Introduction

I often get questions from buyers inquiring how to use current shunts and whether or not a current shunt is the right choice for their application. In this guide, I will explain what a current shunt is, how it is used, and precautions to consider. This advice is for reference only. Please consult a professional before connecting a current shunt.

What is a current shunt and how does it work?

A current shunt is basically a very low Ohmic value, high wattage resistor that is connected in series with a load. Figure 1 shows an example of a current shunt. The shunt, like any other resistor, has a voltage drop. By calibrating the shunt's resistance, it can be made to drop a known voltage at a given current. This is referred to as the full scale current. The voltage drop across the shunt can be read with a millivolt meter and changes linearly with increases and decreases in the current passing through it.

Figure 1: An Example of a Current Shunt

How do I interpret the mV drop across a current shunt?

Current shunts are rated for a maximum current capacity and for mV drop. The mV drop rating is what determines the full scale current of the shunt. At the maximum current rating of the shunt, the voltage drop will be full scale (or the rated mV drop of the shunt).

Example:
Let’s say a current shunt is rated at 50 amps, 50mV. This means that when 50 amps is flowing through the shunt, the voltage drop across the shunt will be 50mV. Since the voltage drop is linear in relationship to the current flowing through the shunt, half the current will result in half the voltage drop (at 25 amps, the voltage drop would be 25mV). Knowing this, it is easy to interpret the mV reading across a shunt. The simple mathematic formula would be:

Current = (Vread/Vshunt) x Ishunt

What is a shunt rated meter?

Shunt rated meters take all the work out of reading the current flowing through a shunt. No calculations required as long as the full scale rating of the meter matches the mV rating rating of the shunt. Say a current shunt is rated for 50mV drop. Choose a meter whose full scale rating is 50mV. The marking on the face of the meter should also match the full scale current rating of the shunt. If your shunt is rated for a maximum current of 100 Amps, then choose a meter marked 0 to 100 amps. The marking can be modified if you cannot find a matching meter. The reason this can be done is because the meter isn’t reading 50 amps when 50 amps is flowing through a 100 amp shunt. Its reading 25 mV or ½ the shunt’s maximum rating. Just mark the half way point of the meter’s face for 50 amps, the ¼ point of the meter’s face for 25 amps, and so on.

How do I choose the right shunt?

The shunt you chose should always be rated higher than the maximum amount of current you’ll be measuring. Shunts can overheat if they are used above their maximum current rating, not to mention they will not be accurate since they are not within their usable range. Also keep in mind the voltage level of the power source. You’ll want to have adequate electrical insulation to prevent shorting of the shunt or injury to anyone accidentally touching the shunt. Another thing to keep in mind is to be sure the connections to the shunt are snug. This will prevent heating of the shunt and inaccuracies in your readings.

Precautions:

Remember, the current is flowing through the shunt and anything that comes in contact with the conductive portion of the shunt will become energized. You can avoid this by placing the shunt inside a ventilated enclosure or making the shunt inaccessible. Another thing to keep in mind is that you should never pass current through the shunt exceeding the shunt's rating. This can cause overheating.

Conclusion:

Current shunts are very handy for reading large currents. They can be used in a variety of applications and can be used with or without a shunt rated meter. Most any DMM has a 200mV range which works great for most shunts. Popular shunt mV ratings are 50 and 100 mV, although just about any mV rating is possible.
I hope you’ve found this guide useful and please check out my auctions on Ebay, user ID: ohwags

Jerry Wagoner