How to measure Watts with two Volt Meters - RC Groups
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This thread is privately moderated by chaosMurphy, who may elect to delete unwanted replies.
Jan 09, 2008, 07:07 PM
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How to measure Watts with two Volt Meters

How to ...
Make a HIGH power shunt for box store DVMs and
Extend the range of most box store DVMs past 10 AMP max

This technique does not apply for AC power measurement

This technique allows different props to be expressed in terms of Watts;

From FIG 2...
First measure resistance and Volts
Second calculate current, I = E1 / R1
Third calculate power, P = E2 X I

lipos exhibit a potential fire and or explosion hazard if output wires short (touch each other).

Never ever allow lipo wires to touch, the lipo will dump MAX AMPs into zero Ohms, weld wires together, chemical avalanche, puff up and may burst into flame and explode.

Although I give two examples below,
DO NOT wire up these example.
DONOT wire up R LOAD equal to zero Ohms.
These examples are for other purpose.

For direct current only

>I< current, Ampere, AMPs
>E< Volt
>R< resistance in OHMs
>P< power in WATTs

I = E1 / R1 … “ / “ …. E is divided by R

P = E2 X I …. “ X ” …. E is multiplied by I

R1 is called a shunt resistor and RL the load resistor.
R1 is usually measured when the shunt is cold (FIG 1).
The shunt will get hot under test conditions and its value will change
This unknown change is critical with respect to repeatability so the shunt power handling character must be addressed.

DONOT wire this example. DONOT connect you lipo at this time.
This is an example only to define your shunt power handling character in order to keep resistance change to a minimum.

Assume MY battery is 10 Volts (yours might be 5.375 Volt), (your shunt might be 3.33 Ohm), do the math, your shunt will get this hot.

My 1.5 Ohm shunt must be capable of dissipating at least;

P(shunt) = E(lipo)squared / R(shunt)

(10 X 10) / 1.5 = 67 Watt …. WOW that is hot

My 10.5 Ohm shunt must be capable of dissipating at least;
(10 X 10) / 10.5 = 6.7 Watt …. Double wow-wow more like it.

Both examples assume R load equal zero Ohm.
A load resistance of zero Ohm is worse case for shunt.

Your ESC (soon to become R load) is not zero Ohm.

Of these two examples #2 is 10 times better than #1 with respect to repeatability.

My 10.5 Ohm (example 2) shunt delivers less Volts to the ESC but no problem…
The reason for this setup is not absolute power measurement or absolute accuracy but only to define different propellers (under same condition) expressed in terms of WATTs, Apples to Apples so to speak, like this example;

From FIG 2...
First calculate current, I = E1 / R1
Then calculate power, P = E2 X I

The power waste in my shunt is taken into account and does not impede this technique because the shunt and load are connected in series with respect to the battery. Current flow through each element is equal in a series DC circuit.

Most box store DVM has a resolution in 1/10 Ohm (1.4, 1.5, 1.6, etc)
The shunt element can take many forms;
Common number 10 house wire wrapped on a broomstick
Common number 8 house wire wrapped on same stick, but you will need more of it.
Nichrome wire is made specific for this purpose.
Radio Shack may be an alternative for high Watt, low Ohm (shunt) resistor…
I can buy a much less expensive RadioShack resistor using example #2 (10.5 Ohm)
BUT I will need a bunch more house wire using same example (10.5 Ohm).
Box stores may offer an alternative, auto dept, appliance dept, etc
An old toaster element for example, cut wire length for desired shunt value
A large rheostat (but not AC light dimmer)

Gramps can assumered absoutootly NO responder/ability for puffed lipos….

My biggest AH-Haaa-Plateau … props turning backward (like in reverse!!!)
Tachometer and weight scale only add to the excitement.

Last edited by Gramps:>); Apr 01, 2008 at 07:53 AM.
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Jan 18, 2008, 07:56 AM
Tom Bass Park RC Nonconformist
Cdr.Instigator's Avatar
My God !!! I just finished reading this tutorial and I got to say that, I'm as lost as when I began reading this... LOL

, I seriously need to take a class on electronics !!!

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