# Low Current Circuits with Car (Automobile) Batteries

#### pandian

Joined Sep 27, 2009
33
Hi frens and pros,
I'm new here. I need some clarification. Hope you ppl will help.

I'm trying to install a low current electronic circuit (eg.: LED indicators, LED based diagnostic panel, LED based room lamp etc.) in my car.
Usual automobile batteries are rated at 40 to 70amps.
Is it possible to use a normal zener/ resistor or IC based voltege regulator to power such low current circuits?
Could the circuit or voltage regulator components stand for such a high current?

Pandian

#### Wendy

Joined Mar 24, 2008
21,840
If you look at Ohm's Law (V=IR) you will see a device only pulls the current it needs, so connecting a low current device to a high current system is not a problem. The reverse is not true.

Voltage is a different issue, but as long as the device is rated for the same voltage no problem.

Cars tend to have dirty power electrically, which has to be adjusted for in the low power device.

#### pandian

Joined Sep 27, 2009
33
If you look at Ohm's Law (V=IR) you will see a device only pulls the current it needs, so connecting a low current device to a high current system is not a problem. The reverse is not true.

Voltage is a different issue, but as long as the device is rated for the same voltage no problem.

Cars tend to have dirty power electrically, which has to be adjusted for in the low power device.
Thanks alot bro.
that clears my doubt.

#### Gustav180

Joined Aug 25, 2009
17
Hello pandian

As Bill says, cars have dirty power electrically, you must think for a higher voltage range than around 12 V.

Normally when everything is OK in the car, the voltage is around 14 V, it is the charging voltage to the battery.

If the charging regulator faild, the voltage can rise to arond 18 V.

If the battery connection faild during charging, the volage from the generator under a short moment gan generate around 35 V! This is the max voltage you must calulate.

If you start the car a cold day, the start engine will take a lot of energy. The battery voltage will drop to around 5 V.

Gustav

#### pandian

Joined Sep 27, 2009
33
wow!
there's alot factors to consider?!
thats a good explanation.
Thanks alot Gustav

#### Von

Joined Oct 29, 2008
65
Mr gustav is technically correct.

But if you bias LEDs for a worst case 35 volts, they probably won't be much use at 12-14v.

Go ahead and design for max. voltage while engine running. That is how they will be used right? If you use these with the engine off mostly they would possibly be just a bit dimmer.

The chance of failure of these other items (reg., etc.) are very rare in today's autos.

#### BMorse

Joined Sep 26, 2009
2,675
You can always use a Voltage regulator to regulate your "auto" voltage to 5 volts, try MC78T05CT from Fairchild Semiconductor, can handle up to 35 volt input. Then power your indicators from that and your LED's should stay the same brightness even if the auto is off.....

#### bountyhunter

Joined Sep 7, 2009
2,498
Hi frens and pros,
I'm new here. I need some clarification. Hope you ppl will help.

I'm trying to install a low current electronic circuit (eg.: LED indicators, LED based diagnostic panel, LED based room lamp etc.) in my car.
Usual automobile batteries are rated at 40 to 70amps.
No, they are rated at 40 - 70 AMP-HOURS which is the battery's capacity.

#### zimbarak

Joined Feb 8, 2009
56
normaly a led require a current of 30 ma and arround 1.5 v for its to glow so u can use the ohms's law to calculate the resistor that might be used in series with the led ! r=v/i (without forgetting the calculation of power dissipation ) p=r.i squar

#### pandian

Joined Sep 27, 2009
33
Any suggestion circuit diagram available?thanks

#### sillyfrog

Joined Sep 29, 2009
4
You can always use a Voltage regulator to regulate your "auto" voltage to 5 volts, try MC78T05CT from Fairchild Semiconductor, can handle up to 35 volt input. Then power your indicators from that and your LED's should stay the same brightness even if the auto is off.....
THis is what I did on my car.