preparing a circuit for a dirty supply

[computer]

Heh,

I'm a little confused on what I need to change - I have a simple 7805 regulator setup with a rectifier before it and a 47u cap after it.

it will run happily and be stable on a 9v battery or a 7.5-12v DC transformer, but I would like to know how I would need to modify this for possible use in a car cigarette lighter.

Can anyone explain what protection I'd need to put in, obviously suitably rated.

thanks,
Ben

 

[Exo]

You should put 2 100n caps, one in front of the 7805 ad one behind it. The 47µF behind it is not required, its better to put a bigger cap (470 - 1000µF) in front of it.

It should work fine then off a car cigarette lighter - I've made a digital voltage meter with a pic running with a power supply like that in both a honda crx and a civic, never had any problems with it.

 

[computer]

Hi,

I didn't have any 100n caps at the time so I just put a 47u one in. Is that bad?

At the moment I'm designing the one for the dirty supply then that goes:

Input -> Rectifier -> 470u Cap -> Regulator -> 100n Cap -> Output

Ben

 

[Exo]

Like said, I would put a second 100n cap in front of it.

 

[Nigel Goodwin]

You should have smallish capacitors on both input and output, as close to the pins as possible - 0.1uF as Exo suggested is fine - but small electrolytics work as well (1-10uF).

The capacitors are there to stop the device oscillating, and even though it 'may' work OK without them, it's likely to oscillate under some load conditions.