Posts: 3
Joined: Mon Nov 05, 2012 12:26 am

Suitable voltage variation for switching mode power supplies

Mon Nov 05, 2012 1:12 am

Hello folks,

New user here. This question may be somewhat off-topic, but I failed to find out a better forum to post it. Also, I could not find another topic about this (but I think it already exists).

What is the normal/acceptable voltage variation for a good switching mode power supply under a given load? I'm using a Kindle 3 charger as power supply for my RPi and, before trying to switch to a Chinese no-brand 1A source, I though it was a good idea to check the voltage output in a workbench. All measures were taken with a simple multimeter, I have no oscilloscope to check for the ripple and other AC noise.

A Kindle charger (rated 5V, 850mA max) with no load measured 4,9V. When powering some resistors and LEDs totaling a load of 85mA, output voltage dropped to 4,84V.

One no-brand supply (rated 5V, 1A) gave 5,14V with no load and 4,94V under 85mA. Another one (rated 5,2V 1,5A) gave 5,5V with no load and 4,87V with 85mA.

I will continue to use the Kindle charger for my RPi but I', really curious about the reliability of such products. What is the normally accepted variation for these off-the-shelf power sources?

Posts: 283
Joined: Wed May 23, 2012 6:48 pm

Re: Suitable voltage variation for switching mode power supp

Mon Nov 05, 2012 3:44 am

You need to check the voltage from TP1 to TP2 on the Pi while running under full load to know if your power supply is acceptable, ideally you want 4.85+ volts.

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