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Old 12-17-2008, 11:12 PM   #1
Quentus
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Default Advice on using a multimeter to determine wattage

I dug this out of my father-in-law's tool box and have very little idea how to use it. But I need to find out the amount of watts that my power system is drawing. Now I read the instructions for the multimeter (ampmeter), but it is not clear as to how and where to use the tool. For example, what I want to do is determine watts, but because I don't have a wattmeter I am planing to measure both current (amps) and volts and then multiply them together to determine watts (Watts = current times volts). Okay, so first I need to load the system right, so I would plug everything in and get the prop spinning. Now this is where I am lost. Assuming I am measuring volts, where do I put the red (positve) clip, and where do I put the black (negative) clip. And am I right in that I need to take a voltage measurement when the system is loaded? Assuming I can get a voltage reading, I would then attempt to measure current. But again, where do the plugs go, same place?

I just don't want to blow anything up or electricute myself, so I am embracing humility here and admitting to not knowing what I am doing. I know I should just go and get a wattmeter, but I would rather spend my money on another plane.lol Any help is much appreciated. Thanks.
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Old 12-17-2008, 11:27 PM   #2
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The short and best answer is to buy a dedicated watt meter. They are not that expensive.

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Old 12-18-2008, 12:05 AM   #3
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"best answer is to buy a dedicated watt meter"

I agree. You could possibly use the multimeter (depending on it's range), but would need lengthy instruction to use it correctly.

Phil

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Old 12-18-2008, 12:11 AM   #4
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Okay, you talked me into it. But I really need one before the weekend and simply can't wait for some on-line rc store to ship it to me. Do you know of a retail store that would carry one? I already checked radio shack and ace hardware and neither of them carry a wattmeter. Wait, don't tell me... no one ever buys them retail, right since they are hard to find? I live in Atlanta, so there's got to be a place that sells them.
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Old 12-18-2008, 12:26 AM   #5
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Why not just go to an RC store?

Brian - AMA 666 72 mhz and glitch free!!!


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Old 12-18-2008, 12:30 AM   #6
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Old 12-18-2008, 04:35 PM   #7
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You can get a close approximation using your multimeter if the current range is high enough (many are not, some limit at 2 amps, some at 10 amps). You first hook every thing up then insert the meter in series with the battery (plus lead to positive battery lead and negative meter lead to motor)--this is aft of the ESC-- and read the current. Make sure the multimeter switch is set to read current. Now, remove the multimeter from the circuit and switch the multimeter to read voltage and connect the positive lead of the multimeter to the positive terminal on the motor and the negative lead to the other motor lead if using a brushed motor. If using a brushless motor, connect the meter leads right at the battery terminals (before going to the ESC), plus of the meter to plus on the battery and negative to negative. When you power up the motor by connecting it, you will see a drop in the voltage reading due to the drop in the internal resistance within the battery. The wattage is as you said, voltage reading times current reading.
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Old 12-18-2008, 05:02 PM   #8
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Thanks Rodneh for the reply. This is really all that I was looking for in terms of a little experiement with voltage and current. I really do want to get a wattmeter, but I simply can't rationalize spending 50$ for one when I could use that for a new plane battery. Anyways, first how do you determine whether the multimeter can handle the test. I mean do you compare the amp rating on the multimeter with the amp rating on the battery? For example, I think the multimeter I have has a 20 amp limit, however the battery can spit out 20C (43.0A) continuous, 30C (64.5A) burst. This suggests it will not work, am I right?
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Old 12-18-2008, 10:41 PM   #9
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Volts X Amps = Watts

That simple.

"Better to be looking up in the air wishing you were flying, than looking down at the ground wishing you weren't."
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Old 12-18-2008, 11:08 PM   #10
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Be careful. Most multimeters can only handle current readings of 200 to 400 mA. Read mA as "milli-amps" which is 0.2 to 0.4 amps. If you run 20 amps through such a meter it will smoke or blow an internal fuse. A multimeter that can handle 5 amps or more is not common. There are ways of using a multimeter to read high amperages using a shunt circuit. Some of the other guys may be able to offer some advice here.

Dedicated clamping amp meters can read high amperage ranges but work differently than multimeters. Clamping amp meters have a circular ring that that snaps around the wire and measures the magnetic field surrounding the wire. Clamping amp meters are not inserted into the circuit like a multimeter. Also, I'm not certain that clamping amp meters can read DC current.

I'm echoing the previous advice .... buy a watt meter made for RC use. Watt meters measure amps plus volts, watts, peaks, lows, and averages. They are small enough to leave in a medium size plane during flight. If you save one ESC from destruction you will have paid (or nearly paid) for the cost of the meter.

Rick
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Old 12-18-2008, 11:45 PM   #11
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Right after you blow up that multimeter, be sure to come back here and tell us why you thought it should be able to handle 40 Amps of current, and how much smoke it let out before it burst into flames. And why your father-in-law is going to kick your $$$ now. Okay?

(Here's a hint - If the test leads on that multimeter aren't at LEAST as large as your motor-to-esc leads, don't try it!)

Dennis Fishback AMA# 162410 AKA# 120592
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Old 12-19-2008, 12:46 AM   #12
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Okay, okay... Point taken.
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Old 12-19-2008, 12:53 AM   #13
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Originally Posted by Dorsal View Post
Right after you blow up that multimeter, be sure to come back here and tell us why you thought it should be able to handle 40 Amps of current, and how much smoke it let out before it burst into flames. And why your father-in-law is going to kick your $$$ now. Okay?
LOL!

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Old 12-19-2008, 01:27 AM   #14
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Sorry to be nasty, but it did get your attention. It would take a multimeter that costs A LOT more than a WattMeter to be able to run that test. btw, http://tinyurl.com/3w5afu or http://tinyurl.com/4l543q are 2 great meters at prices you probably can't beat.

Dennis Fishback AMA# 162410 AKA# 120592
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Old 12-19-2008, 01:52 AM   #15
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most multimeters is max 10 amps, and that isnt going do to do it. but i wouldnt discourage you from trying just for the sake of experimentation.
the voltmeter is fused, the worst thing that happends is that you need to find a screwdriver and a soldering iron and change fuse.
watts is found in several ways. u^2/r = i^2*r=u*i=p
more ways is possible, thats jsut somethign to get your thougts starting.
so depending on wich data is easiest to obtain for you, choose the formula that suits best.
of course there is an option of buying a wattmeter, but all a wattmeter does is really just measuring volts & amps, and multiply them. you can do the same ting yourself with a multimeter, plus a whole lot more. and it costs about the same.
so if i were you, i would spend the money on a descent multimeter rather than a wattmeter. and there is way around that too high amp problem, either just by mathematics, as you see there are formulas not demanding amp input, or simply by buying a multimeter capable of high amps. (there are those multimeters with a claw on it, that you clamp around a cable. those can measure amps by the thousands of amperes. not sure what its called in english though, but in norway we call them "tangampermeter". literally meaning something like clawmultimeter).
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Old 12-19-2008, 05:16 AM   #16
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We call them clamp on CT's. Usually for measuring AC current.

http://www.dentinstruments.com/CurrentTransformers.htm

But, I think you need a shunt to measure DC current.

http://www.cshunt.com/pdf/la.pdf


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Old 12-19-2008, 06:04 PM   #17
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Originally Posted by Quentus View Post
Thanks Rodneh for the reply. This is really all that I was looking for in terms of a little experiement with voltage and current. I really do want to get a wattmeter, but I simply can't rationalize spending 50$ for one when I could use that for a new plane battery. Anyways, first how do you determine whether the multimeter can handle the test. I mean do you compare the amp rating on the multimeter with the amp rating on the battery? For example, I think the multimeter I have has a 20 amp limit, however the battery can spit out 20C (43.0A) continuous, 30C (64.5A) burst. This suggests it will not work, am I right?
I do not know which multimeter you have, you may be lucky and have an older Simpson or one of the same vintage. If there is a dial setting for 20 amps, then you should be safe in using it for that amount of current. There is probably a separate jack (female hole for a bannana plug) for current measurements and another for voltage measurement with the ground connection common for either of them. Just make sure that you do not try to measure voltage when the positive lead is plugged into the current jack or you will blow that fuse. Trying to measure current with the voltage jack won't damage anything, just won't work. If you are in doubt about whether or not you can measure peak current, just have the throttle at low when you connect the current probe/meter and slowly advance it while reading the meter. If the meter approaches full scale and you are on the highest current range, just don't increase the throttle any farther and no problem. I salute you for not being afraid to try out experiments on your own, you appear to have a lot of common sense and should make out well. After all, nothing ventured, nothing gained. If you meter will not read a high enough current for your application, there is another work-around that will give you results that will be very slightly in error. Just insert a precision 1/2 ohm resistor in series with the battery and measure the voltage across it while at full throttle. The current will be equal to the voltage your read divided by the resistance of the series resistor. Now your actual current (without the resistor in series) would have been a bit higher but you will still have a pretty good handle on what the peak current will be. Congradulations for some good thinking, charge ahead.
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Old 12-19-2008, 06:33 PM   #18
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switch mode dc can be measured as well with clamp on multimeters (that is, the current out of the regulator, not out of the battery).
it has to be a multimeter measuring absolute current though, otherwise it will calculate teh current as it it was a sine ac. and its not. but if its beasures absolute current, its fine to use for brushless motors
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Old 12-22-2008, 01:02 PM   #19
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I've always wondered if it would be possible to use the Battery's internal resistance to determine the power being used with a volt meter.

You could measure the battery voltage (unloaded) - then measure the voltage drop when attached to a known load. From this you could work out the battiery's internal resistance.

Once you have this you just hook up the battery to the unknown load, measure the voltage drop and then work out the current being drawn and then the power.

it might not be that accurate and the internal risistance will probably change as the battery discharges but it might give a reasonably good indicative measurement.

Personally, I went for the watt meter option as it a lot easier.
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Old 12-22-2008, 07:26 PM   #20
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yep buck, you can use internal resistance and measure volt drop.
i se no reason why that should be less accurate than a wattmeter, so why not. the wattmeter has fault margins just as the multimeter, and the multimeter tends to be more precise for less money as its a much higher volume production.
wattmeter is the easiest, true, and thats alsy the number 1 reason why im against it.
its too easy, and it stops people from thinking themselves.
being a total gearhead sitting in a garage having the equippment do all the work is no challenge.
even a monkey can build a car when it has all the robots and computers that modern car factories has.
but its still an art to handcraft a car.
i think its the same here too. too much advanced equippment and the builder doesnt earn the credit. his equippment does.
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Old 12-22-2008, 10:48 PM   #21
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"even a monkey can build a car when it has all the robots and computers that modern car factories has"

If you happen to work in the auto manufacturing industry, he's not calling you a monkey.

"No user serviceable parts inside" means I have to crack it open to see what they think I can't fix.

I'd have a lot more planes if I didn't fly so often.
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Old 12-23-2008, 11:05 PM   #22
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The multimeter can actually give you a good guess of the draw by watching votage, IF you know how the batt holds up under load, and what the setup should draw. EDF is really the point where not having a wattmeter gets old. Still, so many setups are nearly PNP now, with better batts and a wealth of recorded info out there, that you can almost go by that info alone, if you trust the sources.
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