What kind of device is needed to measure current draw from a motor? I've spent enough money on electrics to warrent learning in more detail how this stuff works. :]

Thanks for any help!

Rob

It's called an ammeter. It is placed in series with the circuit path to be measured.

It requires a very good ameter to measure currents that our servos use. most analog meters will measure a steady state current reasonably well but; our servos draw current in big gulps followed by a lesser holding current. About the only way to see this is to use a recording ameter or an oscilloscope. Even using the oscilloscope needs some finess to measure peak currents and not have the measuring device effect the accuracy of the readings, i.e. most methods require inserting a finite resistance in series with the load being measured and this will lower the current drain the device would see otherwise. Of course almost any method of measurement are usually better than none and you can, at the very least, pick up relative changes under varying conditions. Just don't trust a normal ameter to give you true data unless you know how to adjust for the added impedance.

You are correct, but Shoruk3n didn't ask about measuring a servo... He wants to measure a motor (electric powered flier). If he measures it with a prop on the motor, he will be able to measure the current during maximum load while the plane is on the ground at full throttle.

Thanks guys! At this point I'm only concerned with measuring the motor draw, but later when I have more time and money I'll do the rest.

Can an ammeter be purchased at a typical hardware store? Something like Orchard? Or is it something I'd have to stop at an electronics store to get?

Availability in a hardware store will depend on the store.

A typical multimeter consists of an ohmmeter, voltmeter, and ammeter. Some have more functions as well. When measuring amps, most are limited to 10Amp maximum though. I would recommend a digital multimeter with auto-ranging capabilities.

When you start wanting to measure current draw for servos or other electronics which consume current in short bursts, you will need a unit which can "capture" the information to memory, and then display it after the event is over.

How much should I expect to pay for something capable of measuring motor and servos etc, with the memory feature?

The memory feature that strato911 is referencing will likely cost you more than you care to invest. A typical casual user DMM will run around $25 to $40 USD, depending on the source and features. I tend to use one quite a lot, but except for a very few exceptions, normally use a ~$30 meter -- on occasion, I have had meters go ffffffsssssttttttt, let the smoke escape and quit working. This was due to operator error and not the meter's fault.

Some typical e-sites that will give you some guidelines on $$$:

www.jameco.com
www.mcmelectronics.com
www.web-tronics.com <--- great deal on a Bag O' LED's
www.allelectronics.com

Regards,

Andrew

I'm not older than dirt, but I can remember when it was Patent Pending

The perfect device for you is the Astro Whattmeter. It measures current, voltage, and power and has many other uses.

Rob

You can get a simple multi-meter at the local hardware store too. Pricing is dependant on many things: Functions, Range, Analogue/Digital, Durability, etc...

Check out the ones your local Home Depot carries at www.homedepot.com
Goto "Electrical", then select "Testing Instruments", and select "Digital Testers".

Most hardware store meters either don't measure current or, as Dennis says, will only measure up to about 10A. This is not enough for most electric flight motors. Even a Speed 400 may top 10A from time to time. Anything larger will certainly use more than 10A.

If electric flight is your main interest I'd go for the Astro Whattmeter, it's an excellent device.

Steve

Does anybody know the burden imposed by the Astro Whattmeter when it is inserted in a circuit (What is it's resistance)? I would really like to know.

Where's a good place to purchase an Astro Whattmeter?
Bill

I don't know, but it will be VERY LOW because it handles very high currents. I don't think you need to worry about this as it was designed for this purpose...

Rob

Here is the meter I use. It is a clamp on and works just great!

I suggest the meter at the Craftsman site (www.craftsman.com) and check for article number 82062.

BTW, it does measure both AC and DC current. Check the detailed specs.

Take a look at this one ; http://www.web-tronics.com/cirspectec29.html

It has a memory function and will measure up to 20 amps.

The meter SteveV has referenced looks like a pretty good buy -- lots of functions and a data hold function. You would have to capture the amperage during the servo transit, but unless the meter takes time to stabilize, it might work sufficiently well for your needs. As noted, it also has a 20 amp max load -- most less expensive meters will manage only 10.

Circuit Specialists (aka: web-tronics) has a wide range of items -- some priced a little higher and some priced a little lower than other E-retailers -- but generally they have a good selection and are competitive.

Regards,

Andrew

If your really lookin' to get serious....check this site out
www.fluke.com