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Old Nov 07, 2008, 07:26 AM
7000mw of raw power!
rich smith's Avatar
New Hampshire (not the old one)
Joined Dec 2006
5,986 Posts
Mini-HowTo
$3 Wattmeter

Those who browse the RCG threads are probably convinced as I am that #1 cause of motor/ESC failure is lack of wattmeter. Here's a solution that is so inexpensive there's no excuse anymore for not checking run current. Also good for seeing how much juice that charger's putting back in your battery or watching voltage creep up to the magic 3.8v/cell for storage.

It is based on the Harbor Freight multimeter that is always on sale for $2.99 in at least one of the active catalogs. Any low cost meter will do. Only other component is any size resistor from 1k-10k or thereabouts. Here's wiring diagram and pics. Takes about 1 minute to build.

Switch to 20v to read volts and 10a to read amps (label says 10a but actually it reads up to 20a). Using calibrated laboratory references I've noticed this meter is considerably more accurate than commercial wattmeters and dataloggers costing much more.



update:

It's good up to 20a using the regular banana plugs and no modifications to meter. Much higher current can be monitored if you open the meter and wire directly to the shunt instead. See 3rd pic below. Before doing this measure a reference load (car lamp w/12v supply). After the mod adjust position of wires on the shunt until it reads 1/10th as much. They are about 1/16" (1.5mm) apart. Now full scale will be 200a instead of 20a.

Below is a pic of the bulb I used to calibrate. Handy for discharging batteries for storage or disposal too. It's got a JST connector because my motors are small. A constant DC supply like an extra drive cable from your PC is best but a battery will work too.

Remember, you need heavier connectors and wire to handle higher current. Note that the meter still works with original probes and scales yet does up to 200a if the new ESC/Batt connectors are used.

Like Curtis says you can add solder to the shunt between the wires to reduce readings for fine tuning. The extra solder allows it to handle a bit more current too.



More update:

Jackerbes and Critterhunter in post 75 and 295 respectively have put together much more detailed how-to with better pictures and diagrams. Thanks guys!
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Last edited by rich smith; Jul 13, 2009 at 06:13 AM. Reason: 200a mod
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Old Nov 07, 2008, 09:43 PM
If it's R/C, it's all good.
hpiguy's Avatar
USA, WI, Wausau
Joined Jul 2005
1,048 Posts
I have that exact meter and never even thought of that. Locally only $2.79 this week.

It's that simple? Jeese, off to make some test leads for JST and micro Deans. Will make a connector and hard wire into the board with better (thicker) wires.

Where do I get the resistor needed? Exactly what do I ask for? 1K what?
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Old Nov 07, 2008, 10:20 PM
Southern Pride
everydayflyer's Avatar
Haralson County GA. USA
Joined Oct 2004
34,833 Posts
Or just make a very simple,inexpensive shunt.

http://www.rcgroups.com/forums/showp...05&postcount=7

Charles
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Old Nov 08, 2008, 05:01 AM
7000mw of raw power!
rich smith's Avatar
New Hampshire (not the old one)
Joined Dec 2006
5,986 Posts
HP,

Ask for cheepest 1k resistor. Wattage, type, etc. don't matter.

PS You can also dig just about any resistor out of an old piece of junk electronic gear kickin' around the house. Or go dumpster diving. Value is non-critical.


Quote:
Originally Posted by hpiguy
I have that exact meter and never even thought of that. Locally only $2.79 this week.

It's that simple? Jeese, off to make some test leads for JST and micro Deans. Will make a connector and hard wire into the board with better (thicker) wires.

Where do I get the resistor needed? Exactly what do I ask for? 1K what?
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Last edited by rich smith; Nov 08, 2008 at 05:14 AM. Reason: junk
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Old Nov 08, 2008, 05:07 AM
7000mw of raw power!
rich smith's Avatar
New Hampshire (not the old one)
Joined Dec 2006
5,986 Posts
Quote:
Originally Posted by everydayflyer
Or just make a very simple,inexpensive shunt.
http://www.rcgroups.com/forums/showp...05&postcount=7 Charles
Yes, Charles, I built many shunts using numbers from your excellent thread. Thanks. However this is even simpler and inexpensiver because no shunt required. With added feature of voltage at flick of a switch.

-Rich

PS another even simpler shunt implementation in my Poor Mans Datalogger thread:

http://www.rcgroups.com/forums/showthread.php?t=921407
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Old Nov 08, 2008, 05:23 AM
Newbie but not too new!!
Lexington NC
Joined Dec 2004
297 Posts
Quote:
Originally Posted by rich smith
Those who browse the RCG threads are probably convinced as I am that #1 cause of motor/ESC failure is lack of wattmeter. Here's a solution that is so inexpensive there's no excuse anymore for not checking run current. Also good for seeing how much juice that charger's putting back in your battery or watching voltage creep up to the magic 3.8v/cell for storage.

It is based on the Harbor Freight multimeter that is always on sale for $2.99 in at least one of the active catalogs. Any low cost meter will do. Only other component is any size resistor from 1k-10k or thereabouts. Here's wiring diagram and pics. Takes about 1 minute to build.

Switch to 20v to read volts and 10a to read amps. Using calibrated laboratory references I've noticed this meter is considerably more accurate than most commercial wattmeters and dataloggers costing much more.

It's good up to 10a using the regular banana plugs. Much higher current can be monitored if you open the meter and wire directly to the jacks and add solder to the shunt to recallibrate. Also remember to use heavier wire and better connectors if you go over 10a.
Hey I like both of these setups and will try them both as I have several of the cheap meters from Harbor Freight Co.

Can you show what you mean by "add solder to the shunt to recallibrate"?

I am not sure I get that part. I see the solid wire inside the meter and am guessing that is the shunt?


On Charles' "Shunt" setup... If I am reading the MilliVolts at 28 on my setup with the shunt but both my watt meters show 26amps what needs to be done to the shunt setup to get that more accurate?
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Last edited by charlie hepler; Nov 08, 2008 at 07:50 AM.
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Old Nov 08, 2008, 11:36 AM
If it's R/C, it's all good.
hpiguy's Avatar
USA, WI, Wausau
Joined Jul 2005
1,048 Posts
Quote:
Originally Posted by rich smith
HP,

Ask for cheepest 1k resistor. Wattage, type, etc. don't matter.

PS You can also dig just about any resistor out of an old piece of junk electronic gear kickin' around the house. Or go dumpster diving. Value is non-critical.
Thank you, that will be easy to find then.
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Old Nov 08, 2008, 12:00 PM
7000mw of raw power!
rich smith's Avatar
New Hampshire (not the old one)
Joined Dec 2006
5,986 Posts
Charlie,

You guessed right. Wire the battery and motor leads to the middle of the shunt and range will be increased. Make sure you have a reference like 12v bulb hooked up before you recalibrate or won't know how much to change. I.E. if was 2a before recal and .2a after then you know range has been increased 10x (200a full scale).

PS I tested half dozen different makes of wattmeters and none of them very accurate. Not even in the same ballpark as el cheapo multimeters.


Quote:
Originally Posted by charlie hepler
Hey I like both of these setups and will try them both as I have several of the cheap meters from Harbor Freight Co.

Can you show what you mean by "add solder to the shunt to recallibrate"?

I am not sure I get that part. I see the solid wire inside the meter and am guessing that is the shunt?


On Charles' "Shunt" setup... If I am reading the MilliVolts at 28 on my setup with the shunt but both my watt meters show 26amps what needs to be done to the shunt setup to get that more accurate?
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Last edited by rich smith; Mar 14, 2009 at 12:45 AM. Reason: ps
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Old Nov 11, 2008, 08:33 AM
7000mw of raw power!
rich smith's Avatar
New Hampshire (not the old one)
Joined Dec 2006
5,986 Posts
Note that the original banana jacks were used in my case. Old leads were cut off right at the end of the connectors and new JST cables soldered on. It is important that a physical strain relief be used for the new wires. A glob of epoxy or silicone around the end of the banana jacks where they attach will do. Another way is to zip tie the wires to the body of the connectors.

In case of extended current versions a notch is cut in each half of the case and small rubber grommet protects wires coming out. Don't forget to put the grommet on BEFORE you solder wires on.
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Old Nov 11, 2008, 09:10 AM
Newbie but not too new!!
Lexington NC
Joined Dec 2004
297 Posts
Quote:
Originally Posted by rich smith
Note that the original banana jacks were used in my case. Old leads were cut off right at the end of the connectors and new JST cables soldered on. It is important that a physical strain relief be used for the new wires. A glob of epoxy or silicone around the end of the banana jacks where they attach will do. Another way is to zip tie the wires to the body of the connectors.

In case of extended current versions a notch is cut in each half of the case and small rubber grommet protects wires coming out. Don't forget to put the grommet on BEFORE you solder wires on.

Man I must be sleepy my 1st day back on day shift! LOL

What does this part mean?
" In case of extended current versions a notch is cut in each half of the case and small rubber grommet protects wires coming out. Don't forget to put the grommet on BEFORE you solder wires on."

I plan to remove the old wires and use heavy 12G noodle wire and solder them to the inside of the meter and add the resistor as you have shown.

If I chose to read Volts and then also read Amps what proceedures will need to be done? Will it need to be unhooked and hooked back up before switching them or turn the meter off and switch from V to amps?
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Old Nov 12, 2008, 07:54 AM
7000mw of raw power!
rich smith's Avatar
New Hampshire (not the old one)
Joined Dec 2006
5,986 Posts
I found that instead of drilling a hole it's quicker to cut a notch or half circle in each case half to bring the wires out. Also makes it easier to remove them.

A rubber grommet (Lowes, HomeDepot, RS, etc. ) keeps the wire from being cut by sharp edges. Couple layers of shrink also works. Or if you're really lazy like me just bevel the plastic and no grommet.

To read volts just twist the dial. No need to disconnect. That's what the resistor is for.

Quote:
Originally Posted by charlie hepler
Man I must be sleepy my 1st day back on day shift! LOL

What does this part mean?
" In case of extended current versions a notch is cut in each half of the case and small rubber grommet protects wires coming out. Don't forget to put the grommet on BEFORE you solder wires on."

I plan to remove the old wires and use heavy 12G noodle wire and solder them to the inside of the meter and add the resistor as you have shown.

If I chose to read Volts and then also read Amps what proceedures will need to be done? Will it need to be unhooked and hooked back up before switching them or turn the meter off and switch from V to amps?
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Last edited by rich smith; Nov 12, 2008 at 08:28 AM. Reason: typo
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Old Nov 12, 2008, 01:40 PM
jrb
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Edina, MN, USA
Joined Oct 1999
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Why is the 1k resistor needed?
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Old Nov 12, 2008, 01:45 PM
7000mw of raw power!
rich smith's Avatar
New Hampshire (not the old one)
Joined Dec 2006
5,986 Posts
Quote:
Originally Posted by jrb
Why is the 1k resistor needed?
It is a current limiter. Without it damage to the meter and battery can occur when changing settings. I.E. dead short while in ma ranges. Not good for ohms range either. This way it's %100 safe.
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Old Nov 12, 2008, 02:03 PM
Registered User
Raleigh, NC
Joined Apr 2007
4,773 Posts
Rich, I have the DVM pictured below. This meter will go up to 20 Amps. Can I use this meter with your circuit using the 20 Amp input?

Bill
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Old Nov 13, 2008, 12:11 PM
7000mw of raw power!
rich smith's Avatar
New Hampshire (not the old one)
Joined Dec 2006
5,986 Posts
Quote:
Originally Posted by BloomingtonFPV
Hi Rich,
I appreciate you making the instructions for the wattmeter available. I made the $3 wattmeter using the 200a modification by soldering the two black wires along the shunt. However, my escs on several planes won't arm when the meter is in series with the battery. I know it is working correctly when I test it on another load. Is it possible that the ESC doesn't like the shunt on the ground? My red wires are tied together and go to the solder point of the fuse using a 1k resistor.
Thanks, Tom
I hope you don't mind posting instead of PM because of limited space there.

If the ESC fails with meter inline it's probably due to wires being too thin or too long, bad connectors, or a wiring error. Properly executed the meter should look like a very short extension and never interfere with normal operation. If you still have trouble clear photos might help us diagnose your problem. Keep us posted.
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Last edited by rich smith; Oct 19, 2009 at 05:48 AM. Reason: accidental edit
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