as external uses.

if you search the web for pi temp controlled fan many people have done this.

To get the Pi’s core temp:

/opt/vc/bin/vcgencmd measure_temp

We typically see 40-45C for Ham appliances Max is 85C

Bryan K7UDR

On Jun 15, 2017, at 3:04 PM, ynotssor via Groups.Io <ynotssor@...> wrote:

Greetings,

I have a udrc-2 on RPi3, powered by 12V to the udrc-2. Through which physical gpio pin is the RPi3 powered please, pin 2 or 4 according to "gpio readall"? Would drawing some small current from one of these affect either the udrc-2 or the pi?

The question has to do with using a 5V pin for power supply for a small 5V fan that would use switched power from the pin to the fan according to cpu temperature.