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Modifying A $3 Harbor Tool Multi Meter To Measure High DC Amp Draws
Posted by: Critterhunter
Date: October 28, 2010 09:25AM
The following instructions will show you how to make a great, cheap, and simple home made shunt using the $3 multi meter from Harbor Tool & Freight. In the RC crowd there are several threads along these lines. Some of them are using an external shunt which gets messy with wires, while others lack the finer detail to walk people with zero knowledge about electronics through the modding process using a meter method, which meters can be used, and etc, so I wrote a step by step guide using a cheap meter from Harbor Tool & Freight.

Buying equipment capable of measuring high DC amp draws can be costly, and most meters can only do this on very small scales with out spending a bunch of money on meters capable of reading large current levels, so this easy mod to a cheap Harbor Tool multi meter will save you a bunch of money, not to mention that it has proven more accurate than many commercial units.

A device like this is very useful in RC planes where you can get an accurate measurement of the amp draw of high powered brushless motors to avoid undue battery stress, but why would you want one otherwise? Well, for example, it can be very handy when troubleshooting things like car problems. For instance, not long after I built this my sisters car kept burning up the fan relay. Before replacing it once again I decided to see if the fan motor was within specs in terms of its amp draw. I quickly found out that it was within reasonable parameters for what it was supposed to draw, but rather the original relay specs were border line in terms of handling it. I found a replacement relay that was rated for higher amp draw and the problem has been solved for a few years now.

I wrote this how to article for the RC crowd, so you might find the connectors I used to be a bit strange for general applications. These Dean Ultras are standards for battery power connections. Simply replace them with alligator clips or whatever else you want to use.

Also, if you dont have a charger or some other source of known amp rates to reference when calibrating it, you can using something like a car light bulb as your reference point. From memory most draw around 1.5 amps. Before modifying the meter use the stock meter to measure the amp draw of whatever you are using so that you have a reference point to calibrate to. Power it with a large enough source (such as a car battery) that the voltage output wont be dropping fast on you, changing the amp rate.

One other minor detail. With the way I show to wire the thing up the display may show a negative number. Doesnt matter, just ignore the -. Lets get to it

First picture is of the meter ready to be opened. Before doing this youll need to check the amp draw of a load using the stock test leads so that you can use this as a known reference load to calibrate the unit. I set the meter dial to 10 amps DC, hooked it up to a nimh/nicad charger, and set the unit to a charging rate of 1.5 amps. My load is my nicad TX (RC radio) battery pack. As you can see both numbers jive very well.

The configuration of the amp meter needs to be in series with the supply and load. Meaning, take the positive wire from your power source (my charger) and connect it to the positive test lead of the meter. Now take the negative lead from the meter and hook it up to the positive wire going to the load (my TX). The negative wire from my charger then is connected directly to the negative input lead on the TX.

Next picture is of the opened case. You can see the shunt marked with an arrow.

Once you have your reference load figured out you can begin wiring things up. Solder red and black wires to their perspective terminals on your Deans Ultra plugs. I used 14 gauge wire to handle the amp draw of my planes up to the 30 or 40 amp range or so. This is the same wire I use to connect my ESCs to my lipos on my planes. If reading higher amps youd probably need to go to 12 gauge or larger diameter wire, or at least keep your testing to seconds at a time to avoid false readings or over heating the wire.

I twisted the two black wires together and also twisted the lead from a 1K resistor (other sizes will work too) to this, then soldered them together. After this I slide some heat shrink tubing over the wires and out of the way for now to avoid any heat, then soldered the other end of the resistor to an existing solder point on the circuit board. This spot is noted in the picture. Once done slide the tubing all the way down and use your soldering iron to heat and shrink it.

Now take your positive (red) wire from the male Deans (Source) and solder it to the shunt. Anywhere will do but best to avoid any calibration grooves done from the factory (see notes in the picture).

Now its time to calibrate the unit and solder the other red (load) wire to the shunt as well. I used a mirror so I could easily see the display on the meter, as its flipped upside down to allow easy access to the shunt. Its also a good idea to tin the wire first with solder so that you wont have to add more when soldering it on, avoiding mis adjusted numbers. Slide the wire lead across the shunt with decent pressure to avoid mis calibration by way of induced resistance. This way when you find the proper spot your solid solder connection wont throw the numbers off should the resistance be reduced.

The stock meter (even though it says 10) will read up to 20 amps. This mod will be good up to 200 amps (with the right gauge of wire used), but the display can not handle numbers this high. The placement of the shunt wires properly will reduce the numbers on the display by a factor of 10. What this means is that I want my 1.5 amp reference load to now read .150 on the display. Thus, 30 amps will now read 3.0 on the unit. Also note that the display may indicate negative numbers. Just ignore that because it only means the meter was hooked up in reverse polarity in the circuit. It wont matter at all.

Once youve found the spot (pushing with decent force to insure a good connection) where the meter reads what your source is saying hold the wire exactly in place and heat it so that the solder flows and connects it securely to the shunt. Re-check your amp reading. If its off you can add a little solder between the wires on the shunt, or groove the shunt to go the other way. Id rather just unsolder the wire and re-adjust myself.

By the way, if in the initial (stock) calibration check using the stock leads your meter numbers differ greatly from what your source is saying, I would try another source to compare. If the meter still isnt real close then Id suspect the meter calibration to be off. Since my charger and the meter were both giving me the same numbers they are confirming that both are calibrated properly from the factory. Its also important to note that amp draws within tenths of an amp of each other are close enough. As the old saying goes, this isnt computing a rockets orbit so you dont need to be exactly percise.

The resistor tapped into the negative wires is to allow voltage readings to take place, protecting the meter from damage as you flip the dial around. With voltage and amps read you can then compute wattsVolts X Amps = Watts. I mostly go by the amps to make sure props are within specs of my motors.

This unit can be more accurate than commercial watt meters that cost $60 or more. Not only is this a great mod, but its a cleaner install (less wires hanging out) than my homemade shunt, and its a bit more accurate. In the trash with the old shunt!

After doing a bit of testing with the modified meter I've found out a few things that should be considered. First, 14 gauge wire is good up to about 32 amps or so. If you are planning to read amps above that for any more than a few seconds at a time I'd increase the diameter of the wire. Dig up a wire gauge chart off the net and compare the numbers.

For this reason, it's also a good idea to increase the thickness of the shunt between the two wires in the same situations by soldering wire in parallel to it or adding solder to the shunt it's self to thicken it's diameter. After that's done you can adjust the wire gap to the proper calibration as before. It's hard to exactly tell the diameter of this solid wire shunt, so I'm only going to trust it up to thirty some amps or so. With higher end amp readings I'll limit my testing to ten or twenty seconds or so.

The induced resistance of too small gauge of wire for the amps you are wanting to read could cause errors in the amp readings. I found testing in the 40+ amp range was showing a slight error in the numbers. Also keep in mind that if you are flying a plane that is drawing well over 30 amps it's a good idea for your battery to ESC wire to use the proper gauge, something larger than 14 gauge. From memory I thought 14 gauge was good to about 40 amps or so. Looks like I'll have to re-wire the power systems on the EDFs I'm building platforms for.

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Re: Modifying A $3 Harbor Tool Multi Meter To Measure High DC Amp Draws
Posted by: Critterhunter
Date: October 28, 2010 09:27AM
More pics that wouldn't fit the first post...

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Re: Modifying A $3 Harbor Tool Multi Meter To Measure High DC Amp Draws
Posted by: Critterhunter
Date: October 28, 2010 09:27AM
Last two...

mining members for own forum

Re: Modifying A $3 Harbor Tool Multi Meter To Measure High DC Amp Draws:thumbup:N/T
Posted by: w6pea
Date: October 29, 2010 12:59PM

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Re: Modifying A $3 Harbor Tool Multi Meter To Measure High DC Amp Draws:thumbup:
Posted by: Critterhunter
Date: December 20, 2012 09:43AM
Recently this came up in conversation so I figured I'd bump it up again. :throw:

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