Please help me understand this soft latching MOSFET switch

[mianchen]

#!Noob Alert!#

I love this little circuit and have used it in a few of my projects. Minimal parts count and easy to make. I'm even thinking of designing a tiny board for it and had the PCB made so I can use it in my other project easily.

Anyway, when I connected this to a headphone amp (TDA1308) I made for my wife, it won't turn off.

I managed to make it work by connecting an LED between the OUT and GND of the switch circuit and increased the cap to 200nF.  I don't want to use an LED in this case, and I don't quite understand why it has to be there to make it work. Please enlighten me.

[cowana]

The LED is acting as a load for the circuit. You should be able to connect the circuit you want to use this for in its place - and you could always add an external load resistor to pull a bit more current if required.

[mianchen]

It worked after I remove the 100uF cap from the TDA1308 circuit...... no LED or load resistor needed 

[Jon86]

It worked after I remove the 100uF cap from the TDA1308 circuit...... no LED or load resistor needed 

You shouldn't remove that cap, an amplifier needs decoupling. Did it not turn off at all, or did it turn off after a while?

[mianchen]

It worked after I remove the 100uF cap from the TDA1308 circuit...... no LED or load resistor needed 

You shouldn't remove that cap, an amplifier needs decoupling. Did it not turn off at all, or did it turn off after a while?

It didnt turn off at all with the 100uF cap. When the 100uF is replaced with a 1uF cap it works. The amplifier is powered by battery - is 1uF enough of decoupling?

[Dave]

The problem is that the decoupling capacitor has enough charge turn Q1 back on after you switch it off with S1.
You could try putting a diode after your switching circuit, so the charge from the decoupling cap can't flow back. Might as well use a schottky, to reduce the voltage drop.

[mianchen]

The problem is that the decoupling capacitor has enough charge turn Q1 back on after you switch it off with S1.
You could try putting a diode after your switching circuit, so the charge from the decoupling cap can't flow back. Might as well use a schottky, to reduce the voltage drop.

  Thank you Dave.  why didn't I think of that? I knew it's something to do with the cap but a diode never came across my mind 

sorted