EEVblog Electronics Community Forum

Products => Test Equipment => Topic started by: ballsystemlord on October 26, 2023, 03:15:49 am

Title: How do I get the resitance of the DCA ranges?
Post by: ballsystemlord on October 26, 2023, 03:15:49 am

Hello,
Often higher end meters specify the burden voltage of the amps range. For example, 100mV/mA would give us a 100ohm resistor. But some meters only list the voltage drop as a < X.XXX number. Like the attached image.

How do I get/approximate the resistance of the meter's amperage ranges?

Thanks!
[attachimg=1]

Title: Re: How do I get the resitance of the DCA ranges?
Post by: bdunham7 on October 26, 2023, 03:49:11 am

Ohms law, R=V/I.  So the 10A range states "<0.5V", 0.5/10 = 0.05, that implies the resistance is 0.05 ohms or less.  The total resistance includes more than the shunt--there's the internal circuitry and the fuse.  That range likely has a 0.01R shunt.

Title: Re: How do I get the resitance of the DCA ranges?
Post by: ballsystemlord on October 26, 2023, 04:25:04 am

I figured ohms law was the way to go, but 11mV burden on 1nA seemed a bit high.

Title: Re: How do I get the resitance of the DCA ranges?
Post by: bdunham7 on October 26, 2023, 04:31:23 am

Quote from: ballsystemlord on October 26, 2023, 04:25:04 am

I figured ohms law was the way to go, but 11mV burden on 1nA seemed a bit high.

The burden voltage shown is the maximum at full range.  At partial range readings, it should be proportionately less if it is using a regular shunt system.  If it is an active system, then the input can't be characterized as a resistance.  This meter likely uses a 100R shunt for the low current ranges and a gain amplifier for the very lowest.