Author Options:

Powering multiple devices with different voltage using one wall wart. Answered

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!



8 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 />

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.

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.)

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.

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 />

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.


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.

I think he means the <a href="https://www.instructables.com/community/The_Laws_a_rebranding_for_newer_members/">Zeroth Law</a>.<br /> <br /> <small><small><small><small><small><small><small><small>(I thought you were leaving?)<br /></small></small></small></small></small></small></small></small>

That is in fact what I meant. "The zeroth law" makes me think of <a href="http://en.wikipedia.org/wiki/Zeroth_law">this zeroth law</a>, which I consider to be kind of pointless because it's just restating "if A=B and B=C then A=C".

It's not as pointless as you think. It is actually making a statement that temperature is a well-defined, quantifiable, and universally applicable state of a system. It is also the definition of "thermal equilibrium," which is a core concept in thermodynamics and heat engines.<br /> <br /> The zeroth law does <em>not</em> say "A=B, B=C => A=C". It says "T(A)=T(B), T(B)=T(C) => T(A)=T(C)." It tells you that there is a property (temperature) of an isolated system which you can measure, and which will tell you unequivocally what will happen if you bring different isolated systems into contact.<br /> <br /> If you measure the temperatures of all three systems T(A), T(B), T(C), using whatever technique you like, and the values are equal, then you <strong>know</strong> that if you bring them into thermal contact with one another, you will not be able to extract any useful work through the (thermal) connection.<br />

Think about that the next time you want to go trash a law of physics. :D

How come you're telling me, instead of PKM?

;-(  Now Kelsey sad...

I got the message anyway, as reply to is a transitive relation for the purposes of emailing me notifications (ie LiRa's post was a reply to a reply so I got a "reply" email).

Stupid zeroth law, got me set on thinking about transitive closures now >_<

awww, I hope you get <em>closure</em> soon ;-) <div id="refHTML"> </div>

Oops, seems I hit REPLY on the wrong comment!

Awww, don't be sad! I fix it!

<a href="http://www.planet-scicast.com/view_clip.cfm?cit_id=2871">I see you!</a><br />

:-) Luckily, I don't have to wear a tie, even <a href="http://www.pbs.org/wgbh/nova/einstein/">when I'm on TV</a>.<br />

P.S.  Congratulations on a successful hijacking! ;-)

Look under "The Equation Today", and I'm one of the three "young physicists" who wrote <a href="http://www.pbs.org/wgbh/nova/einstein/toda-kelsey.html">essay</a>s for the Web site. If you happen to have TiVo'ed the episode, you can just fast-forward to the last fifteen minutes and see me in all my Hawaiian-shirted <strike>laziness</strike> glory :-)<br />

I still say that was one of the weirdest things ever to stay up late watching that episode and like the next week meet a guy I saw on it. :D

>smirks< I DVRed it. IT'S ON MY HDD YO!

Oh, dear, not anymore, I checked and it seems someone deleted it...

That is the reason for the naming, and the wording of the full set.

(II thought i cleaned up the bloody mess...)

Just a rehash of mever's laser vortex... but I certainly intend on making some improvements/modifications so post it I will (per the addendum).

Try 2 voltage regulators, one 9v and the other 3v.

A single nine volt regulator would work for both. Think about it.

I admit, I hadn't really considered that option (didn't have my "two jug problem" hat on).  That said, in this specific case I'd be a bit unhappy about connecting a fan (electric motor, lots of electrical noise) and a sensitive solid-state device together in this way.  Using two regulators is more proper and should help isolate the two power supplies from each other.

Most laser modules have built in protection circuits to filter out the noise. Usually.

alrighty, this is what you could do

first off, that's a laser pointer, right?  not just a laser diode or anything, like it's a complete module that was powered off of batteries or something before, right?

PC fans are usually 12 volts, so you can just connect that directly to the wallwart.

As for the laser question, you have to answer my above question first :P

I think I got an answer somehwere above but I'm not 100% because they went all non sequitur on me... so I'm interested in hearing your input.

Yes, it is a laser pointer. And your also probably correct about the fan being 12v instead of 9.