Hi,

I am building an effects pedal at the minute that requires a mains supply but also has the option to run it on a 9v battery. I've also got a circuit board that I'm trying to reverse engineer to work out how it operates but doesn't make any sense to me as the mains power supply goes into the circuit one way from the positive pin then the battery clip positive goes from the side pin of the barrel jack through the battery then out to the input TRS jack on the Ring that isn't connected to anything else. I'm not sure how the power gets in to the circuit from the battery. Looking at it there's a diode coming out from positive mains via a resistor, the diode in parallel with a capacitor. I didn't know if this had anything to do with it but I just expected this to be to breakdown over 9v and the cap for noise rather than for any sort of current running through the battery from the mains.. Here's a picture of the circuit. It looks very basic but there's something I'm missing...

Many thanks (English-only site -Moderator.)

 

Welcome to AAC.

It sounds like they are using the switch built into the barrel jack to disconnect the battery if you plug in a mains adapter.

 

Welcome to AAC.

It sounds like they are using the switch built into the barrel jack to disconnect the battery if you plug in a mains adapter.

Ah I see, very clever. So when the adapter isn't plugged in the current from the battery runs back through the jack and into the circuit.

 

Here is an image from Fuzz Central.



The lower jack is for the guitar cable. The upper jack is for external mains power.

As shown, the battery powers the circuit through the switch built into the DC power jack. When the power plug is inserted, its barrel breaks the switch contact to the battery while making contact with the +9 V wiring to the circuit board.

The battery negative terminal and the external 9 V negative wire coming from the power jack center pin are tied together and connected to the Ring contact of the guitar jack. When the guitar cable is plugged in, the long sleeve on a standard guitar mono cable connector makes contact with both the sleeve and ring contacts inside the jack. This completes the circuit from the battery negative / external mains negative to the pc board GND. Thus, the guitar cable acts as the on/off switch.

ak

 

A.K. presented just what I was going to present, and it is exactly correct. So AK beat me to it.
I recently repaired one of those pedals and that is the circuit arrangement used, and that is exactly why.

 

Using a stereo jack with a mono plug is a very common way of eliminating an on/off switch that can be forgotten. One of the first things everyone does when finished playing is unplug the guitar.

ak

 

Certainly AK is correct. The other benefit is that using that scheme there is no on/off switch that accidentally gets switched on while handling and the resulting dead battery next time. But it would certainly be confusing to those not aware of the scheme.