Cheap DC Volt/Amp meter that measures on positive leg

[PhysicsDude55]

I have a few projects coming up where I want some of those cheap small panel mount Volt/Amp meters like these:



I have about a dozen different ones that I've ordered from China.  The problem is that they all measure current on the "negative" side, like this:



For some of my projects I want these for, there will be a common negative to many devices (automotive stuff, PC bench power supply, etc).  Does anyone know of a relatively cheap volt/amp meter that's roughly the same form factor that can measure current on the "positive leg" of the circuit?  I understand why they design them to measure off the negative, and why that's a simpler/cheaper design, but wanted to see if anyone knew of some different models that measure differently.

Thanks!

[bob91343]

You probably need two units instead of one.

[Ian.M]

To do high side current sensing, one needs either an isolated sensor like Allegro's range of ACS... Hall effect current sensing ICs, or an 'over the top' OPAMP + precision matched resistors to implement a differential amplifier, with sufficient gain to keep the shunt burden voltage low enough, but also with excellent common mode rejection.  Either option significantly increases component costs, so no cheap meters.  Low side sensing doesn't have the same problems as there's no common mode voltage to reject, so no need for precision matched resistors, and any 'jellybean' OPAMP with a common mode range including its negative supply is suitable to amplify the shunt voltage to a suitable range for the ADC.

[ledtester]

You might see if this (5-year old) video is applicable to your displays:



It is quite likely that the design mentioned in the video is not available anymore.

[Ian.M]

Unfortunately,without a datasheet for the differential ADC + PGA IC, there's no way of telling whether or not its inputs have an 'over the top' common mode range.  If it doesn't odds are taking them outside the rails (which is only 0V to 3.3V if it shares Vcc with a 3.3V MCU) will let the magic smoke out.  Also current measurement accuracy may be compromised before it reaches 3.3V common mode.

One approach that would work even if the modules no longer use that circuit, would be to remove the shunt, and build an over the top capable unit gain differential amplifier to allow the shunt to be inserted in an arbitrary positive rail, and translate the  differential voltage across it down to an equal zero referenced voltage that can be injected at the load terminal of where the shunt was.   Its differential rejection could be hand trimmed, with lets say a 1mA test current through the shunt, while varying the rail voltage, so it could be built with only 0.1% resistors.