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

 

well of coarse after i post this i find some info.
I found this site:
http://www.southwest.com.au/~jfuller...regulators.htm

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.

 

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.

__________________
There are more ways to get to Rome.

Electricity, Electric clocks, Meters and Trains are great.

 

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!.

__________________
PIC programmer software, and PIC Tutorials at:
http://www.winpicprog.co.uk

 

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.

__________________
If is too complicated.....most of the times is the wrong way.

 

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.

 

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.

 

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?.

__________________
PIC programmer software, and PIC Tutorials at:
http://www.winpicprog.co.uk

 

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.

__________________
If is too complicated.....most of the times is the wrong way.

 

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.

__________________
Uncle $crooge

 

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

__________________
If is too complicated.....most of the times is the wrong way.

 

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 also post at the following sites:
http://www.stop-microsoft.org http://www.heated-debates.com
Screen name: Aloone_Jonez
And http://www.silicontronics.com, same screen name as here.

 

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.

__________________
If is too complicated.....most of the times is the wrong way.

 

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.

__________________
PIC programmer software, and PIC Tutorials at:
http://www.winpicprog.co.uk

 

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...