Viewpoint

Build a high power amp meter for less than $10

DougSmith


...much less if you already have a digital meter.


If you have an electric powered model, the number one thing you need to know is: How many amps does your motor pull from the battery? If you don't know this fact, chances are you're gonna burn up motors, speed controls, and batteries. Here's a handy adapter for a digital meter, good for 200 amps that you can build cheap. Most digital meters have an amps range good for only 10 amps, usually not good enough, plus the meter leads included have too much loss and give you a lower reading. With this one, you can make the leads any length with no loss at all. Technically speaking, it's called a meter shunt. What you're doing is measuring the tiny voltage drop across a length of #18 wire. You set your meter to Millivolts and it reads out in Amps. Just plug this adapter in between your battery and speed control, run your motor up to full power, and read out the current in Amps. I used a cheap meter from Harbor Freight for $4.00. If you already have a meter, all you will need is Deans plugs, banana plugs, and wire. If you have to buy a meter, you can use the banana plugs that come with it. See fig 2.
 

fig 1. Meter Shunt

The first thing you need to calibrate your meter is a Wattmeter. Borrow one, we already know you're cheap or you wouldn't be reading this far. You're actually measuring the voltage drop along a three inch length of #18 copper wire. Set your meter to the 200 millivolt position. Solder one of your meter leads directly to the Deans plug as shown in fig 1. Solder the other lead near the other end of the #18 wire. Run the motor up to speed, anything over about 10 amps and see what your meter reads. If it reads the same as the wattmeter, you're done. If your meter reads high, move the wire down the length of #18 wire and solder it again. Sooner or later you will find a place along the wire where both meters read the same. One of the laws of electronics states that the current in any part of a series circuit is the same as any other part. Once you have both meters reading identically, cover your adapter with a piece of heat shrink and you're done. Return your borrowed Wattmeter, your cheapie meter will be exactly as accurate as the one you borrowed.
 

fig 2. Here's how it all hooks up.

Your new meter has another advantage over a Wattmeter - you're not limited by the little short wires you must use with a Wattmeter. I made mine about 2 feet long, but if you prefer 20 feet it will be just as accurate because you're just measuring millivolts and the length of wire to the meter won't matter. Be careful if you get one of the $4.00 Harbor Freight meters, they have another model that sells for $10 that looks identical. You don't want the $10 one because it has a backlight and runs the battery down faster, get the $4 model. Yes I know I'm cheap but I don't care.

Doug Smith
The Duckmaster.
 


 


 


 


 


 

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