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Author Topic: Cheap ammeter wire-up - advice welcome  (Read 3517 times)

RayW

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Cheap ammeter wire-up - advice welcome
« on: January 01, 2014, 08:25:18 PM »

I bought a cheap ammeter and shunt to measure amps for a scale model boat.


After reading a lot of info on this site about ESC's and Brushless Motors, I am very nervous about attempting to wire it up.


I have put a few details on my web page at [size=78%]http://schoolroad.weebly.com/rpgmodel_13.html[/size]


One thing I am unsure about, is whether to wire it to the motor, or into the circuit before the ESC.


Any suggestions would be very welcome.
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Tug-Kenny

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Re: Cheap ammeter wire-up - advice welcome
« Reply #1 on: January 01, 2014, 10:18:31 PM »


For measuring the total  Amps drawn then you need to connect it between the battery and the esc.


Hope this helps


Ken

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Seaspray

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Re: Cheap ammeter wire-up - advice welcome
« Reply #2 on: January 01, 2014, 11:02:48 PM »

Yep to measure Amps (current) you need to have the meter as part of the positive wire system As Ken says


battery  +ve ----------meter----------------ESC   +ve

Mind and connect the meter wires correctly as you may get a minus reading instead of positive, Change wires around on the meter you'll get your positive
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nick_75au

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Re: Cheap ammeter wire-up - advice welcome
« Reply #3 on: January 02, 2014, 12:07:57 AM »

You need to pretend the ESC is the motor (on the Chinese diagram)


To wire the shunt, the Shunt goes on the NEGATIVE side of the circuit between the ESC and battery, the black small wire goes to the battery side of the shunt, the yellow to the load side of the shunt, and the red wire to the positive side of the battery. The smaller red wire supplies power to the meter, as long as your battery voltage is less than 30 volts, connect it to the same pos as the thicker red wire

Its much easier to use a proper RC wattmeter, http://www.hobbyking.com/hobbyking/store/RC_PRODUCT_SEARCH.asp?strSearch=wattmeter

Nick


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nick_75au

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Re: Cheap ammeter wire-up - advice welcome
« Reply #4 on: January 02, 2014, 12:28:04 AM »

Having a look through your webs, looks cool, I would point out that that motor is not really a good motor to use, its RPM is way too high. Its designed for fast boats and surface piercing props. I'm guessing that you are looking to develop efficient low current draw drive? I can provide more info if you want.


Cheers
Nick
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malcolmfrary

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Re: Cheap ammeter wire-up - advice welcome
« Reply #5 on: January 02, 2014, 11:16:11 AM »

You cannot measure the current taken by a brushless motor without having the ESC in circuit, that measurement has to happen between the battery and ESC.  With brushless, the ESC is effectively half of the motor, and usually will do nothing until it has a valid control signal.  This normally implies either having the radio hooked up and fully working, or using a servo tester as a signal source (like the one done by Action).
The simplest down and dirty way of determining current draw under load is to insert a fuse in the battery lead, and experiment with different values - when you get the borderland between blowing and not, you know.


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RayW

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Re: Cheap ammeter wire-up - advice welcome
« Reply #6 on: January 04, 2014, 03:23:47 AM »

Thanks guys, some really good comments.


I wasnt even aware you could buy specialised wattmeters for RC models.


They look really good.


I want one that can display the Amps drawn at various speeds, as I have an onboard video camera that records GPS Speed, and knowing the draw of the motor may give me an idea of the HP on the fullsize boat required.


Does anyone have any experience with those wattmeters and which is simplest and easiest to setup ?


Nick75 comment about the motor may be really important. I bought the outboard because it was especially cheap, but I can change the motor quite easily I would have thought. I think the shaft is 4mm - but I will have to check. It has an adaptor, so it would be easy to change.


I anyone can  suggest a motor or what the best RPM/Power would suit a model that may weigh up to 20 kilos, that would be great.


I havent got into it yet, but having an RPM display would be really great as well.


Thanks again everyone




PS administrators - you better change the current year in your Verification scripts. Its no longer 2013
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nick_75au

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Re: Cheap ammeter wire-up - advice welcome
« Reply #7 on: January 04, 2014, 04:28:27 AM »

Hi Ray,


if you want to go real technical there are several data logger type units that will give you any info you want, some even live, I've just got an Eagletree system, the base unit does amps and volts and with a cheap add on does RPM as well, another add on allows GPS data to be recorded. I wouldn't call it easy to set up as I HAD SOME WIN8 ISSUES but once its configured its pretty much plug it in, get the data then download the data. Most of the cheaper units don't have any recording, just live data, some will record peak data only


20 Kgs seems rather heavy ?


What performance do you need, you probably need to put the biggest prop you can to the outboard then use a lower RPM motor, the motor will depend on the size of prop and how fast you want it to go


AS an example though a 40 mm scale brass prop with a 1000 Kv 28 mm diameter motor will on 12 volts push a light 2kg hull to about 15-20 KPH


Ive got another hull that is about 7Kg called Axebow,by Damen Shipyards that does about 12 Kph on two 750 Kv motors also on 12 volts.


A 20Kg hull woild be heavy tug boat or similar and would use 60-100 mm props at from about 1500-5000 rpm max (smaller prop higher RPM)
http://www.rcgroups.com/forums/showatt.php?attachmentid=4261513&d=1315289844


The above link might be of use to help with the numbers
 


Nick
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RayW

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Re: Cheap ammeter wire-up - advice welcome
« Reply #8 on: January 04, 2014, 05:45:12 AM »

Hi Nick


Thanks for the info you sent.


The boat is a 1;5 scale model, with a fully loaded design weight of  ~ 2700 kilos, so ~ 17 kilos at that scale. It is also a sailing boat, so much of that weight is ballast. Top motoring speed wont need to be achieved at that total weight, but around 12 kilos.


The full size boat is planned to travel around 25mph, so I need to to go  over  5 mph in testing at ~ 12 kilos.


That link you sent me didnt work for me, but it may be like  a file I already have with prop data like


http://www.marinespecialties-rc.com/rpmpitchspeed.htm


but that was for fuel powered props, probably surface piercing.


I will have a careful look at those measuring boxes, and see if i can make any sense of them.

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RayW

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Re: Cheap ammeter wire-up - advice welcome
« Reply #9 on: January 04, 2014, 06:02:14 AM »

PS Nick - got that link to work OK
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malcolmfrary

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Re: Cheap ammeter wire-up - advice welcome
« Reply #10 on: January 04, 2014, 09:58:06 AM »

Quote
The boat is a 1;5 scale model, with a fully loaded design weight of  ~ 2700 kilos, so ~ 17 kilos at that scale. It is also a sailing boat, so much of that weight is ballast. Top motoring speed wont need to be achieved at that total weight, but around 12 kilos.The full size boat is planned to travel around 25mph, so I need to to go  over  5 mph in testing at ~ 12 kilos.

Going off the numbers, at 1:5, a 2700Kg boat weighs in at 21.6Kg.  Again, at 1:5, speed, being related by the square root of the scale, 25mph becomes 11 and a bit.  Any pictures of the original?
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RayW

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Re: Cheap ammeter wire-up - advice welcome
« Reply #11 on: January 04, 2014, 11:32:54 AM »

Thanks Malc


Yes, I was being a bit vague about the figures, just working on memory.


I have attached the full weight table in case it helps anyone with info.


This model hasnt been built full size, but is developed from an existing boat
http://macgregor26.com/


Thanks for the interest.


Cheers


Ray
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malcolmfrary

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Re: Cheap ammeter wire-up - advice welcome
« Reply #12 on: January 04, 2014, 05:19:21 PM »

It is reasonable to assume that the max speed figures were obtained just before the fuel ran out, and with the bare minimum of fittings, so a top model weight of 9.5Kg when used as a fast boat.  The blurb mentions water ballast, presumably for when it's in its other personality as a sailboat.  I would use a detachable keel weight on a model.  Top speed mentioned is 22+mph, so I would guess at 22.0001, scaling to just under 10 mph.  This is fast for a boat that size and weight, and implies a healthy power to weight ratio.
I imagine that Nick is vastly better informed that me what would likely do the job.
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More Coffee

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Re: Cheap ammeter wire-up - advice welcome
« Reply #13 on: January 23, 2014, 02:22:10 PM »

I purchased  pretty much the same setup . It didn't work so well. The shunt a appears to have been totally dipped in a conductive anticorrosion  plate.
I had to clean it up with a wire wheel to get any sort of reading ...and it wasn't right.
The shunt was way out on the lower current .
 
The RC style power analyser , is merely a hall effect sensor to measure current , some power calculations to get the rest of the info. The ones I have taken apart appear to have a Atmel MCU , a 16x2 display, hall effect sensor , power control board and that's all.
 
If you adventurous a really cool accurate ammeter could be made up in power analyser form.
http://www.ebay.ca/itm/111040360152?var=410166550831&ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649
 
Good luck with that arrangement , But I really think it wont work well
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RayW

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Re: Cheap ammeter wire-up - advice welcome
« Reply #14 on: January 25, 2014, 12:02:37 AM »

Many thanks MC - that is really valuable information.


I will look into your suggestion for a more reliable way to get the info.


I appreciate your help, along with all the other contributors.


MalcomFray with the fuse idea seems the most brutally efficient :-)
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Re: Cheap ammeter wire-up - advice welcome
« Reply #15 on: January 25, 2014, 03:39:06 AM »

This lays out what Im saying a little more clearly.

http://www.hacktronics.com/Tutorials/arduino-current-sensor.html

http://www.instructables.com/id/DIY-Amp-Hour-Meter-Arduino/

Its basically a hobby power analyser . But you can build it to your own requirements
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