Friday, 3 October 2014

The Impossible Circuit

In the course of experimenting with different wiring configurations I accidentally discovered that its possible to get sound from a pickup even when only one of the ends of the coil is connected.

After the initial WTF and some checking of my wiring I realised what was going on. You can see from this diagram that one end of the pickup is connected to the positive terminal. The other is disconnected and the wire connected to the earth is connected to the metal pickup cover.

Testing with a multimeter confirmed that there was no electrical connection between the case and the disconnected wire and yet when I plugged the guitar into my amp I could hear sound at a much lower volume than normal.

So if the pickup is not connected to earth how is the sound getting through? What is not shown on this diagram is that all three wires are in a single two core shielded cable. The close proximity of the wires meant that together they were acting like a capacitor. A capacitor blocks DC which is why the meter didn't detect any electrical connection, but it does let alternating current pass.

So the circuit actually looks like the diagram to the right. It's kind of interesting but not really of much practical use. I only discovered it because my switching rig lets me configure circuits that should never exist and I accidentally configured it with the lead going to the positive terminal instead of the negative.

What it does illustrate is that sometimes you have to be careful of  cable placement because it can have unintended consequences.

No comments:

Post a Comment