5V Linear power supply design

[MagPal]

Hi,

I am designing a 5V linear power supply for an audio-controller with a 7805 regulator and do need some final design tips. The circuit is supplied with 12V DC from an external supply. The 5V are used for driving an Atmega microcontroller with multiple LEDS and transistors and a MAX232 interface. Approx. Continuous current draw is 200mA, worst case.

The current design is without any protection circuits. Two caps in parallel on the input of the 7805. 330nF and 47uF. Two caps in parallel on the output side, 100nF and 47uF.

However I do need some tips for the protection of the regulator:

Should i add a protection diode from the output of the regulator to the input to protect the regulator from having higher output voltage than input voltage with these capacitor values? Some do this, and some do not. What is the correct way?

Would it be a good idea to add a zenerclamp on the input to protect the regulator from transients? What is the best way of doing this? Any documentation would be great!

Thanks in advance

[sofaspud]

If there is a possibility of shorting the input, a diode across the regulator is a good idea. I don't think clamping the input is necessary; I'd prefer to deal with any such transients at the source. Unless it's a poor 12V source, I don't think they will be any problem.
Your concerns are addressed in Texas Instruments document SNOSBT0H, which is the NatSemi LM340/LM78xx datasheet.

[Mooly]

You add protection to suit the intended use and user/s.

If you know you will only connect it to a 12v supply then there is no need to protect against other scenarios. If you know you will never short or abuse the supply then short circuit protection needs to be basic and safety related only. ie a fuse to protect the transformer.

[MagPal]

Thanks for good advices and documents.

The input is not likely to short, so i think the regulator will do fine without in that case.

The 12V supply is a standard Mean Well supply, so i assume it have transient suppression built in. I was just curios if this could be useful in other psu circuitry as well.

About the 47uF capacitor values. What is the best way of calculating these values? I have found many ways of doing this with unregulated rectified DC, but there are less info about regulated DC. Are there any rule of thumbs in a design process to make this calculations quick and reliable?

[Mooly]

I suppose the best answer is to consult the data sheet for manufacturer of the regulator used.

In practice its not critical. A small cap ensures stability of the regulator (again it's regulator specific). 1uF, 47uF or 100uF. That wouldn't matter. A larger cap could be said to improve the transient response of the supply within limits.

Remember any decoupling is only really effective where it's applied on a PCB. Add a few cm of wire or PCB print and things can start to look very different.

So don't get hung up on caps and sizing. Small caps close to the reg in and out pins and some local decoupling on the main PCB it feeds is all that's normally needed.