The “Cheapo” Ammeter

cheapo-ammeter-002Figure 1: The “cheapo” ammeter for keeping tabs on SoC/SBC power usage. Click to enlarge

I have a number of SoC/SBC based “homemade tablets” and “homemade server” boxes that consume anywhere from 300 mA to several amps at 5 VDC.  I wanted an easy way to check the power consumption under various load scenarios (GPU=on versus GPU=off, WiFi=on versus WiFi=off, eight cores versus four cores versus one core, etc).

I had been connecting my volt-ohmeter-ammeter occasionally to do this task, but that’s a) a hassle, b) ugly, and c) consumes the usage of one of my testing tools.  In figure 1 is shown my #2 “homemade” tablet – which is a Pi2 powered device.  Hutch my dog was adorning the screen area of the device at the moment the pic was snapped.

Remember ArcherKit?  I remember one such ArcherKit science board full of dozens of coil-springy connectors and an assortment of gadgets.  I think an ammeter was included in the bunch.  Well, I managed to hook a nice supplier for these things (BGMicro.com) – who is selling the 5 ampere meter at just over $2.  Wow.  The included 2 watt shunt resistors are worth that!

Now, I know that it may not survive frequent bashings (although I’ve already dropped it on the floor once and it survived).  Also, the 5 ampere range is just a little too much for typical SoC/SBC current levels.  Figure 1 shows about 425 mA coming from the 12VDC power supply that is feeding the tablet (the tablet has a built-in DC-DC converter). The reason for the converter is that the tablet also contains a battery charger, and additionally I like the “protection/isolation” provided by the converter.

So, (425 mA * (12.5V/5.0V)) is just over 1A, which would push the needle a little further upscale if I were using a 5V supply.  I have a high powered Odroid XU4 that will push the meter up closer to mid scale, thus I’ll probably not make any adjustments to the meter setup.

Adding up my totals, I get:

  • 2 Two-dot-one mm power connectors x .25 = $.50
  • 1 Plastic box = $4.00
  • 1 Five ampere analog meter = $2.35
  • 1 Off/On switch = $.69
  • 1 Reverse voltage protection diode = $.10

Total = *** $7.64 ***

What a deal.  I know I could have purchased a digital in-line ammeter for about the same money, but there’s something nice about a swinging analog meter needle.

I purchased a couple of the meters, so I can leave them inline for full-time monitoring.  The tablets all have ADC converters, so I suppose I could set up a current meter inside of each device, and run a little app to display the values all of the time.  Or maybe it’s more convenient to have the outboard meter, which can read the level of current when the Pi is powered, but not running an operating system.

BTW: Fingernail polish for the “I” input and “O” output labels was perhaps not the best choice.

Note: the author does not have a recent, applicable background in circuit building, or battery related issues, so this is presented as the work of a hobbyist, and is not meant for duplication by others. Readers should look elsewhere for design advice and info.

Note: This author and site is not affiliated with the Raspberry Pi in any way. For information about those projects visit  http://www.raspberrypi.org. “Raspberry Pi” is a trademark of the Raspberry Pi Foundation. ArcherKit is a product likely trademarked by RadioShack Corporation (now operating as General Wireless Operations Inc. dba RadioShack). Neither company is affiliated with this author or site in any way.

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