The rectifier problem, backwards...

[KalTorak]

I've got the opposite problem of the guy with the homework problem to take 120VAC and get 6VDC.

I've got a lovely Christmas wreath with some lights. It's got a power cord, which implies it wants 120VAC.
I wish to hang it on my front door, where there's no convenient outlet, so I'd really prefer to run it off a 9V battery, and I'm willing to make the assumption that the wreath really doesn't draw much power.

So I need to take 9VDC and step it up to 120VAC.

I can figure out how to take a 555 timer and some FETs to make a 9VAC squarewave (er, 18V peak to peak, I think). What I can't figure offhand is whether it'd be more efficient to take that squarewave and run it into a 12x transformer to get a ~120VAC squarewave, or whether I should charge-pump up to get 120VDC and then switch THAT back and forth to get a 120VAC (240V p-p) squarewave. Whichever way I go, I should probably 60Hz filter the result...

Any strong feelings one way or the other?

 

[TuxDave]

That sounds awfully strange. Are you sure there isn't some type of DC converter attached to your power cord? Kinda like what they have for laptops etc... for some strange reason, I didn't believe lightbulbs ran directly off AC power.

 

[FrankSchwab]

Well, assuming standard Mini-lights on the wreath, assume 1/2 watt per bulb, or about 25 watts for a string of 50 (count'em up).

For a 9V power source with 100% step-up efficiency to 120VAC, you'd need about 3 amps (continuous) of current. Your poor little transistor battery is going to expire rapidly. Looking at the chart on Page 3, a 9V is only rated at about 0.570 A-Hrs, at low currents (much, much less at high currents).

If you really want to jury-rig it, you'd need an oscillating circuit capable of driving 5-6 amps peak, into a 120VAC->9VAC transformer that's rated for 3 amps on the 9V side. Your square wave proposal would probably work, but you'd need a big old MOSFET to switch the current, and a 9V power source that could source 5-6 amps. Assuming real world values of efficiency and internal resistance of the battery, I'd be surprised if it even lit. I'm not sure you can get 3A out of a 9V battery even by shorting the terminals together.

Besides, this would not be a reasonable circuit for someone who is not an avid electronics hobbyist to build.

Look through the hot deals forum for a cheap low-wattage power inverter, hook it up to a 12V gel cell battery, hang it on your door behind the wreath, and you're done.

/frank

 

[TuxDave]

hm... weird... so it really does run directly off AC power? That's news to me.

 

[KalTorak]

okay, that makes sense. I hadn't considered that 120VDC would be sufficient, but I should have - that saves me the trouble of doing the 555 timer, but I'd still have to do one hell of a charge pump.
As for actually building the thing, no problem - I do know how to do that sorta stuff.

3A through a MOSFET's not that much, really - I think I could push that much through a TO-220 or a D2PAK, depending on whether I'm surface-mounting stuff or not. As I mentioned, I haven't looked at either how much energy one gets out of a 9V bat, nor how much power the bulbs take.

it'd probably be fastest to take 12 9V bats, hook them in series, and connect the power cord to the ends.

 

[Geniere]

Originally posted by: KalTorak
okay, that makes sense. I hadn't considered that 120VDC would be sufficient, but I should have - that saves me the trouble of doing the 555 timer, but I'd still have to do one hell of a charge pump.
As for actually building the thing, no problem - I do know how to do that sorta stuff.

3A through a MOSFET's not that much, really - I think I could push that much through a TO-220 or a D2PAK, depending on whether I'm surface-mounting stuff or not. As I mentioned, I haven't looked at either how much energy one gets out of a 9V bat, nor how much power the bulbs take.

it'd probably be fastest to take 12 9V bats, hook them in series, and connect the power cord to the ends.

Re-read FrankSchwab's post again. The typical 9volt battery can't deliver enough power for your needs. Connected in series, the light set will be only at decent intensity for a few seconds with zero intensity in 10's of seconds.

To get something to work for a resonable period of time you'd have to think in the order of an automobile battery with an 12vdc to 120vac inverter. An auto battery should be good for 2-3 hours of decent light intensity.

 

[FrankSchwab]

Actually, KalTorak is on to something - 12 9v batteries would give you 108VDC, and should be able to drive the lights for an hour or two. That just might work!

Not exactly environmentally conscious (throwing away 12 9v batteries every evening), nor particularly cheap ($12 / night at EBay prices). Maybe 10-12 9.6V RC battery packs? Roughly $10 / ea on Ebay, with about 2 A-Hrs each. Should run about 10 hours; pop'em on a makeshift charger in the morning (250V Bridge rectifier and a big enough resistor to limit current to about 100 ma) and you're done!.

/frank

 

[damonpip]

First, there is no way you will be able to run them on a 9v battery, the wattage of the lights is simply too high. If you must use batteries, I would reccomend a 12v lead acid battery and a inverter, like you'd use in a car. It would probably be easier to just run an extension cord though.

 

[FrankSchwab]

What part of my derivation did you not understand?

One 9v battery - not a chance in hell.

12 9v batteries - you're asking for 250 ma out of each battery - where's the problem?

/frank

 

[TuxDave]

Originally posted by: FrankSchwab
What part of my derivation did you not understand?

One 9v battery - not a chance in hell.

12 9v batteries - you're asking for 250 ma out of each battery - where's the problem?

/frank

I think it's for the very problem that you mentioned above. You're throwing away 12 batteries every 2 hours. I agree, you should either find a better power supply or find a lower power xmas wreathe.

 

[RaynorWolfcastle]

Umm... Maybe this isn't the highly technical answer you're looking for, but how about using an extension cord or two?

 

[damonpip]

Another idea: String together some new lights with LEDs. 4AAs should be able to run 20 LEDs for about 24 hours.