Adafruit: using Multimeter

by davidh on Fri Jul 27, 2012 9:52 pm

Hi,

I found a USB charger rated at 5v @ 2.1A and wanted to test it. I wanted to see if I the settings I want to use for voltage and ampere is correct.

for voltage

IMG_0406.JPG
IMG_0406.JPG (555.92 KiB) Viewed 1375 times


and for ampere

IMG_0407.JPG
IMG_0407.JPG (558.55 KiB) Viewed 1375 times


Are these settings correct?

Cheers
David
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Re: Adafruit: using Multimeter

by franklin97355 on Fri Jul 27, 2012 11:48 pm

Those look OK to me but reading the manual would be your best information.
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Re: Adafruit: using Multimeter

by adafruit_support_bill on Sat Jul 28, 2012 4:50 am

Yes. Reading the manual is a good idea. Also, don't try to measure the current (amperes) without a load.
http://www.wikihow.com/Use-a-Multimeter
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Re: Adafruit: using Multimeter

by davidh on Sat Jul 28, 2012 4:35 pm

So, strip the leads of a usb cable, plug the adapter in the wall, and measure the output?
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Re: Adafruit: using Multimeter

by adafruit_support_bill on Sat Jul 28, 2012 6:39 pm

So, strip the leads of a usb cable, plug the adapter in the wall, and measure the output?

That is OK for measuring voltage. :!: NOT :!: for measuring current.
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Re: Adafruit: using Multimeter

by adafruit_support_mike on Sat Jul 28, 2012 10:58 pm

You need a load: something that sits between the positive and negative wires from the USB cable besides the ammeter.

An ammeter is basically a short circuit (it has very low resistance), so you want the voltage difference across the probes to be very low. Putting just an ammeter between wires whose voltage difference is 5v Would Be Bad. You need something else (the load) to fill up that 5v voltage gap while still letting all the current through.

For 2.1 amps of current at 5v, you'll have to find a load whose effective resistance is about 2.4 ohms. That will take some effort.

The simplest way would be to get 18 1/4-Watt 10-ohm resistors. Make two bundles, each with 9 resistors in parallel. Connect one end of the first bundle to the positive wire from your USB cable, and one end of your second bundle to the negative wire from your USB cable. Then connect the ammeter between the bundles.

Each bundle will be equivalent to a 2-1/4-Watt 1.1-ohm resistor, and the series resistance across both bundles will be about 2.2 ohms. 2.1 amps of current should produce 4.65v across that resistance, which is close enough to the regulator's 5v nominal voltage that it shouldn't cause any problems.
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Re: Adafruit: using Multimeter

by davidh on Sat Jul 28, 2012 11:34 pm

I'll probably need a bigger breadboard for this :D
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