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# LED flashlight circuit

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#### davidg58

##### New Member
Hi,

I am trying to build a led flashlight for astronomy.

The circuit has the following:

6v power supply, 3 ultrabright led's (1.8v, 30ma), 22 ohm resistor, and 1k pot for dimming (all in series).

The website that I got the information from suggests that a voltage regulator should ideally be used.

This site suggests a 7812 regulator for a 12v power supply, I used a L7806CV that I understood should be used for a 6v supply.

When I connect the circuit without the regulator the led's are MUCH brighter than when it is used.

I can build circuits, but I'm definitely not a designer. Would it be possible for someone to post a circuit with the regulator connected properly, the site that posted the info suggested two ways of connecting the chip, a voltage regulator and a current regulator, I think that I may have made a mistake with the layout.

Hi David. I'm not an expert but, for dropping the votage going to the LEDs, I use the formula R= (supply voltage - LED operating voltage)/LED current

Since your LEDs and everything else is in series, the current of the circuit would be 30 ma and the voltage for the LEDs would be 5.4 volts. So you have to get rid of 0.6 volts with a resistor to avoid blowing the LEDs. Using the formula above: (6 - 5.4)/0. 3 = 20 ohms So their 22 ohm resistor should work well to drive the LEDs.

As to the variable resistor in the circuit, I would use a 100 ohm instead of the 1k. The 100 ohm, running full, added to the fixed resistor (22 ohms) would absorb 3.7 volts leaving only 2.4 volts for all three LED or 0.8 volts each. This would probably be pretty dim if not below the cut-off voltage of the LEDs. With the 1000 ohm unit, all the useable control would be compressed at the low end - making adjustment prety sensitive.

I regret I have no idea as to the purpose of a 12 volt regulator used with a 6 volt battery. Perhaps you would like to post the web site where you got the circuit.

Hi,

I hope you have solved your problem. I saw your question a few minutes ago and I hope I can help you anyway.

Well, when you use a power regulator to feed your circuit, if you use a 7812 (12 Volt regulator) for example, you must feed this regulator with at least 13,5 Volt (or so).

When you tell that you get much brighter light without the regulator, I think the reason for that is that the voltage you are applying to the regulator is the same you are expecting on the output. This is wrong because you'll get a smaller voltage output. So, if you use a 7805 voltage regulator you should apply 6,5 or 7 Volts to get 5 Volt at the output, otherwise you'll get a smaller voltage.

Regards,
Pedro[/quote]

By spec sheet, the L7806CV has a dropout of at least 2 (typical) to 2.5 (worst case) volts.. Thus it will only regulate if you have a voltage source of 8.5V.

Whoever told you to use a voltage reg in this case is mistaken. LEDs use current regulation achieved with ballast resistors, voltage regulation doesn't work very well. LED properties vary with lot and temperature, and have a really sharply nonlinear IV curve. So at exactly 5.4v, the LEDs may pull 30mA, 10mA, or 50mA. Or, another way to say it, the LEDs at 30mA may have a voltage of 5.4V, 5.2V, or 5.6V.

22 ohms may not be enough to guarantee the current is regulated, particularly if the supply voltage varies (as with a battery).

why not use a very simple PWM circuit?
this way you have a better efficency, and the light from the LEDs will be much better.

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