Ammeter

by Dwaine

The 'Eska Electra' project, chronicled on this site, required the temporary use of an ammeter in the range of 30 amps. These are available readily for automobiles, but being the project/DIY kind of guy, I reasoned I could save myself a trip to the store by building it myself.

Credit for much of the information needed goes to Google and the many sites I read before starting this project. I also must point out that I planned to build it for many months. During this time I had the opportunity to hold onto junk that I typically would have tossed.

Firstly, lets make it clear how an ammeter works. We want to measure how many amps are traveling through a certain circuit. The ammeter is placed into circuit to be measured and must be able to safely pass at least as much current as the circuit to be measured. The ammeter contains two paths for the electricity to travel through. The majority passes through the 'shunt'. A shunt is a high amperage conductor with a known low electrical resistance. This slight resistance creates a low voltage difference between the two ends of the shunt. The second path, a voltmeter to be precise, is placed across the ends of the shunt. As more amps are pushed through the shunt, the voltmeter sees a higher and higher voltage. Presto! Instant DIY ammeter. Let's look at the specifics.

When I started the 'Eska Electra' I made sure to hold onto any voltmeters that crossed my bench. Just three were in the junk pile when the need for the ammeter could not be held off. The one with the most promise was an apparent wind angle gauge manufactured by Brookes & Gatehouse Ltd. (now known as B&G). This gauge, which I hoped to be a voltmeter, was probably 20 years old.

As the photo shows, the gauge reads from about 45 to 0 to 45. But that is wind angle and not volts. To figure out the volts to degrees ratio I put on of my multimeters on the leads and attempted to measure resistance. The multimeter does this by applying a tiny voltage. I watched the gauge move to 22 degrees. A second multimeter was enlisted to read the tiny voltage, .07 volts, across the same leads.

Ohm's Law states that resistance equals volts divided by amps. I imagined applying this law to my shunt which didn't exist yet. If 22 amps were passing through my shunt and the voltage difference was .07 volts what would the resistance of my shunt be? R=V/I=.07/22=.0032 or about .003 Ohms. I _could_ have gone looking for a store bought .003 Ohm shunt, but that would require the dreaded car trip.

I had some 10 gauge wire in the bin and found, with a quick google search, that it was approximately one Ohm per 1000 feet. Three feet would give the .003 ohms I was in search of and would be easy enough incorporate into this tool. I started with about 42 inches of wire. I then measured, stripped, bent, and tightened accordingly until I had the mess being shown by my little helper.

My test was simple enough. I dug into a different junk box and located a 65 watt headlight bulb for my wife's Subaru. Ohm's law was used to compute expected amperage based on advertised wattage. Hooking the home made ammeter and bulb in series had the needle swing up to the expected 5 amps. Groovy.