# Help me measue current.May 26, 2010 7:05 AM   Subscribe

Can I install an amp meter in the positive side of a circuit?

I want to install this amp meter in my pickup. I want to keep an eye on the add-in equipment I have installed. I have grounded some of the items to the frame, so I don't have an easy way to insert this on the negative line. Here is the install guide. (PDF)

Can this work in the positive line? I can't get a good answer from the web site.
posted by Climber to Technology (7 answers total)

I don't know specifically about car wiring, but ammeters in general should work identically at any point in a loop. Just make sure that you connect the measurement (+)/(-) terminals to the shunt resistor such that the (+) terminal is at a higher potential (closer to the positive battery terminal) than the (-) terminal, so that the polarity of the current reading is correct.
posted by caaaaaam at 7:28 AM on May 26, 2010

If it's a well-designed, modern piece of electronics, it will have high-impedance differential inputs and it will work fine on the positive side. If it's a piece of crap, the two negative wires (black and blue) will be connected internally and it will bridge the shunt across your battery (equals hot shunt). If you have an ohmmeter, you could check that the inputs (green and blue) have high resistance to the other wires. My guess is that it will work for you; if you want to be sure, you could always make a test circuit with some spare wire before installing in your car (battery -> shunt -> spare headlight -> battery). Also, caaaaaam's point about polarity is important.
posted by drdanger at 7:41 AM on May 26, 2010

Different designs for current meters exist; some would do what you want, some wouldn't.

For example, some current meters have power terminals and sense terminals, but expect the sensed voltage to be within a few volts of the negative power terminal - so if you connect the sense resistor on the positive side of your circuit, then connect the sense terminals to the same power source (so the sensed voltage is within a few volts of the positive terminal, rather than the negative terminal) they may not like that.

Other current meters, however, would be just fine with what you want to do.

Unfortunately the website and install guide do not contain sufficient information to definitively say whether this current sensor would have problems or not.

If you already have the sensor, I could suggest some tests which might help you work it out. If you haven't yet purchased the sensor, you could e-mail the retailer and ask them for more details on it.
posted by Mike1024 at 7:44 AM on May 26, 2010

High-side current metering is not at all unusual, it's just a question of whether the sense inputs of the meter can tolerate being that far away from ground. (In a specification this might be listed under "common-mode input voltage range", but the installation guide you linked to isn't that detailed.) You could presumably power the meter from a separate, "floating" supply (a separate battery, eg) but that'd be a hassle.

FWIW, can you insert the shunt in the wire that connects the frame to the battery's negative terminal? My car uses the frame for the current return for lots of things, but it all eventually has to go through one wire to get back to the battery.
posted by hattifattener at 9:42 AM on May 26, 2010

It should work fine. Don't expect much in the way of precision, but as far as breaking anything, everything seems to indicate that it will work as you intend.
posted by spiderskull at 1:09 AM on May 27, 2010

I have a diesel with two batteries, so measuring via the ground from the frame to the battery won't work.

I'm thinking I will keep looking for a meter that states it will work that way. Powering the meter with a separate supply would be a hassle.

Thanks for all of the input.
posted by Climber at 11:23 AM on May 27, 2010

I don't know of any that are easily dashboard-mountable, but if you google "clamp meter" you'll find tools for measuring current which sense the magnetic field from the wire - so they don't have to be wired in, just looped around the cable.

Most of the cheap (\$20) ones have the clamp built into the display so they wouldn't go well on your dashboard, but if you just want to run an occasional test they could do the job.
posted by Mike1024 at 12:58 PM on May 28, 2010