Ten Minute Ammeter Shunt

I was recently working on a project that required me to observe the current spike of a charging battery. The only way I know how to do this is with a low resistance shunt and an oscilloscope. I have the scope but every time I use this technique I end up piecing together some sort of resistor monstrosity on a breadboard just to take it apart when I am done...

Today I decided to make a permanent multi use shunt.

So, I just happen to have these parts laying around:
- four 1 ohm resistors
- various sizes of heat shrink
- two alligator clips

And you may require:
- screwdriver(s)
- soldering iron and solder
- heat gun

But this is Instructables, so really use whatever you have on hand. These are just the steps that I followed to make this shunt for this project; build it however it will make the most sense for you.

The most difficult part of this build is making the resistors fit in between the two alligator clips. I could have put the resistors on the outside but I wanted them to be protected by putting them on the inside of the wire crimp area. I decided to make the crimp area of one clip fit inside of the crimp area of the other clip, and be insulated of course.

Your going to want to twist the resistors together in a way that they will fit into the crimp part of the alligator clip, but also so that the leads of the different resistors cant short the others out. A small amount of solder will hold them together for easier assembly.

Without stabbing yourself with the screwdriver, pry open the crimp part of one of the alligator clips. Its probably a good idea to pry the second one open too but not as far, it will help with assembly.

Last but not least, be sure the twisted together ends of the resistors will fit through the corresponding wire holes in the alligator clips. Mine had to be drilled out a tad.
 

Ok first lay the resistor bundle into the alligator clip with the smaller diameter crimp end, it wouldn't hurt to shrink wrap the resistors to prevent shorts.

Electrically connect the resistors to the first alligator clip however you choose, I used the screw terminals on the back of the clip and then soldered it for good measure.

When your satisfied with the first connection, place shrink wrap around the outside of the crimp part of the alligator clip and shrink it.

Next feed the other end of the resistor wire into the second alligator clip. Solder or tighten the second electrical connection as necessary, just be careful not to melt through the heat shrink.

Lay the shrink wrapped end of the first clip into the pried open end of the second clip and re-close the pried opened end.

It should be more than sturdy enough to use but I added a zip tie just in case.
 

The type of resistors you used will determine the final resistance of the shunt, four 1 ohm resistors in parallel will give you a total resistance of 1/4 ohms.

Using Ohms Law E = I * R : one amp of current through the shunt will result in a voltage drop of .25 volts, just multiply the voltage drop times 4 to end up with your current.

Just be careful how much current you expose your shunt to, it can only dissipate a watt so: 2amp * .5vdrop = 1 watt (my shunts continuous limit or 20 amps at a 10% duty cycle)

Thanks for reading.