Ammeter Weirdness - Page 1
Forum section: electronics / beginners

author: wilheldp
author_rank: Regular Contributor
timestamp: July 28, 2014, 12:17:12 am
content: I am trying to measure the voltage and current when charging an electronic cigarette battery. I have the charging plug wired into one side of circuit and an e-cig plug on the other side. I have a panel ammeter installed bridging the positive terminal on the charger and e-cig, and a panel voltmeter installed across the e-cig terminals. The ammeter has a Vcc and Common terminal for external power, and the voltmeter just has a Vcc terminal (takes the common from the measured load). I have it all wired up, and the current seems to measure correctly, but I can't get the voltage to show the correct value. I did some probing around for continuity and I found a direct short between the positive terminal on the charger and the common terminal on the external panel meter power supply. I measured a spare ammeter, and this seems to be a "feature." Can anybody explain why the measured value and external common terminal would be shorted on an ammeter?

author: IanB
author_rank: Super Contributor
timestamp: July 28, 2014, 12:23:57 am
content: I would guess the external common terminal on the ammeter is going to be common with one of the current sense terminals? Unless the ammeter has sophisticated power isolation circuitry inside it must be so.

author: wilheldp
author_rank: Regular Contributor
timestamp: July 28, 2014, 12:28:42 am
content: Well, if it's common with one of the sense terminals, it's common with both of them (plus ~50 milliohms). It seems really stupid to me to tie the positive terminal of the source you are trying to measure to the common of your Vcc power supply. And I think it's causing issues trying to measure the voltage of the battery (not to mention affecting the charging circuitry that I'm trying to measure).

author: IanB
author_rank: Super Contributor
timestamp: July 28, 2014, 12:49:48 am
content: If we think about the internal circuit of the ammeter, there is going to be a current sense resistor and then an amplifier that measures the voltage difference across the resistor and scales it for display. One rail of the amplifier is going to be common with one end of the sense resistor in order to provide a return path for the voltage sense current entering the amplifier.Once you know what the polarity of the common rail is (+ve or -ve) you can put the ammeter in the appropriate side of the circuit being measured so that it doesn't cause an unintended short.Alternatively you could use independent power supplies (e.g. batteries) for each meter.

author: Asim
author_rank: Regular Contributor
timestamp: July 28, 2014, 01:11:47 am
content: connect the measuring terminals from the low side, connect yellow/white to the low side of the load and the black to the ground

author: Richard Head
author_rank: Frequent Contributor
timestamp: July 28, 2014, 08:34:22 am
content: or just quit!

author: sleemanj
author_rank: Super Contributor
timestamp: July 28, 2014, 09:26:25 am
content: Quote from: wilheldp on July 28, 2014, 12:28:42 amYou say this like it's not important. It sounds like you are measuring the current sense resistor.A meter like this is not isolated, the top of the sense resistor is sampled by the ADC, and is relative to the microcontroller (well, ADC's) ground, and therefore the "current out" is connected to the negative supply directly, and the current in is connected to the current out and the negative supply via the sense resistor.

author: wilheldp
author_rank: Regular Contributor
timestamp: July 28, 2014, 11:24:11 pm
content: Asim's suggestion fixed it. Thank you, sir.Richard Head, GFY.Sleemanj, I guess that makes sense, but why have a separate common conductor on the ammeter board if they are just tied together?

author: sleemanj
author_rank: Super Contributor
timestamp: July 29, 2014, 12:49:09 am
content: Quote from: wilheldp on July 28, 2014, 11:24:11 pmThink about current paths, and Ohm's law.Or more specifically, that you don't want to add to current flowing through the low-side of the high current path as this will offset the measurement.