Powering multiple devices with different voltage using one wall wart.

I really hate to post such a remedial question here but after searching the better part of the entire weekend for an answer I'm not much further along than I was at the start. I even went as far as hauling my butt to the radio shack to ask them.... I don't know what I was thinking there... It's always been obvious to me the guys there don't know much more than how to push cell phones - not to say they are all like that but the one by me has never been any help in terms of electronics.

Anyway, I want to power a 5mw (3 volt) laser and one PC cooling fan (9 volt?) from one spare 12v wall wart. I've never been much for electronics but I'm thinking I can run the power to a breadboard, then run 2 seperate feeds through various resistors and/or regulators... just can't get any straight answers as to the best approach for this.... what's the best way to cut the voltage to a safe level for each device.

I'd be happy with a good instructable for something like this if someone can point me toward one.

Thanks for your help!

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PKM8 years ago
I used to think "3V + 9V = 12V, just put them in series". Sadly, it doesn't work like that.<br /> <br /> The easiest way is probably to find two voltage regulators that can run off 12V, set them up to produce 9V and 3V, and then use those supplies with the appropriate resistors etc. to power the laser and fan.<br /> <br /> Making something involving lasers and spinning things? Remember the first law!<br />
Padlock PKM8 years ago
Or you could just use one 9 volt. For 9 volts, obviously use 9 for positive and 0 for ground. For 3, use straight out 12 for positive and 9 for ground.
Some (much) electronic equipment doesn't like floating ground.  You are also coupling noise and voltage spikes between the two circuits, which could lead to performance problems.  For a fan and a laser, that may not be a big deal, but it's a poor approach to learn from.
Depending on the quality of regulator, the voltage spikes would be the same as with normal ground, or would even dampen them.

A perfect regulator, one that stayed consistently at exactly nine volts, is comparable - and equivalent - to ground. Not in terms of electrical potential, but it terms of stability - how much it changes (or, in this case, stays uniform). Thus, theoretically there is no difference between our perception of "ground" and so-called "floating" ground.

Cheap regulators that will most probably be used, like the 7809, are acceptable in terms of stability of the output. Hook one up to an oscilloscope, and you might see minor fluctuations. This, however, may be only noticeable in situations dealing with audio or other analog waves.

Not so with a fan, nor with a laser.
gmoon Padlock8 years ago
When taking this approach, shouldn't a negative regulator (7909) be used instead? That way, the negative voltage is controlled relative to the ground, instead of the ground relative to 0V (avoiding ripple on the elevated ground.)

However, IMHO, the laser is far more susceptible to any spikes or overvoltage than is the fan. When subtracting 9 regulated volts from an unregulated 12V supply, there are no guaranties what remains is 3V...

Better to use a 3V regulator for the laser diode (or a constant current regulator), and a 9V zener circuit with the fan (if it is indeed 9V and not 12V.)
Padlock gmoon8 years ago
A negative voltage regulator would not leave 3 volts, unless -12 was used as well.

It is true - the laser is more susceptible to voltage spikes then the fan. However, a laser is still not that sensitive. Most of them have protection and regulation circuits built in - and if (he/she) were using a straight out laser diode, I doubt they would be asking how to drive it straight from voltage instead of first running it through a driver.

Treating a laser as sensitive audio equipment is very juvenile - makes it more complicated, even though it is completely unnecessary.
gmoon Padlock8 years ago
Ah well, I was thinking of a 12V transformer. And correct, that still wouldn't give you a stable 3V without a separate regulator. Which takes us right back to the beginning--if anything should be regulated, it's the laser.<br /> <br /> <em>Treating a laser as sensitive audio equipment is very juvenile</em><br /> <br /> I believe the word you meant is <em>correct</em> ;-) (I kid, I kid.) Laser modules are FAR more susceptible than audio equipment to overvoltage and spike damage. Modules have no protection at all (other than a heatsink, which may or may not be present.)<br /> <br /> Laser <em>pointers</em> <strong>may</strong> have some protection circuitry, but I'd be leary of connecting one to any wall wart or inductor-type power supply....but clearly a lot don't have decent regulation:<br /> <a href="http://holography.dragonseye.com/holoworldforumarchive/326791981.html">holography.dragonseye.com/holoworldforumarchive/326791981.html</a><br /> <br /> The bane of audio equipment is ripple, not so much spikes or over-voltage (although clearly, you can fry an opamp just as easily as a laser diode.) I.E., feeding it the correct DC voltage, but with an AC noise component. <br /> <br />
Padlock gmoon8 years ago
Laser diodes in themselves do not have any protection circuits at all.

The lasers modules used, like ones below, do. This is the most common type of laser, and would be the type someone would buy unless they specifically bought a "laser diode". The one below, which was 4 dollars, is rated 3 - 4.5 volts. However, it runs fine on approx. 6.02 volts - which is what I'm running it at now. Thus, laser diodes are extremely sensitive, but laser modules have "driver" circuits in them to run the laser efficiently.
gmoon Padlock8 years ago
Many modules aren't regulated. Anyway, if he has a module with circuitry, I hope he's got the specs, too...

Either way, I don't think we're helping feign3 here, so I'm letting it drop.
As far as being a poor approach to learn from, until someone has actually studied electronics enough to know why that matters, the other way will most likely provide more of a learning experience - one that's easily understood in the eyes of a beginner.
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