Ground Loops Explained

A ground loop is a phenomenon that occurs when a device has more than one path to ground. "Ground" can mean earth ground or, as in the case on motorcycles and cars, the negative terminal of the battery. Signals looking for ground will take the path of least resistance, and that path can change from moment to moment with circuit conditions. In a system that handles audio, this condition can lead to unwanted noise.

This shows examples of how ground loops can form and three of the ways to eliminate them:

The diagram at the upper left shows what happens when a radio and intercom system are connected with a grounded cable. The dotted lines show the possible paths to ground. From the perspective of either device, there are two ways to ground, one directly via the bike's negative rail and the other by way of the audio cable. Electricity will take the path of least resistance, and which path that is can change many times every second. Those changes are what lead to the noise associated with ground loops.

The way to eliminate ground loops is to make sure every device has exactly one way to find ground. There are a few different ways to accomplish this:

• Break the ground line between the radio and comm system as shown at the lower left. A signal in either device has only one way to find ground, and that's via the device's connection to the negative rail.

• Break the negative connection on one device or the other as shown on either of the diagrams on the right. Signals looking for ground will find only one way to ground, and that's through whichever device is grounded. The down side to this approach is that the current to power the ungrounded device has to be carried through the radio cable and the other device, and that usually precludes the use of anything that requires high power.

• For lines carrying AC signals such as audio, an isolation transformer can be installed. The transformer will induce the signal coming from one side onto the other with no DC connection, keeping the grounds separated.

-- MarkFeit - 08 May 2010

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