Voltage regulator

[markland556]

I was wondering if anyone know of any schematics out there to regulator an input voltage. I need it to accept from 12v to 14v. And output 12v only. Thanks

Its for leds in my car and the voltage in it changes from 12v to 13.8v and the LED's will fry if they get above 12v.

~~Thanks

 

[markland556]

well of coarse after i post this i find some info.
I found this site:
**broken link removed**

From what that site says i can just hook up an LM7812 in line with my 12-13v source and the output will be 12v only.

 

[RODALCO]

Your 7812 regulator may not work perfectly because it needs about 2.5 to 3 volts more at the input to regulate properly.
So for 12 Volts out you need 15 Volts in.

I don't know what the leds in your car draw but normally 20 mA per led.

An other option you have is to put 3 x 1N4001 ( 1 amp) or 3 x 5400 ( 3 amps) diodes in series with the 12 volts supply to the leds.

When the alternator is charging the car accu the voltage will be than 13.8 - (3 x 0.6) = 12 volts for the leds.

Or use series resistors for the LED strings, bear in mind the current draw and calculate the appropiate value and wattage for the resistor.

 

[Nigel Goodwin]

I was wondering if anyone know of any schematics out there to regulator an input voltage. I need it to accept from 12v to 14v. And output 12v only. Thanks

Its for leds in my car and the voltage in it changes from 12v to 13.8v and the LED's will fry if they get above 12v.

If your LED's fry at 13.8V then you're doing something SERIOUSLY wrong, the change from 12V to 13.8V should make no real difference.

Try posting your circuit!.

 

[Tarsil]

I was wondering if anyone know of any schematics out there to regulator an input voltage. I need it to accept from 12v to 14v. And output 12v only. Thanks

Its for leds in my car and the voltage in it changes from 12v to 13.8v and the LED's will fry if they get above 12v.

~~Thanks

How about rearanging the leds and using a common LM317 or 7809 for geting let's say...9V? I use (for a economic lamp) leds powered at 3.5 V in pulses an 2.8V continuos without problems about heating. Try puting 3 white leds in series and powering from a 9V source without any limiting resistor. If are geting too hot ..reduce the voltage or use a res. A variable suply with 317 might come in handy.

 

[markland556]

Well the leds fry at 13.8 becasue i factored the resistor for only 12v and if they are getting more the led itself is probably getting about 4v, in my hundredes of experiances modding xboxs with leds they tend to die if the get too much power.
I could just factor the resistor for 13.8v but then when the car is off the leds will be somewhat dimmer..

All this is for is replacing bulbs in the cluster of my car with leds. Also in the 3rd brake light.

 

[OutToLunch]

instead of relying on the input voltage, you should create a constant current circuit to drive the LEDs - that way the input voltage can vary without affecting the intensity of the LEDs.

 

[Nigel Goodwin]

Well the leds fry at 13.8 becasue i factored the resistor for only 12v and if they are getting more the led itself is probably getting about 4v, in my hundredes of experiances modding xboxs with leds they tend to die if the get too much power.
I could just factor the resistor for 13.8v but then when the car is off the leds will be somewhat dimmer..

The difference will not be noticable! - the difference between 12V (which you shouldn't use anyway, you should use 13.8V) and 13.8V is only just over 10%, not enough to blow an LED unless you were massively over driving it in the first place!.

LED's don't get 'voltage' they take 'current', and that's what you calculate, assume 12V or 13.8V either would be fine with the other. Personally I would suggest using 13.8V in the calculations, as that's the nominal voltage of a car battery, so why would you choose anything else?.

 

[Tarsil]

Well the leds fry at 13.8 becasue i factored the resistor for only 12v and if they are getting more the led itself is probably getting about 4v, in my hundredes of experiances modding xboxs with leds they tend to die if the get too much power.
I could just factor the resistor for 13.8v but then when the car is off the leds will be somewhat dimmer..

All this is for is replacing bulbs in the cluster of my car with leds. Also in the 3rd brake light.

That's why u should use a voltage or current regulator instead of using a simple resistor. In this way u will keep the same brightnes for the leds.
The voltage regulator will alow the same power to be disipated by the LED. Using the current reg the power will be variable (not too much tought) but u might disipate (and waste) less power on the regulator.

 

[audioguru]

With a 560 ohm current-limiting resistor, a 2V red LED will have 21.1mA with a 13.8V supply. With 12V it will have 17.9mA. You won't see it dim.

Too many LEDs in series?
Five 2V red LEDs in series with a 180 ohm current-limiting will have 21.1mA like before with a 13.8V supply. With 12V they will have only 11.1mA. They will look dimmer.

 

[Tarsil]

So...use the voltage or current regulators. U can use a cheap LM317 with a heatsink and few other parts for both.

 

[Hero999]

You can use an LM317 constant current source to power LEDs from a wide voltage range.

But you can make a 12V regulator from a 13.8V source, you need a very low dropput regulator.

 

[Tarsil]

You can use an LM317 constant current source to power LEDs from a wide voltage range.

But you can make a 12V regulator from a 13.8V source, you need a very low dropput regulator.

I was thinking about geting a 9V or so voltage.
About the circuit above....can't I just use a zener diode and a logic mosfet?
Or maybe I'm wrong about the role of the 555 circuit.

 

[Nigel Goodwin]

Jesus! - this is getting silly!.

Bear in mind that the original incandescent lamps on the car vary in the same way as the voltage varies - it's NOT a problem.

EPE did an article on car replacement LED lamps in the May 2006 edition (including nice PCB designs to make replacement easy).

They used four chains of four LED's, with 150 ohm resistors in each chain, for the high stop lamp, with a series diode (1N4004) for each pair of chains. The diode is to block any high negative spikes which might kill the LED's.

 

[markland556]

Jesus! - this is getting silly!.

Bear in mind that the original incandescent lamps on the car vary in the same way as the voltage varies - it's NOT a problem.

EPE did an article on car replacement LED lamps in the May 2006 edition (including nice PCB designs to make replacement easy).

They used four chains of four LED's, with 150 ohm resistors in each chain, for the high stop lamp, with a series diode (1N4004) for each pair of chains. The diode is to block any high negative spikes which might kill the LED's.

So pretty much just run 4 LED's a 150ohm resistor and a 1N4004 diode in series and all should be well?

Id really like to keep this as simple as possible.

I can figure out the resistance of the resistor myself. Now that i know i should just wire it for 13.8 (diddnt even cross my mind before)
all should go well. What exactly is the whole point of the doide, you said a spike but im not too sure what that really means...

 

[audioguru]

So pretty much just run 4 LED's a 150ohm resistor and a 1N4004 diode in series and all should be well?

If the LEDs are red 1.8V ones like I have, when the battery voltage is 13.8V (it might be higher) then I calculate the current will be more than 39mA that will kill 30mA max LEDs pretty quickly.

 

[markland556]

hmm, im running some blue 5mm LED's not sure what voltage they run at or the reccomended mA for them.

 

[audioguru]

hmm, im running some blue 5mm LED's not sure what voltage they run at or the reccomended mA for them.

Use a 470 ohm resistor in series with a 9V battery and measure their voltage!
If you don't know their voltage then either they won't light or they will burn out:

Four 3.5V blue LEDs in series is 14V, plus more volts for the current-limiting resistor. A car doesn't have such a high voltage.

If you don't know their voltage then how are you going to calculate a safe current-limiting resistor value?

Most LEDs have a max continuous current of 30mA.

 

[markland556]

iv figured out its 3.3v at 20mA. So ill run 4 leds with a 150ohm resistor, what does the diode do really and is it needed?

 

[audioguru]

Four 3.3V LEDs need a minimum of 13.2V, plus 0.7V for the diode= 13.9V. For 20mA through the 150 ohm current-limiting resistor, it will need 3V across it and the total voltage needed is 16.9V. You don't have such a high voltage.

If you use three 3.3V blue LEDs in series with a diode and a 150 ohm resistor then the current will be 21.3mA with a 13.8V supply, but only 9.3mA with 12V.

Cars have voltage spikes on the supply voltage that destroys sensitive parts. The absolute max reverse voltage for an LED is only 5V. A diode prevents any reverse voltage.