A wall-wart alternative

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I finally got tired of replacing the 2 C-cells in a clock with a moving pendulum, so I measured the current (3 volts at 340 microamps) and built a charge pump like this, viewed in a fixed font:
0.047 uF @400 V || | -----------||--------------------->|------------> || | | | | | 3V 120 VAC --- --- to clock ^ _ 0.047 uF @400 V | | || | | -----------||-----------------------------------> ||
I used two caps in case one shorts and for some ground isolation. The diodes are 5.1 V zeners ($1.29 for 2 at Radio Shack) to limit the clock voltage if someone unplugs the batteries. This circuit supplies 390 microamps. I hope to avoid replacing the batteries until their shelf life runs out in 2012 :-)
This could be useful for lots of low-power stuff, eg smoke detectors.
Nick
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snipped-for-privacy@ece.villanova.edu wrote:

We used to use a similar circuit for powering electronic thermostats when I worked for Potterton Boilers here in Blighty. We used just one Class X capacitor feeding a 24V zener and smoothing capacitor.
I also used to use one for charging the NiCads in my caving lamp. Of course I made the classic mistake of plugging it in then picking up the crocodile clips to connect onto the battery (rather than connecting onto the battery then plugging in). Rectified 240V doesn't half hurt.
--
Malc


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I used a ac adapter of the proper voltage to charge a bunch of D sixed nicads to run my answering machine during power failures. it was like a UPS uninterruptiple power supply.
my old machine would spaz with any power failure:(
The batteries went bad after 6 years, my new machine a different voltage with a battery backup of memory, so I dont use this anymore
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I hope you are using rechargable batteries!
Regulart alkaline or others may dry out, overheat and start a fire!
USE ONLY RECHARGABLE BATTERIES IN THIS APPLICATION!
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NO!

How much will they overheat if overchaged at 3Vx50uA = 150 microwatts? :-)

NO!
Happy new year,
Nick
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I like the concept, but it's hardly worth doing if you value the portability of the object and batteries can be had so cheaply.
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What is your child's life worth when they need to steal a battery for their latest toy?
That's just idiotic.

you value the

cheaply.
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Rich Komp replies:
Nick, You are right. I have used non-rechargable batteries for years with solar battery chargers. The current is controlled by the number of photons going into the PV cells, not the internal resistance of the batteries. I once deliberately grossly overcharged a regular alkaline battery in a solar charger and got it to explode like a firecracker, but it took almost an amp for a single AA cell, not the microamps we are discussing. I will admit that after 5 years or so in a solar clock, some of the batteries have leaked, but the battery contacts, weren't too hard to clean (and I've never seen a fire from this). I like breaking rules, especially when they are so FORCEFUL. Rich

Nick
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snipped-for-privacy@ece.villanova.edu wrote:

Caps do have a failure mode, so a series safety R is normally included with these type circuits.
If your ac supply is polarised, you'd be a bit safer putting one cap in the live than one in each pole, as the output will then be at apx earth potential, though not isolated. As it stands its semi-live.
NT
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It should be noted that such *series capacitor* devices were outlawed for commercial products long ago in Europe. As the Big Cat notes, without a resistor to limit things, failures are usually dramatic.
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2 Lithium batteries would be safer, cheaper, less work, last longer and much more reliable.
Just a moronic idea
V | |

type circuits.

devices
Europe.
things,
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Looks like a shock hazard to me.
V | |

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Clark wrote:

And at that low a current, why not ferget the caps and just use a couple of 150K resistors? The additional power loss will hardly spin your meter off the wall and you'll avoid the possible cap failure problem.
Jeff (Ducking and slinking off...)
--
Jeffry Wisnia

(W1BSV + Brass Rat \'57 EE)
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V | |

It seems counter-productive to build booby-traps into smoke detectors and the like.
Unlike a wall-wart, it will require special precautions not to accidentally touch anything when you go to change the battery or maybe even set the time on a clock. You will never know whether a capacitor has shorted and whether you might be killed the next time you provide a path to earth ground.
There's nothing to limit the current when capacitors fail, so that failure will cause a fire. There's no transient voltage protection, so the odds of capacitor over-voltage failure is pretty high.
Don
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And it has battery backup :-)

I doubt that. The diodes will probably open, and I used really skinny wire.

Or diode damage. So... I added a 1K series resistor. This would also work:
0.022 uF @400 V* || | -----------||--------------------->|------------> || | | | | | 3V 120 VAC --- --- to clock ^ _ | | 10K | | -----------www----------------------------------> *Digikey's $29.95 M400-KIT-ND 400 V capacitor assortment contains 10 of these, along with 10 of 14 other values from 0.001 to 0.1 uF.
Nick
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snipped-for-privacy@ece.villanova.edu wrote:

same precautions as any mains device, all nonisolated parts must be protected from touch. The danger with this system lies not in the design itself, but in putting into the hands of people that dont know what theyre doing with it, and might use it like a wall wart. Since the safety R has been added, it can be used perfectly safely, but only when its issues are understood and appropriately addressed.
Smoke detectors could be used hapily with it if a screw were added to prevent opening.

X rated capacitors use fusible coatings, so a short causes a burn out of the conductor around the short. The overall capacitance is not noticeably affectde, IOW its self healing.

Very fine wire sounds like effective fusing to me.

Yes, though again you need the R and C in the live. The days where you could scatter proetction devices between L and N lines is long gone - at least in our part of the world anyway.
If youre using non polarised plugs on this, you'd need an R in both lines imho. These need to be safety Rs, not the kind that catch fire and carbonise.
Personally I'd make the R as high as is possible, so it responds effectively to any fault throughput incease.
NT
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wrote:>> >There's nothing to limit the current when capacitors fail, so that failure

You only need 20mA of AC current to cause ventricular fibrillation. If the capacitor fails, someone could get killed touching whatever your circuit connects to.
Don
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Don K wrote:

It would be a foolish designer that enabled users to touch the output of one of these, so thats a non issue in a competent design. If for some reason they did, Y rated caps would be more appropriate than X - but still, liability like that isnt something I'd want to take on unnecessarily.
As far as fusing, if a cap shorts, enough i will flow to pop a fine wire, assuming it really is fine. A single strand of a thin stranded equipment wire etc. So the fuse concept is good.
I suppose if one wanted to improve the safety of an unsafe device (ie nonisolated supply with outputs touchable) one could put a cap in both poles and earth the supply output. Would need to ensure R is significant to avoid RCD problems.
Note this would work as a 1 cap supply, as the E connection would effectively short out the neutral capacitor. Ick.
NT
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Don K wrote:

It would be a foolish designer that enabled users to touch the output of one of these, so thats a non issue in a competent design. If for some reason they did, Y rated caps would be more appropriate than X - but still, liability like that isnt something I'd want to take on unnecessarily.
As far as fusing, if a cap shorts, enough i will flow to pop a fine wire, assuming it really is fine. A single strand of a thin stranded equipment wire etc. So the fuse concept is good.
I suppose if one wanted to improve the safety of an unsafe device (ie nonisolated supply with outputs touchable) one could put a cap in both poles and earth the supply output. Would need to ensure R is significant to avoid RCD problems.
NT
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The rest of this stuff is cheap...
$13.65 for 100 1N5231BD1CTND 5.1V zeners, $15.86 for 1000 P10KBACTND 10K resistors, and a line cord cut off an old appliance.
Anyone else want to burn their house down? :-)
Nick
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