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Bugger

Limiting voltage

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Well, I have this project in mind and and got things figured except for one part...

I have an input of 9v and I want to output 2.2v or 4.0v...

Any help?

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You can either use a voltage regulator, or a resistor.

A voltage regulator gives the advantage of supplying a constant voltage, no matter what the resistance or current draw of the circuit is. This is the best way to do this, you can get an LM317 anywhere that sells this stuff.

With a resistor, you can calculate what current your circuit will draw and find a resistor value to cause a large enough forward voltage drop to bring it down to the desired voltage. This resistor will also dissipate a lot of heat, so remember your P=IV and choose a resistor that won't burn out and be aware it can get really hot, so don't start any fires.

When you're stepping voltage down, you're always wasting energy. If possible, use a smaller battery to begin with or something.

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The 'best' way depends on how much current you need at the low voltage, is this powering a circuit or just acting as a reference point?

For a reference voltage, you can use a zener diode with an inline resistor. In nasty ASCII art:

+9V --==R==--+----{<|--- 0V (where the '+' is the reference voltage)

The zener will regulate to it's marked voltage providing that the current through it is correct (normally around 5 to 10mA), the resistor provides most of the voltage drop (at the chosen current) and the zener will 'fine tune' the voltage smoothing out any fluctuations in voltage/resistance.

This only works if there is a minimal/constant current draw from the '+' point, as too much variation in current here will affect the voltage, hence this is a 'reference circuit".

The amount of energy used is the 'power lost' by the circuit (normally as heat). If the currents are low, the power loss will be low too, even if the voltage drop is large.

Cheers,

Mungewell.

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A voltage regulator gives the advantage of supplying a constant voltage, no matter what the resistance or current draw of the circuit is. This is the best way to do this, you can get an LM317 anywhere that sells this stuff.

LM317, can i configure that to regulate the voltage to an output i want?

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Yes, it's an adjustable voltage regulator.

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Well, here's a crappy sketch I drew of what I'm thinking of...

Please throw as much suggestions and ideas as you like

diagram.jpg

The idea of this sketch is, well, I'm too broke to buy a flash for my dSLR camera, so I thought of making one by using white LEDs... I don't want it to flash neither connect it to the camera's battery... I want it to have it's own switch and DC power source (Battery)

But then again I came up with the idea of using RGB LEDs so I could choose the color I want... I know it won't give good light and all, but I'm doing it for the fun of it...

The Red's voltage should be in the 1.4v~2.2v

The Green and Blue voltages should be in the 3.somethingv~4v

The variable resistors (I drew'em wrong... I'z sorry) should control whether allow the whole voltage to pass or block it...

I hope you get my idea and all... So please help me out pulling this through (It's an idiotic idea... But it's PHUN)

Edited by Bugger
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Well first, your idea for changing the brightness of the colors might not work. Each color diode has a different voltage range, so they'll require different voltages. The brightness curve is extremely sharp. At just above the operating voltage, they'll already appear quite bright. They'll continue getting a bit brighter until finally they reach their maximum voltage, in which case they get really bright and burn up.

Also, each diode should have its own resistors. This is not necessary for simpler projects, but since you want an even light throughout all resistors, it will indeed be necessary here. If you don't have individual resistors, any diode that's just a little bit out of spec will appear significantly brighter or dimmer than the others on the same voltage source. If they are far enough out of spec, only one will light and the others won't light at all. To avoid problems like this (which are a pain to sort out), you have to use individual resistors. That means you can't use a single variable resistor for each color anyway.

The only way to reliably change the brightness of an LED is to use pulse width modulation. Though you can still do what you want. Get a small microcontroller with 3 analog inputs and either 3 pwm outputs or bit-bang the pwm yourself. There are probably PWM controllers that can do similar things as well, but I've never used them.

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I actually want to avoid microcontrollers as much as possible...

Well, I split the line to three lines, each line has it's own variable resister and regulator...

If that sketch is so annoying, you think using 3 different LEDs (Red, Green, Blue [Or maybe 4; RGGB]) would be a better idea?

Edited by Bugger
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Using 3 discrete LEDs is exactly the same as using an RGB LED. An RGB LED is just 3 differently tuned diodes on the same chip.

You also only need a single voltage regulator. The regulator should be tuned for the to the ideal voltage for the highest voltage drop of your diodes. In this case, that'll be the blue LED, which probably needs around 3.5v (it'll be in the datasheet for your RGB LEDs). Each diode that needs a voltage less than that needs a resistor to bring the voltage down to its ideal voltage.

Without PWM, you're going to have a hard time controlling the exact brightness of your LEDs. It has just occurred to me that you can use a 555 for PWM (duh? I've done that numerous times :P), so no microcontroller is needed. However, the component count of your circuit just went up. You'll need 3 555 chips, plus the 2 resistors and 2 capacitors required for them to operate in astable mode. The 555 also can't source much current, so an additional beefy transistor will be required for each output.

If you were to use 2 556 chips (a 556 is just two 555s on the same chip), you could configure the extra 555 to be a one-shot triggered by a button. This could trigger the output for about half a second (or less, or more, you could add a potentiometer to configure that) to give a good flash.

Though with a microcontroller, your chip count and other component count would be lower. A single ATtiny2313 could be used, with 3 or 4 potentiometers, a button and some transistors (micrcontrollers can't source much current either). There would be no need for the resistors and capacitors for the 555s, as well as only one microcontroller instead of 2 556 chips. Don't be afraid of microcontrollers, the programming is very easy and it really would make this project easier to build.

But there's an easy way out. Replace the potentiometers with switches. You'll only be able to select from 8 colors, but that's better than no control at all. It'll be quite easy to build, requiring only a single voltage regulator and resistors for the diodes. One 555 could provide the timing for the flash, activating a single transistor all the LEDs can sink through. This would be the easiest, but certainly not the best, way to build a flash that can flash any color.

One last thing you have to consider: brightness. There's a reason camera flashes use xenon bulbs, they really have to put out a lot of light or they're useless in all but the darkest situations. You're going to need a lot of LEDs here, and a good reflector. You're going to have to do some experimentation to see if 9 RGB LEDs (27 LEDs really) is going to be enough. If it's not, you might have troubles actually constructing this. Too many LEDs and it becomes a wiring nightmare, especially if you're trying to cram it all into a small enclosure. Look around for the brightest LEDs you can find.

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Well Ohm, can you fix my sketch because you made me initialize my "i'm totally lost" mode >.>

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