# Regulated DC to DC car battery converter

Discussion in 'General Electronics Chat' started by Lanz, Jun 25, 2009.

1. ### Lanz Thread Starter Active Member

Oct 30, 2008
153
0
I want to light up a LED from the car battery.
Because car battery have high current,i need a regulator to drop the voltage to 3V to drive the LED.
I just need a simple circuit.
Anyone know how to do it?

2. ### SgtWookie Expert

Jul 17, 2007
22,182
1,728
Are you going to be operating the LED while the car is running?
If so, the electrical system of the car will measure around 14v. In the formula below, this will be called Vsupply.

Let's say that the LED you wish to power has a Vf (forward voltage) of 3v @ 20mA.
You can use a simple resistor to limit the current through the LED.
Rlimit >= (Vsupply - VfLED) / DesiredCurrent
Rlimit >= (14v - 3v) / 20mA
Rlimit >= 11v/0.02A
Rlimit >= 550 Ohms.
The closest standard E24 resistor value is 560 Ohms.
Table of standard values: http://www.logwell.com/tech/components/resistor_values.html

Let's determine the actual current.
I = E/R (Current = Voltage / Resistance)
I = 11v / 560 Ohms
I = 19.64mA
Let's determine the power rating necessary for the resistor.
P = EI (Power in Watts = Voltage x Current)
P = 11v x 19.64mA
P = 11v x 0.01964A
P = 0.21604 Watts
This is nearly 1/4 Watt. For reliability, we always double the wattage requirement; otherwise the resistor will be very hot to the touch.

You will need a 1/2 Watt, 560 Ohm resistor.

If you want the current regulated, you could use an LM317 regulator with a 62 Ohm resistor.
1) Connect the IN terminal of the LM317 to the electrical system's + voltage using a 1/4A fuse.
2) Connect the 62 Ohm resistor from the OUT terminal to the ADJ terminal.
4) Connect the cathode of the LED to ground.
Formula for using the LM317 in current regulator mode:
R1 = Vref/DesiredCurrent, where:
10mA <= DesiredCurrent <= 1.5A
1.2v <= Vref <= 1.3v; Vref is nominally 1.25v.
Alternatively:
Current = Vref/R1

Last edited: Jun 25, 2009
3. ### Lanz Thread Starter Active Member

Oct 30, 2008
153
0
Thanks for the info Sgtwookie.
I was exposed to LM78xx all this long.
I dont know abt other regulator especially adjustable one.
But i would like to learn some other regulator to increase my knowledge.
Can you explain to me the difference of LM78xx and LM317 and the benefit using LM317 on this circuit?
Actually i want to light up many LEDs.
I want to set up lights under the body near the side skirts,like neon light used in most of NFS Underground cars that looks cool in the night.
And i just thinking to use purple LED but never heard of that colour LED available at the shop.So i want to combine blue and red LED to bring out the purple colour.Is it possible?

4. ### Skeebopstop Active Member

Jan 9, 2009
358
3
Many LEDs now a days can be wired straight up to a supply. They will not drop more than their 2V forward voltage, or whatever it is and won't let too excessive currents.

Check it out. You could probably find some that you can just hook up.

5. ### Lanz Thread Starter Active Member

Oct 30, 2008
153
0
But i want to hook up the LED to a car battery which has high current and voltage.
Wont the high current spoil the LED?

6. ### THE_RB AAC Fanatic!

Feb 11, 2008
5,435
1,305

I'll have nightmares now about the poor OP touching LED after LED directly onto a 12v car battery terminals!

"well they DID work! really bright for about 1 second but now they stopped working... how do I make them start working again?"

7. ### Skeebopstop Active Member

Jan 9, 2009
358
3
http://www.farnell.com/datasheets/59322.pdf

like I said, a lot of em now a days come able to be slapped onto high voltage sources. This particular one has built in resistor, most of em probably do. I imagine you can even get some surface mount small package ones with inline resistor on the silicon.

8. ### Skeebopstop Active Member

Jan 9, 2009
358
3
Further to my last post, you could just go for one which has a Vf-max of 48V or something and be safe with it.

Check you'll get some decent luminosity at 12V though. From my experience, LEDs are pretty good over a wide range. I have one on a design now which maintains a reasonable luminosity when it is shining from 400V or 50V. Hardly notice the difference.

9. ### SgtWookie Expert

Jul 17, 2007
22,182
1,728
The LM78xx series are designed to be simple fixed voltage regulators, only requiring external capacitors to prevent oscillations and stabilize the output voltages. They don't require external resistors. However, they can't be used (practically) as current regulators.

The LM317 (1.5A) and variants LM317L (100mA), LM317M (500mA) require at least one external resistor from the OUT to ADJ terminal to function as a current regulator, and two (one from OUT to ADJ, and another from ADJ to GND) to act as a voltage regulator. National Semiconductor's datasheet for the LM117/LM317 contains a great deal of information about this regulator.

When used as a current regulator, the LM317 has a minimum "dropout" voltage of 3v; when used as a voltage regulator the minimum dropout is 1.7v.

The red and blue will look like red and blue, which may get you into trouble with the police. Avoid colors that are used by fire and police vehicles.

10. ### Lanz Thread Starter Active Member

Oct 30, 2008
153
0
Ya,i want to use LED rather than neon because dont want trouble with the police.I think want to use blue only as its very bright.
Sorry to ask SgtWookie,can u draw a schematic diagram for this circuit u mentioned.
Im confused with the In and Out and the Adj connection of the regulator n my LED had blow.
I measure the output voltage n it shows 12V eventough i move the VR knob,so i think im wrong somewhere.
And the regulator is very very hot.

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11. ### SgtWookie Expert

Jul 17, 2007
22,182
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Here is a link to the datasheet:
http://www.national.com/ds/LM/LM117.pdf

See page 2 to determine which terminals do what.

See the schematic on the bottom of page 19 entitled "Precision Current Limiter".
You don't need a potentiometer, just a fixed resistor.
62 Ohms will give you roughly 20mA output.

You cannot use a 78xx type regulator as a substitute for the LM317 for this regulation scheme.

12. ### Lanz Thread Starter Active Member

Oct 30, 2008
153
0
You mean this LM317,doesnt have ground pin?
So,the ground should directly connected to the LED?

13. ### Audioguru New Member

Dec 20, 2007
9,411
896
Why do kids shine blue LEDs on the ground under their cars where they cannot be seen??

14. ### SgtWookie Expert

Jul 17, 2007
22,182
1,728
No, it does not. It has IN, OUT, and ADJ. You're thinking of the 78xx/79xx series, which have IN, OUT, and GND.
The cathode of the LED should be connected to ground, and the anode to the ADJ terminal of the LM317.

15. ### Lanz Thread Starter Active Member

Oct 30, 2008
153
0
Just for fun.
Take a look at the picture below.Its neon light.I dont want mess with a cop because neon is very bright,so i tought LED as an alternative.
FYI,im not a kid.

Now i understand.Thanks for the info SgtWookie.

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16. ### Søren Senior Member

Sep 2, 2006
472
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Hi,

Apart from the voltage drop of 5V against the 1.25V for the LM317, they can be used in the exact same way - a 220 Ohm resistor is about right for 23 mA.
Not effecient, but that's howe we did before the LM317 was available.

Lanz <- You can get LEDs in just about every color and you'll get a more even purple if you just went with purple LEDs, rather than red and blue ones. Or you could go for 1W RGB LEDs and dial in whatever color suits your current mood (or make the color based on RPM or speed).

17. ### Lanz Thread Starter Active Member

Oct 30, 2008
153
0

I had searched in a nearby shop,the purple LED is not available.I will try search.Tell me more about "1W RGB LEDs".Is this RGB LED is in single LED?Im interested especially on this "make the color based on RPM or speed".

Last edited: Jun 27, 2009
18. ### Lanz Thread Starter Active Member

Oct 30, 2008
153
0
I had connected the circuit like what Sgtwookie said.
I think this is how it should be connected.
I asume that LM317 and LM117 are the same.

I had test the circuit with VR and fixed resistor(62 ohm).
No change in the voltage eventough when i move the VR knob or 62 ohm resistor.
The voltage still shows 12V when i connect to my car battery.
I dono where is the problem?

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19. ### SgtWookie Expert

Jul 17, 2007
22,182
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Aside from the LM117 having a broader temperature range, they are basically the same.

The meter has a very high impedance. The regulator is attempting to source 20mA current from the ADJ terminal, but it can't raise the output voltage high enough.

To simulate a load, use a 10 Ohm resistor from the ADJ pin to ground, and then measure the voltage across the resistor. Since I=E/R (Current = Voltage / Resistance), you should read 200mV across a 10 Ohm resistor. A 100 Ohm resistor with 20mA flowing through it would measure about 2v.

20. ### Lanz Thread Starter Active Member

Oct 30, 2008
153
0
But the voltage shows 12V,what will happen if i fix a LED at the output?
It will burn rite bcoz my first trial LED already blown.
How to get 3V 20mA via this circuit?I want use this circuit to light up LEDs.