Power From Car Battery

[AtomSoft]

Just wanted to ask can i simply use a 7805 regulator to power a PIC from a car battery?

Or do i need a rectifier ? because of alternator?

 

[Hero999]

The short answer is yes.

If you're running this from a car's automotive system, then I'd recommend adding some overvoltage protection.

Connect a 15V zener across the input and put the whole circuit in series with a 5W 2.2Ω resistor. The zener will absorb any large voltage spikes and the resistor will limit the zener current to a safe level, as well as reducing the power dissipation in the regulator.

 

[jimlovell777]

To add to Hero's suggestions, only use the 7805 for powering a PIC while the car is running. A 7805 will leave you with a dead battery one day if it sits too long. For projects you intend to always have on, even with the car off, you need an automotive LDO and a close eye on overall current usage.

 

[AtomSoft]

I will be sure to add a switch in the car to turn the device ON or OFF. This way it will cut the circuit completely. Ill be sure to use a 15v or 9v zenor and a nice 5w resistor. Should i use resistor on ground series or vcc? or it doesnt matteR?

 

[DirtyLude]

You should use a regulator that is spec'ed for automotive use. Like an LM2931. These regulators have built in protection from reverse voltage and load dumps.

 

[AtomSoft]

to source that would take too long. I need the supply done in 2 days. And also... if i get it i lose money since it wasnt in the original price.. $1 isnt alot but shipping is heh I rather goto RS and get a 5w resistor...

 

[ericgibbs]

I will be sure to add a switch in the car to turn the device ON or OFF. This way it will cut the circuit completely. Ill be sure to use a 15v or 9v zenor and a nice 5w resistor. Should i use resistor on ground series or vcc? or it doesnt matteR?

hi atom,
IF you do fit the zener and resistor across the battery supply be sure to fit a fuse in the +V supply lead.
DONT use a 9V zener across the battery supply!!

The best advice is from DirtyLude, use an automotive regulator

 

[DirtyLude]

It's not that important. I've got stuff running in my old car that has no protection at all and has run for years.

I did blow an ignitor by reverse connecting a battery once, though.

 

[ericgibbs]

It's not that important. I've got stuff running in my old car that has no protection at all and has run for years.

I did blow an ignitor by reverse connecting a battery once, though.

hi,
Long before the automotive regulators were on the market, I used the 78xx series of regulators in vehicles and boats with no problems.

Connect the battery to the 7805 via a 2amp diode and 2amp slow blow fuse.
Use a minimum of 25V rated 100nF ceramic & 100uF electrolytic on the input and output of the 7805.

 

[AtomSoft]

WOW THANKS!!! i have some 1A diodes only tho. I guess i should get a 2A Diode... The fuse ill buy since its cheap also. Thanks GUYS!!! As for CAPS i have some 25v and 50v so im set!!! thanks

 

[ericgibbs]

WOW THANKS!!! i have some 1A diodes only tho. I guess i should get a 2A Diode... The fuse ill buy since its cheap also. Thanks GUYS!!! As for CAPS i have some 25v and 50v so im set!!! thanks

hi,
How much current are you drawing from the 7805.?
If its less than about 500mA continuous then a 1amp diode will be fine, say a 1N400x type.

 

[AtomSoft]

yeah it will have 1 PIC, 4 595's but 1 led on at a time

 

[AtomSoft]

like 300mA MAX!!! at any givin time

 

[Oznog]

hi,
Long before the automotive regulators were on the market, I used the 78xx series of regulators in vehicles and boats with no problems.

Connect the battery to the 7805 via a 2amp diode and 2amp slow blow fuse.
Use a minimum of 25V rated 100nF ceramic & 100uF electrolytic on the input and output of the 7805.

I second this. In truth, blowing the reg due to a spike is "unlikely". If you want something working in a few days, it's no problem at all. Well, 97% sure it won't blow within the first month.

Be sure your reg can take the heat. If your load draws 300mA (bunch of LEDs or whatnot) and your car is running with the batt at 14v, that's 2.7W of heat in the reg. That's too much without a sink, and "a bit much" for a small clip-on sink, esp if the case is enclosed without convection airflow. But, a PIC alone is far under 300mA even at max clock speed.

 

[Gayan Soyza]

I burned lots of zenner diodes in automotive industry & short circuited the supply .I gave off zenners & moved to regulators like Mark Higgins suggested.

 

[AtomSoft]

300mA was a example ... i might only draw 100mA but 300 was just to show its way under 500mA

 

[3v0]

You should use a regulator that is spec'ed for automotive use. Like an LM2931. These regulators have built in protection from reverse voltage and load dumps.

I have to agree with Mark. Others have used non automotive regulators but I would not do so esp on a device I was selling. Murphy will get you. If there were no possibility of problem they would just use common regulators on cars.

The LM2931 are under $1 each. I have purchased them in 25 unit lots for maybe 35 cents. It is 100mA but I have found similar regulators in the TO-92 case up to 300mA. Next time you order pick up a few.

3v0

 

[AtomSoft]

I agree that using a special part made for the purpose is best but since this is a single time thing and i would only need 1 it makes no sense to spend $10 because of shipping to get 1 part for 1 project.

 

[AtomSoft]

Heh i found a cheap quick solution. I used a old car cell charger. Replaced/fixed fuse and now will use it to power the device

 

[yfx4]

Does anyone have product numbers for automotive-rated regulators (5v out) that will source 500mA?? If so what are some suggestions?