5 volt regulator circuit question

lboucher

Hi All

Attached you will find a typical 5 volt regulator circuit.
A 3 terminal 5 volt regulator with 2 capacitors to control voltage ripple.
My question is fairly easy i am sure. (I am an ME after all.)
What is the Diode D3 for?

Let me take a guess and say that this is to make sure power is drained quickly once the power switch is turned off? If this is true, does it matter if you switch the power on the input side compared to the ground side?

Long story short. I followed plans from others when building my christmas stuff. Recently at work i started using the Parallax SX 28 Protoboard and i have one board which is giving me flaky behaviour. Almost like it isn't turning off when i flip the off switch. I am switching it on the Vin side. So for the longest time i wondered what that diode is for, then when i saw this problem at work it dawed on me that maybe this is what it is for and i need to add a diode to my board.

Thoughts?

Thanks
Lucas

lboucher

After doing some more thinking.

I don't think the diode is what i need, i think i just need to enable brown out protection.

Probably BOR42.

I think that would explain the odd behaviour i saw at work. (Hummm??)

Mike Green

The diode is to protect the regulator when there's a large capacitor on the output side of the regulator. If you quickly turn off the input power supply, the output capacitor can discharge quite a lot of current quickly through the regulator and damage it. If you have a diode from output to input, most of the current passes through the diode which can handle it. Some regulators don't have this problem. The manufacturer's datasheet usually will give you some parameters to decide whether it's necessary.