Safe and Stable 5V Automotive Circuit ?

[iso9001]

I have pic set up to drive some accessory stuff in a auto application, I have a 7805 5V regulator, but I'm not sure about some things...

Can I hook a 12V lead directly to the regulator or is it going to need a resistor or such ? (regulators current handeling is 1amp, but my pic will draw MUCH less then that, or is it going to draw the batteries current to convert... I dont know much about this stuff)

Also, I notice I'm not getting a steady voltage, If off a 9V I'm getting about 4.50V, if from the car battery its ~5.17V. I dont have any caps on this circuit mainly b/c I have no idea where they go, anyone have any advice on this ?

Thanks,

 

[brodin]

Can I hook a 12V lead directly to the regulator or is it going to need a resistor or such ? (regulators current handeling is 1amp, but my pic will draw MUCH less then that, or is it going to draw the batteries current to convert... I dont know much about this stuff)

You can connect 12V directly to input on a 7805.

 

[Nigel Goodwin]

Also, I notice I'm not getting a steady voltage, If off a 9V I'm getting about 4.50V, if from the car battery its ~5.17V. I dont have any caps on this circuit mainly b/c I have no idea where they go, anyone have any advice on this ?

You MUST use capacitors, the regulator will be unstable if you don't - either consult the datasheet, or look at any circuit which uses a regulator IC. My tutorial processor boards use a 7805 regulator, you could look there.

 

[Russlk]

The 7805 won't last long if you connect the +12 volts directly to the 7805 input because there can be up to 60 volt spikes (due to starting motor mostly) which will blow it. Use a hash filter as available from Radio Shack.

 

[iso9001]

Thats the reason I ask about the 7805... I blew one.

What is a hash filter?

Got a pn by chance ?




ALSO: Nigel I cant find those diagram you talked about anywhere, do you have a link ?

 

[andrew2022]

ALSO: Nigel I cant find those diagram you talked about anywhere, do you have a link ?

click the link in his sig and go to P16PRO40

 

[iso9001]

Thanks, I found it, but i think its going to take some time to figureo out what is going on there and whay of that ' I ' need to incorperate into my designs.

 

[Russlk]

270-050 is a Radio Shack filter, but it is expensive for what it does. The old 270-030 was reasonable, but it is not carried any more. Radio Shack has hardly anything useful any more. A series inductor and shunt capacitor is what is needed.

 

[Nigel Goodwin]

Thanks, I found it, but i think its going to take some time to figureo out what is going on there and whay of that ' I ' need to incorperate into my designs.

Have a look at the circuit from one of my tutorials, it uses a 7805 and a couple of capacitors.

 

[iso9001]

Sorry, I've ben out for a few days...

Um.... I went and found the radioshack 270050A and..... its like a 2lb noise filter. WAY too big for what I'm doing.

I have no idea what a series inducer and shunt capacitor do.

Is that what I need to stop voltage spikes from buring my 7805 ?

Suggestions ?


Btw: I put the caps on and they dont appear to do anything.... Its ~5.24 w/ them off or in place. Does it matter where in the circuit they are (ie: before the feed to the pic or after?)

 

[Nigel Goodwin]

Btw: I put the caps on and they dont appear to do anything.... Its ~5.24 w/ them off or in place. Does it matter where in the circuit they are (ie: before the feed to the pic or after?)

The capacitors need to be as close as possible to the 7805.