# Actual resistance of an Ammeter

• andrew700andrew

#### andrew700andrew

Hi, I want to use a digital ammeter to measure the current flow though a Raspberry pi. The Pi uses about 700mA and is 5V so its got a resiatance of about 7 ohms. Therefore something like 0.5 ohm ammeter will have a large voltage drop of about 0.3V which is way too large and would lead to the Raspberry pi crashing.

The issue is that I have no clue what the resistance of an actual ammeter is and googling it doesn't turn anything up. I understand that the resistance will vary from ammeter to ammeter but is there some sort of general figure which I could use? I could just try it and see but I'd rather have a number to work with. Also, these are ammeters at school and so I don't have access to them at the moment, and there's no indication on them and no technicians know either.

Also, I have access to a multimeter too, will this have a lower resistance or be more accurate so preferable for use?

Thanks!

you could always use a .1ohm resistor in series with the circuit and measure the voltage drop, then calculate the current from there.

Put the multimeter on RX1 and use it to measure the resistance of the digital ammeter.
Be aware a good quality multimeter on RX1 might apply 150 milliamps to the digital ammeter, so set ammeter scale appropriately.
Then reverse the process and you'll learn something about both meters.

A 200 uA meter movement has a typical resistance of about 600 ohms. So full scale current will get you a voltage drop of about .12. This is the best you are likely to do without using a smaller shunt resistor and amplifying the voltage.
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Many years ago in school we had to design voltmeters and ammeters based around some movements that were laying around in the lab. It was considered very important to fully understand the spec's of voltmeters and ammeters and the loading effects on various schemes.

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Many years ago in school we had to design voltmeters and ammeters based around some movements that were laying around in the lab.
Likewise. Once it occurred me an idea I could use shunt like this one in combination with digital ammeter. Well, I could but measurement error in high current range of my device was like ± 100 %

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