Stompbox Power Anatomy
Most effect pedals can be powered by a battery or by an AC adaptor. Additionally, the battery and AC adaptor jack are wired in such a way that when you insert the AC adaptor plug, the battery is automatically disconnected. (It wouldn’t make much sense to have a circuit where battery continued to drain when it was plugged into and AC adaptor.) Finally, you’ll want a way to disconnect the battery when you are not using it so you don’t end up with a dead battery.
A special note here: there are two types of grounding used in pedals. Negative ground and positive ground. Negative ground (meaning the ground is connected to the negative side of the power supply) is the most common. Positive ground is the opposite. All the stuff on this page assumes a negative ground. If you are working with a positive ground circuit, hold this web page up to a mirror. (Just kidding). But really, this page is about Negative Ground Power Wiring.
Stompbox Power Anatomy
The Standard AC Adaptor Jack
The AC adaptor part of the switching is accomplished through the use of a three-lug AC adaptor jack. The most common type used in pedals is the 2.1 mm, tip-negative jack and plug arrangement. This is what you’ll find on typical commercial pedals like those from Boss, Ibanez, etc.
Here are the standard views and pin-outs for the standard three-prong 2.1 mm AC adaptor jack:
And here’s what the plug looks like:
But if the power supply (battery or adaptor) only needs positive and negative connections, why the three-lug jack? That’s the magic of the jack: one lug is for negative, and two are for positive. When you insert your AC adaptor plug, the jack automatically disconnects one of the positive lugs for you. When there is no plug inserted, pin 2 and 3 are connected together internally in the jack. When you insert the jack into the plug, the connection between pins 2 and 3 is broken, the sleeve (+) of the plug is connected to pin 2, and the tip of the plug (-) is connected to pin 1.
Wiring in the Battery
So we can use this switch with a simple wiring scheme to disconnect the battery when the AC adaptor plug is inserted. Here’s the diagram of how this works. First we’ll look at how the circuit works with no plug inserted in the jack:
Remember how pins 2 and 3 are internally connected inside the jack? So when the plug is not inserted, power flows from the battery, through pins 2 and 3, and on to the circuit board. What happens when you insert the AC adaptor plug? The “jumper” inside the jack is disconnected, which effectively removes the battery from the circuit and replaces it with the positive signal from the AC adaptor plug:
Using the Input Jack As a Switch
So now we have the circuit set up so that inserting an AC adaptor plug will disconnect the battery. But what about disconnecting the battery when there is no AC adaptor? In other words, how do you turn your pedal “off”? As you’re well aware, you do this with almost every pedal out there by simply unplugging your cord from the guitar input jack. Done. That’s all there is to it.
But how does this work. You’ve probably already guessed that it uses the same mechanism as we discovered in our review of the AC adaptor jack: a three prong jack. The ¼” input jack on a pedal is actually a stereo jack—that’s why it has three lugs. But our guitar outputs mono, not stereo. So we can use the third lug for a switch. The following diagram shows how this works:
The Tip carries the signal, and the ring and sleeve act as a switch and carry the ground. When you have nothing inserted in the jack, there is no connection between the ring and the sleeve. However, as soon as you plug in your guitar (or any ¼” plug) the ring and sleeve lugs are connected. This is the “switch” you use to disconnect the battery when you unplug your guitar.
So now we have the complete power handling circuit in place. It accounts for battery, AC, and turning the stompbox on and off.
The following diagrams show how the negative terminal of the battery is connected to the ring of the input jack. With no plug inserted, the negative line from the battery has nowhere to go, it essentially disconnects the battery. Plug in your guitar and the battery’s negative line is connected to the input jack’s sleeve, which is connected to ground.
No Jack Inserted:
And there you have it: the anatomy of stompbox power.
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