sgarst
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Wall power for RPi, Servo

Mon Mar 30, 2015 3:12 pm

I'm trying to figure out a stable, safe and low-cost means to provide power for a raspberry PI and a small servo. Almost everything I read talks about providing power for the servo separately, and always recommends batteries. But I really need a way to power my device with one wall plug (US), that presumably provides 3V for the PI, and 5V for the servo. My only thought is a power strip with two wall warts... but oh goodness this seems wrong.

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joan
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Re: Wall power for RPi, Servo

Mon Mar 30, 2015 3:20 pm

The Pi needs 5V unless you are powering it in an unconventional way.

I feed 5V into a breadboard. I use the breadboard power rail to power both the Pi (via the expansion header) and servos. There is no reason you can't do the same, electrically if not physically.

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B.Goode
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Re: Wall power for RPi, Servo

Mon Mar 30, 2015 3:23 pm

sgarst wrote:I really need a way to power my device with one wall plug (US), that presumably provides 3V for the PI
I would expect the RPi to fail to boot if supplied with 3v power. The usual spec quoted is 5volts to within 5%. (ie 4.75 to 5.25v) Perhaps that was a typing error?

In the UK it is not uncommon to find mains powered usb adaptors that have 2 5v usb connectors. Obviously you would need to check that the adaptor can supply the current demanded by the servos.

sgarst
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Re: Wall power for RPi, Servo

Mon Mar 30, 2015 3:47 pm

Ack! Yes, my mistake. I'm trying to find a wall plug power supply for the RPi (5V) and servo (5V). I keep seeing warnings about supplying separate power for the servo, and indeed I have been having problems powering my tiny servo from the Pi itself. I guess I'll just try a USB hub (one of these: http://elinux.org/RPi_Powered_USB_Hubs).

scotty101
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Re: Wall power for RPi, Servo

Mon Mar 30, 2015 4:26 pm

You need to supply the Raspberry Pi with a stable 5v. This can be supplied over the MicroUSB connector or through the GPIO connector.

If your wall power supply can provide a stable 5v for the Raspberry Pi and for the Servos then power the Pi from the GPIO connector and filter the 5v to cope with any sudden power surges from the Servos.

When I was developing an Arduino robot, I found that the servos could draw so much power that the Arduino would reset. I managed to fix this though by putting a 220uF Capacitor between 5v and ground.
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Pi Interests: Home Automation, IOT, Python and Tkinter

ggerard
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Re: Wall power for RPi, Servo

Sun Apr 12, 2015 6:28 pm

I'm also interested on this question, could you please expand on your answer?
joan wrote:The Pi needs 5V unless you are powering it in an unconventional way.

I feed 5V into a breadboard. I use the breadboard power rail to power both the Pi (via the expansion header) and servos. There is no reason you can't do the same, electrically if not physically.
The problem as I think to see is that if you use a 5v psu, split into two and power from there the rpi and the servos, the servos will drop the voltage for the rpi shutting it down. But I'd have said that the servos can draw intensity, not voltage, so that you could solve that by using a 5v 10a psu for example. I'm wrong then (but why?).

My point is the same as the topic (I'd say), how to use one single power supply for both the rpi and 2 servos? From my electrical ignorance, it seems not logical that you have 220v in the wall and you still have to use 2 psu to power these 2 devices.

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joan
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Re: Wall power for RPi, Servo

Sun Apr 12, 2015 7:35 pm

ggerard wrote:I'm also interested on this question, could you please expand on your answer?
...
My 5V supply can provide up to 4 amps.

The servo(s) and Pi and other stuff on the breadboard take less than 4 amps. Result happiness.

If the servo(s) and Pi and other stuff took more than 4 amps. Result misery.

ggerard
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Re: Wall power for RPi, Servo

Mon Apr 13, 2015 12:55 pm

From the breadboard to the rpi, how do you do the connection? a cut micro-usb cable?
joan wrote:
ggerard wrote:I'm also interested on this question, could you please expand on your answer?
...
My 5V supply can provide up to 4 amps.

The servo(s) and Pi and other stuff on the breadboard take less than 4 amps. Result happiness.

If the servo(s) and Pi and other stuff took more than 4 amps. Result misery.

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joan
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Re: Wall power for RPi, Servo

Fri Apr 17, 2015 8:48 am

ggerard wrote:From the breadboard to the rpi, how do you do the connection? a cut micro-usb cable?
...
I just connect the breadboard ground and 5V rails to pins 6 and 4 respectively on the Pi's expansion header. It bypasses the polyfuse protection so you could add your own fuse in-line if you think it might be useful (I don't).

LinuxCircle
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Re: Wall power for RPi, Servo

Thu May 14, 2015 3:58 am

Use 9V wall adapter, then use UBEC to convert it to 5V 5A.

Best practice and result is to separate Pi and Servos power source by using 2 power supplies, e.g. 2 adapters or 1 adapter + 1 battery.

You could use the same power supply, but do not power servos from GPIO 5V because it has limited current, 200~300 mA at best. If you want to connect, you need to branch them up using a breadboard or Y connector. Otherwise if there is a jump in the current, the Pi will shut down inappropriately.

Better still, use Raspy Juice board which will convert the original power source (wall adapter) into a safe 5V for the servos as well as the Pi: http://www.linuxcircle.com/2013/03/19/r ... ontroller/
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