User avatar
HawaiianPi
Posts: 4750
Joined: Mon Apr 08, 2013 4:53 am
Location: Aloha, Oregon USA

Re: How to measure current draw of the RPi

Thu Oct 12, 2017 8:41 am

Forgot to post links for the meters I use.

DROK USB 2.0 Digital Multimeter, Ammeter Voltmeter Capacitance and Watt Meter, 7 Modes Multi Tester
Amazon US: https://www.amazon.com/dp/B00J3JSEG6
And this looks like the same meter on Amazon UK: https://www.amazon.co.uk/dp/B00J4H5I3E

Note that there is a version without the extra modes or button (only measures V & A) and Amazon in the USA got tripped up and had the 7 mode description on the other meter. So if you order the 7 mode meter and get one without the selector button, send it back. It seems that the other version is no longer available on Amazon USA, but it's still on the UK Amazon site (at a higher price).

I also have one of these: https://www.amazon.com/dp/B01L8QNG6Y
Which appears to be identical to the Drok at a lower price (when I bought it, anyway).

And one of these: https://www.amazon.com/dp/B01H1UW9H8
Which shows Volts, Amps, mAh and time (H:M) all at once. Nice, but the smaller numbers are harder for my aging eyes to read. It was much cheaper too (this is the one I got for less than $5, but it's a bit higher now).
Last edited by HawaiianPi on Fri Oct 13, 2017 3:32 pm, edited 1 time in total.
My mind is like a browser. 27 tabs are open, 9 aren't responding,
lots of pop-ups...and where is that annoying music coming from?

Milliways
Posts: 465
Joined: Fri Apr 25, 2014 12:18 am
Location: Sydney, Australia

Re: How to measure current draw of the RPi

Thu Oct 12, 2017 10:19 am

This thread, like so many similar discussions, is full of half-truths and misinformation.

If you REALLY want to measure the current drawn by the Pi it is simple, just insert an ammeter in series with the power feed. There are inexpensive commercial devices which can be inserted in the USB line to display the current and voltage. These simple approaches all suffer from a drawback - they introduce a voltage drop, typically 200mV.

It is also POINTLESS! What do you expect to do with the reading? To do anything meaningful you would need to capture the peak current as well as average (this is a little harder).

An engineer would approach this from a different direction calculating the worst case from data sheets, possibly augmented by readings. The Foundation has presumably done this, and recommendations based on this are published https://www.raspberrypi.org/help/faqs/#powerReqs These show that ALL models will operate on less then 1000mA - I routinely operate all model with a USB keyboard and mouse on a 1A supply. The Foundation recommendations for "Recommended PSU current capacity" make a generous allowance for other peripherals.

The problems so many people have with power is due to inadequate power supplies (including the "official" supplies, although they are better than most) and/or poor cables. The effort so many people put into worrying about the Pi would be better spent on testing power supplies.

Paul Hutch
Posts: 397
Joined: Fri Aug 25, 2017 2:58 pm
Location: Blackstone River Valley, MA, USA
Contact: Website

Re: How to measure current draw of the RPi

Thu Oct 12, 2017 10:35 am

Be careful when using those inline USB power meters. They can only show the voltage into the cable but for any significant length (10 cm+) there will be a significant voltage drop.

Here's a Google Sheet that calculates the voltage losses for varying cable lengths and gauge.
https://docs.google.com/spreadsheets/d/ ... sp=sharing
The conditional color formatting is a rough estimate showing green for almost definitely OK, red almost definitely bad and plain for depends on the voltage output of the USB power adapter.

Use a voltmeter connected directly to the Pi's header to see what voltage the Pi is actually getting.
Last edited by Paul Hutch on Thu Oct 12, 2017 2:06 pm, edited 3 times in total.

Massi
Posts: 1691
Joined: Fri May 02, 2014 1:52 pm
Location: Italy

Re: How to measure current draw of the RPi

Thu Oct 12, 2017 11:54 am

well, you can put a ina219 sensor between the power source and the pi
so the pi will monitor the pi
that's fun, i did it.. but you should take care of a couple of things:
- since there will be a power drop, it would be better to change the resistor inside on the sensor board with a smaller one (by a factor of 10, at least)
- if you want to build a "full usable" power meter, you MUST take care of the quality of connectors. Chinese u-usb connectors will drive poor current with huge voltage drops.

but yes, it's nice :)

jamesh
Raspberry Pi Engineer & Forum Moderator
Raspberry Pi Engineer & Forum Moderator
Posts: 23891
Joined: Sat Jul 30, 2011 7:41 pm

Re: How to measure current draw of the RPi

Thu Oct 12, 2017 2:08 pm

Something like this...?

https://www.amazon.co.uk/Eventek-Adjust ... wer+supply

Set voltage, tells you the current draw. It's similar what I use.
Principal Software Engineer at Raspberry Pi (Trading) Ltd.
Contrary to popular belief, humorous signatures are allowed. Here's an example...
“I think it’s wrong that only one company makes the game Monopoly.” – Steven Wright

User avatar
FTrevorGowen
Forum Moderator
Forum Moderator
Posts: 5108
Joined: Mon Mar 04, 2013 6:12 pm
Location: Bristol, U.K.
Contact: Website

Re: How to measure current draw of the RPi

Thu Oct 12, 2017 3:10 pm

Milliways wrote: ...
The problems so many people have with power is due to inadequate power supplies (including the "official" supplies, although they are better than most) and/or poor cables. The effort so many people put into worrying about the Pi would be better spent on testing power supplies.
Something I initially investigated prior to receiving my first Pi (I had to wait for one from the "2nd. batch" IIRC) and have continued to do so:
http://www.cpmspectrepi.uk/raspberry_pi ... plies.html
The most common failure is to not understand that a charger is not a power supply and, whilst both devices may be labeled with a similar "specification" they function differently. (In particular a 5V x-amp charger does not have to provide both 5V and x-amps at the same time - to do so could lead to excessive heating of the device being charged)
Trev.
Still running Raspbian Jessie or Stretch on some older Pi's (an A, B1, B2, B+, P2B, 3xP0, P0W, 2xP3A+, P3B+, P3B, B+, A+ and a B2) but Buster on the P4B's. See: https://www.cpmspectrepi.uk/raspberry_pi/raspiidx.htm

MarkTF
Posts: 300
Joined: Tue Mar 03, 2015 4:59 pm

Re: How to measure current draw of the RPi

Thu Oct 12, 2017 5:15 pm

Martin Frezman wrote:
Tue Oct 10, 2017 1:47 pm
Of all the ideas presented so far, I think the "USB Power Meter" idea is probably going to be the most workable.

Having looked at them on Amazon, the thing that is curious is the difference in price between seemingly (more or less) identical devices. For example, the one at the link posted earlier (at amazon.uk) was 74 pounds (i.e., something like $100) (wow!). On amazon.com (i.e., in the US), there are a couple of them at about $11 (which is probably about the same as the 6 pound figure quoted above), but then there is one that is $58 and others even higher than that. Yet, they all look about the same - hard to tell from the text if there is any real difference between them.
Better quality USB power meters have a smaller series resistance (measuring the voltage drop across this gives current) and an amplifier to get that voltage to proper levels for the embedded microcontroller that reads the voltage and calculates current. The cheapest ones go directly to an A/D input hence either sacrifice sensitivity to low currents or require a larger resistance.

Searching for a particular device may yield a review. There are quite a few at this site: http://www.lygte-info.dk/info/indexUSB%20UK.html

Return to “General discussion”