Computer acquisition of analog ammeter reading

**aliaze1** · Nov9-09, 07:47 PM

Hi,

My friends and I are doing a project that involves some very high (multiples of 10 kV and above) voltage, but very low current.

The controller for our transformer has an analog ammeter on it, but we would like to have a computer read and recored the current readings from it. We have a microcontroller that can read voltages, so I was thinking we could take the leads of this analog ammeter, and run it through a 1Ω resistor and get a voltage drop, which will have the same value as the current since the resistor is only 1Ω.

Am I on the right track?

Thanks!!

**berkeman** · Nov9-09, 08:01 PM

Originally Posted by aliaze1

Hi,

My friends and I are doing a project that involves some very high (multiples of 10 kV and above) voltage, but very low current.

The controller for our transformer has an analog ammeter on it, but we would like to have a computer read and recored the current readings from it. We have a microcontroller that can read voltages, so I was thinking we could take the leads of this analog ammeter, and run it through a 1Ω resistor and get a voltage drop, which will have the same value as the current since the resistor is only 1Ω.

Am I on the right track?

Thanks!!

As Norm Abram likes to say at the start of every New Yankee Workshop show, "Let's first talk about shop safety."

Is the ammeter measuring current in the transformer itself, or is it somehow isolated and SELV? You need a safety barrier somewhere between the high voltage and your SELV microcontroller and other circuity.

**vk6kro** · Nov9-09, 08:57 PM

Yes, you can do that, but it would be better to put the resistor in the return path of the high voltage supply.
This way, one side of the output is grounded and the other side is only slightly above ground. So, no high voltages are going to get to your expensive micro chip.

HV current.PNG

You will probably need a bigger resistor than 1 ohm and, if the current is very small, even 1000 ohms probably isn't going to make much difference to your current flow.

This will give you a voltage of 1 volt per mA of current being measured, but adjust R to suit your actual current.