Author Background: I was into RC Flight about 15 years ago and achieved a low-level degree of competency and was pulled away into other things. Now back I have focused on R/C Electric Flight and find re-entry harder than entry. My current technical skills far out weight my flight skills which allow me to take off and usually return safely to the earth and not be a nervous wreck when I am down. While serious about the hobby, I do not want to take food off of the table to get all the nice stuff right away. Hence I have developed a series of 'cheap' ways to have the needed test equipment to help me build better performing models.

The Electric Motor AMPMETER from stuff you have lying around i.e. Cheap:

What you need:

• A fuse holder & fuse (Automotive type correct size for your application).
• A multi meter (VOM/Volt OhmMeter).
• Multi meter test leads/probes with small alligator clip ends.
• A friend's amp meter or your own to calibrate your POOR MAN's AMP METER.

 How it works:

• An amp meter is simply a shunt (a device with very low resistance in the amp range that you wish to measure) with a sensitive volt meter (Milli or Micro Volt) across it to measure the voltage drop and while it is calibrated in amps it is actually reading the voltage drop across the shunt which is directly proportional to the amount of current (amps) passing through the shunt.
• In my poor man's poor excuse for an am meter, I chose a fuse for a shunt because they are made very accurately and do not seem to vary much from one fuse to the next, they are cheap, and I want one in my motor controller circuit to protect my esc and other items any way.

Setting up your POOR MAN'S AMP METER:

Illustration 1b

• Calibrating your AMP METER: You are going to need an amp meter to calibrate your POOR MAN's AMP METER setup. Of course a neat way to do this is to borrow an expensive meter from a friend. Then take readings with the expensive meter compare with readings from your home made meter and set up a table (see table 1) to translate your home made meter's readings into amps.

Illustration 1c

• Alternate method: Use your multi meter which will have a amperage meter scale to measure much lower amps than you want to measure when running your electric aircraft motor. My multi meter is a pretty good one as it goes up to 10 amps. Most are maybe 2 to 5 amps max or less. But any amperage range will work. Just pick something that will draw fewer amps than your multi meters amperage capacity. For me, I like to use my nicad charger as I can set it for a wide variety of charging rates. I hook up my multi meter in series with the charger and a battery pack and measure the amperage. When using my adjustable nicad charger, I can adjust it to a nice even reading like 3 amps (Illustration 1b). Then I connect my fuse holder and fuse in series with battery pack and charger and measure the milli volt drop across the fuse (in my test it read 15 milli volts at 3 amps, illustration 1c). Then with a table (i.e. table 1) I can calibrate POOR MAN's AMP METER.
• The Math/Calculations: The following example is made with one check point, at a low amperage (3 amps) with my multi meter and then validated with a friends expensive meter while running one of my motors. For my purposes I have found that a single point will do because the fuse is quite linear. In my actual example, test setup using my nicad charger and multi meter amp meter showed I was pulling 3 amps and when using the fuse holder & 30 amp fuse, resulting in a milli volt meter reading of 15 milli volts. Divide the amps into milli volts and answer is 5 milli volts per amp. From that, calculate the following table: (Note, I rounded off all my readings as I figured the whole system was not accurate enough to go beyond whole amps and even milli volts rounded to the nearest 5 millivolt).

Table #1

Conclusions & Exclusions etc:

• Before the flame mail pours in, this is the part where I say use at your own risk. No guarantees, promises, warrantees, or liabilities on the part of the author.
• There is great debate as to whether a onboard fuse serves any positive use.

• Yes I know this is sort of crude, but it works and is much better than no amp meter at all.
• Yes I know that 155-milli volt drop at 31 amps and an 8.4-volt flight pack is nearly a 1.5% power loss in the flight power system, but I put the fuse in anyway. Maybe some day I will be 'hot' enough pilot to notice a 1.5% power loss.
• Your results will be only as good as the care you put into getting your calibration readings and you should keep in mind this is not a replacement for high quality test equipment.
• It is a cheap method of having a amp meter that you can take to the field to check how things seem to be working especially if you need to change props etc due to the breakage of your favorite and probably the only prop you have that size.
• This method should never put your electronic equipment at risk as a properly installed fuse probably protects the system anyway.
• If you are running close to the edge of your fuse-rating etc., it could result in an unexpected blown fuse and loss of flight power at a poor time (like take off). But again, it is better than no amp meter at all. Actually much better.