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Inverter for camera battery charger

I need an inverter so that I can run the charger for my camera battery from my car's 12V battery. I want something cheap and preferably made from bits I've got laying around or can buy easily and cheaply.

The charger (a small switcher) is marked:

Output 4.2V DC 0.8A Input 110V - 240V AC 50/60Hz 0.2A (I haven't checked, but I'm pretty sure that the 0.2A is large overestimate - it doen't get warm enough to be that inefficient)

Efficiency isn't a prime concern, but it would be nice if it didn't get too hot. Size isn't a great concern, as long as it's not enormous.

My first thought was to use a 555 timer running at ~55Hz, a single transistor inverter to give two out of phase square waves, a PNP/NPN common emitter pair on each of these two to drive the 15V winding of a small 240V-15V mains transformer. This should give me about 400V pkpk on the output. (240V rms = 679V pkpk)

Questions.

  1. Is this a viable scheme?
  2. Is driving the transformer winding with a square wave OK? Do I need protection diodes on the output of the PNP/NPN pairs?
  3. Is the switcher likely to be damaged by the non-sinusoidal waveform? (I don't think so, but...)
  4. Is there a simpler and better way, bearing in mind the cost constraints?

Any thoughtful comments welcome.

TIA

Jon

Reply to
ZZZjon
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[snip]

Is is a charger or a power supply?

If it's a power supply (ie the charging 'intelligence' is in the camera) then a 4.2V regulator (LM317 with appropriate resistors to set the output) running off the 12V supply would be much simpler.

At full load, the LM317 would be dissipating 0.8 * 8 = 6.4W, so it would need a modest heatsink.

Stepping the 12V up and then back down again seems wasteful and complex if it's not absolutely necessary.

Tim

Reply to
Tim Auton

It's a charger (Panasonic Battery Charger DE-A12A, to be precise). The battery (which has 3 terminals labelled +, -, & T) is removed from the camera for charging.

I agree that going from 12VDC to 240VAC and back again is inelegant and wasteful, but I can see no alternative without breaking into the charger itself, and I don't want to do that.

Thanks anyway.

Jon

Reply to
ZZZjon

Agreed.

If the OP insists on a transformer solution, though, could he get by with a smaller transformer if he increased the switching frequency to, oh, a few kHz?

Michael

Reply to
mrdarrett

Oh, is the transformer built-in to the charger? (Hate it when they do that.)

Michael

Reply to
mrdarrett

I thnk I need to be in the 50-60Hz range as specified by Panasonic. I'm pretty sure that the charger input goes straight to a rectifier, and the DC is then switched at a high frequency, as you suggest. But I'm not 100% sure, and don't want to risk it.

Jon

Reply to
ZZZjon

Buy a proper charger.

That's a Li+ charger. Charge Li+ (or LiPoly) incorrectly at your peril (quite literally - that's quite possibly one of the issues behind the recent Dell etc pyrotechnics).

To charge a Li+ or LiPoly, you should precharge at about C/10 until the terminal voltage reaches about 3V, then at the C (or less) rate until the terminal voltage reaches 4.2V (constant current mode). Then charge in constant voltage mode until the charge current drops below C/10.

Do not, under any circumstances, exceed 4.2V on the battery terminals (although it probably has internal cutouts if it's a consumer item anyway).

If it does not have an automatic cutout, it will be spectacular. If it does, the cutout will operate and you won't charge the battery.

If you want a charger to operate from 12V, there are plenty of solutions out there (but beware of plugging into your car - it's a power supply from hell).

Cheers

PeteS

Reply to
PeteS

I've got something similar to this:

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Is it safe to use this to charge electronics? (Car start-up surges, etc.)

To be on the safe side, on my recent camping trip, I waited until the car was started before plugging in my camcorder charger into it. (Similar situation as the OP: 12V to 110V, back down to 5V or whatever it was the camcorder battery charger wanted).

My camcorder's got an ancient NiCd battery on it, and it's about 5 years old, so I no longer expect the battery to hold a charge for very long.

(Camcorder worked fine after charging on the 4-hour trip to Yosemite, by the way. Just don't want to shorten its life.)

Michael

Reply to
mrdarrett

The charger is a switcher, so it probably is.

It probably also runs on DC, and the square wave will likely not hurt. I've heard there are supplies that run on 230 volts or 110 volts but not on say 170 volts. Does anyone know if this is still common and whether it applies to large or small power supplies??

The 40o volts pk-pk sounds safe if the supply doesn't mind an 'in between' voltage.

FWIW these types of supplies are available for cheap, but not as cheap as building yourself. I have one that uses a swithcer to make DC, and follows with a bridge to make it 'AC'. Got 2 for 5 euro.

Thomas

Reply to
Zak

Which make and model of camera and battery?

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Reply to
Homer J Simpson

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