A little design help...

[L. Daniel Rosa]

I'm lacking confidence in my ability to calculate chokes today, so I'm asking for some help. I'm improvising a vent fan in a bathroom that doesn't have one. I'm mounting six computer fans on a piece of plywood and putting it in the window. The load is 72V 0.55A and the mains voltage here is 120. I'm planning on using a single rectifier and reactive components for all the regulation I'll need. I hope to keep the ripple down to 30Vp-p which requires 191uF minimum (somebody double check this please). I was also hoping to use a choke to limit the charging current, but my figuring told me I'd need 0.13H, which seemss like it'd be a bit big (I definitely need help with this calculation). If the choke would be prohibitively large, then would somebody please confirm the resistor I'll need? I'm guessing about 50R. I was hoping I wouldn't be burning as many watts in heat as the fans will use (or more), but I'd like to keep the circuit simple and cheap. Comments please?

[Externet]

Hi Daniel.
Try an elegant approach with an interesting gyrator circuit. Fakes an inductor with a capacitor.
It's not as simple as a single inductor but it is not as massive either.
The full wave rectification on the circuit here can be instead a single rectifier as you propose :<p>http://www.mitedu.freeserve.co.uk/Circu ... yrator.htm<p>Enjoy,
Miguel

[L. Daniel Rosa]

Thanks for the link to the gyrator circuit, Miguel. Unfortunately it's inappropriate for the application. The fans are rated for continuous duty at 10% high supply, and I've tested them at +20%. I believe that they can withstand even higher, and I plan to run a six pack of them with the ripple peaks at 87V and the troughs at 57V. I did a little studying up in Switching and Linear Power Supply, Power Converter Design by Abraham I. Pressman to see if anything there would help me with my query. The nearest thing I found indicates that at the frequency I'll be using, I'll need a 3.5 henry choke (even worse than I originally believed). So it looks like I'll be using RC instead of LC, oh well.<p>[ August 11, 2002: Message edited by: L. Daniel Rosa ]</p>

[Externet]

Hi Daniel.
What about using a power transformer with multi tap primaries and feeding 120 into the 240 primary, to use it as an autotransformer; then half rectifying the ~60 AC...?
You could drop some volts with a series Zener too...would be well ventilated...
Regards,
Miguel

[dacflyer]

computer fans?<p>they draw almost nothing,,
i'd just use a 12V 1 amp wall wart.. should run them all with no problem,, and its simple as can be.......good luck

[L. Daniel Rosa]

Well, thanks for all the input fellas. I was really looking to limit the current purely by reactive components. Zeners would have the some effect as resistors, except they'd cost more. Transformers aren't cheap enough either, I'm trying to use my accumulated junk here. I'd consider winding the choke if I knew what value to target.<p>Dacflyer: If you want that material that may be relevant to correcting power factor, just let me know. I don't think there'll be any problem with scanning a few pages out of one of my school books.

[MrAl]

Hi there,<p>If i were doing that, i would get a fan that
ran at 120vac and save a lot of headaches.<p>If you have surplus 12vdc fans laying around
you want to use up, that's a different story.<p>Linear Technology makes software you can
use to simulate your series choke + cap
+ half wave rectifier
circuit easily. It's available on the
Linear Tech site for free
You can try different value inductors and
see what happens.<p>Al<p>
Added later:<p>I found that using a 0.2H choke and 100uf cap gets
you close to 0.55amps at about 70vdc. The
choke has to have about 15 ohms resistance, but
you can vary the resistance to adjust the output
voltage if you want if you use a series resistor
of small enough value to act as an adjustment while
not consuming too much power.<p>[ August 12, 2002: Message edited by: MrAl ]</p>

[haklesup]

Correct me if I am wrong but it sounds like you have six 12V, .55A DC fans wired in series for a total of 72V. <p>First, and most importantly, these fans may not be isolated from their cases posing a potential shock hazard.<p>Second, the series configuration is unreliable in that an open in any fan will stop all of them (and put 72V across the open) and a short in any fan will increase the voltage across the remaining fans (increasing their chance of failure) in either case you have six times the chances of failure than if you had only one fan.<p>I highly recommend a transformer and a full wave (4 diode) or half wave (2 diode) rectifier. Passive filtering can be minimal and regulation is unnecessary. Shoot for 12V, 3A and wire the fans in parallel. In this configuration the transformer isolates you from the line and the 12V makes it safe at the load end. Open fan failures do not disturb the other fans and a short condition can be solved with a half amp fuse in seres with each fan. This is much safer for a bathroom where there are plenty of good ground contacts (pipes) and water around. A doorbell transformer is pretty cheap or recycle one from something else.<p>In any case if you go the other way, make sure it is plugged into a GFCI outlet or breaker.<p>One more thought, (and this is may be what you wanted all along) you might get enough inductance from a fluorescent light ballast. They are rated at 120V and are intended to limit current into the load (lamp) widely available in many lamps current and wattage specs should be on the case. I had a heck of a time finding a value for inductance on the web though.

[keymaker]

Could you not use a power supply from an old PC?

[ampedtech]

I like playing with stuff too, but what you suggest is a fire bomb in your house. No disrespect intended. United Labratory and the uniform electrical code exists for a reason. (Go to their web sites and read their mission statements) Now let me whip an old school phrase on you. <p>FAIL SAFE<p>So you control your sprinkler with a solinode. What flavor do you choose? <p>NO (normaily open)
or
NC (normaily closed)<p>If you choose NC you'd be the winner cause when the NO failed, an uncontroled amount of water would dispursed. That might not seem too bad, but what if that sprinkler was in a field sloping to an aniaml pen, and your gone for two days?<p>
Fail safe, when it fails (all things do) do the least amount of damage.<p> Go to a store buy a new fan that is UL listed. Use the fan for it's intended use.<p>If your electrical and electronic experiments use elecricty that has enough power (watts) or has the right current / voltage to hurt or kill living things, do not leave them unattended (Like in your bathroom). (watts. Can you hold it in your hand when it's as hot as it will EVER get?)<p>The killing ability of eletricty is a vary tricky thing. If for intance your skin is broken your resistance (ohms) is incredability low at that point. <p>Get cut on a device with 12v @ 200ma relays. Your cut contacts the open terminals (not uncommon) on the relay and if that current goes through your hart, you are dead. It may not even take that much current. <p>IT IS YOUR RESPONCIBILTY TO KNOW THIS STUFF.<p>Would you build a pipe bomb for the 4th and hammer the end of the pipe?<p>My harsh tone was for those that responed, knew better, and said nothing.<p>This stuff can be fun. But if if you cook the dog, kill your baby sis, or burn down the house it won't be fun no more!<p>I will be happy to engage intelligent dialog about this subject.<p>Mark

[L. Daniel Rosa]

MrAl:<p>Yes, I do have a bunch of 12V fans, at least two dozen. All identical and tested. I salvaged them from equipment that was marked for obsolescence. They're rated to run on 20% overvoltage continuously, I've tested them for 40%, and I believe they can withstand up to 30V as they have the same circuitry as the 24V models (except for the stator poles).<p>I got the idea that if I used a single rectifier on mains voltage, I could use an LC filter to act as a switching regulator. Granted the ripple would be huge, but the fans won't care too much.<p>I looked in Linear Technology, and they've got a lot of stuff. If you could narrow the search a bit, I would appreciate it.<p>
hacklesup:<p>That's right, I am planning on a series connection of six 12VDC fans. These fans are very well isolated, I've looked inside more than one. Thanks for your concern. An open failure would be easily tolerated as a fail safe. A shorting failure would strain the other five units, true.<p>If something causes the current demand to rise, the capacitor will discharge more quickly, bringing down the ripple trough and reducing the average voltage. Not enough of a reduction to completely compensate, but something.<p>Thanks for the ballast idea. It may be hard to implement, though. The plywood is already cut to size, and there may not be enough room to mount one.<p>digitech:<p>I neglected to include the safety precautions I'm taking in the text for a few reasons. Among them are; many people don't want to wade thruogh that much text, many people assume I'll take (some) precautions (this one can get deep and nasty), I didn't feel like typing my safety profile 'cause I save my rabid foolishness for red hot steel.<p>Yes, I will be using a GFCI protected outlet, a three prong plug, and a fuse. The mounting will be secure enough that it is highly unlikely that the fans will fall in the toilet while I'm in the shower.<p>[ August 14, 2002: Message edited by: L. Daniel Rosa ]</p>

[Michael Vickers]

After looking the electrical code, I'm appalled that there isn't anything to stop me from cutting myself in the heart and jabbing a 12VDC wall wart into it. Could a NC solenoid attached to the end prevent this?<p>Digitech, you definitely make some good points, but I think a friendly reminder would have been sufficient, rather than a largely misspelled rant. But you're right, safety comes first.<p>Also, I'm wondering why a UL approved wall wart wouldn't be good enough to do the job. Surely this can't be an efficiency issue? Exactly how much wattage do they waste?

[dacflyer]

danial >> from what i been getting in replies , it is not feasable for me to mess with the power factor corrections...its so minimal anyway<p>still need a address to you so i cAN send you a check for postage on the displays you sent
thanx again

[MrAl]

Hello again,<p>L. Daniel,
Heres the web address of the free spice software:
http://www.linear-tech.com/software/<p>I recommend this to anyone working with
electrical circuits that is interested
in doing some analysis and/or experiments
right on the computer.<p>Al

[haklesup]

"Wall worts" molded transformers with integrated rectifiers and regulators cap out at less than 1A typically. most I have are less than 600mA. Fine for 1 or 2 fans but not 6. Higher current (compact) supplies are actually small switchers (like the ones used for laptop computers and digicams)these are much more expensive.<p>They are only inefficient when left plugged into the wall and with the load turned off.<p>[ August 14, 2002: Message edited by: haklesup ]</p>