Cell phone power supply i-dears

[Rob Mercure]

Folks,

A friend recently gave me a cheap Trac phone which uses a 3.7 VDC wal wart power supply. Now I was thinking about alternative power supplies - such as one for the truck - and wondered if simply putting three carbon-zinc batteries in series, say D cells for the power, with a silicon diode would provide sufficient "emergency" power? Three D cells = 3 X 1.5 = 4.5V less the junction drop on the diode 0.7 V = 3.8 VDC. Pretty dang close to 3.7 - after all, how precise could/should this be?

Alzo, if I were to go the route of cobbling up something that worked on the truck's electrical system would a simple resitive voltage divider feeding a zener be sufficient - need about 350 mA for the unit - or would I need to add some active electronics? Generally it's the range of truck voltage fluctuation - about 12.6-14.3 VDC - that concerns me, not a simple 12 to 3.7 with varying current.

Rob

[Steve Conner]

3.7V is what Nokia cellphones use. You can get a cigarette lighter adaptor for a Nokia for about $20, and they have a switched mode buck regulator chip inside. I'd give you mine, since I don't have a car to plug it into any more, but you can probably buy one locally for less than the shipping.

I believe older Nokias came with an unregulated 3.7V wall wart, and I have got them to charge off a 6V supply with a series resistor, so I don't think it needs to be too precise at all. Measure the unloaded voltage of your wart to see.

I don't think the phone will function without its internal battery, regardless of what power supply you hook it up to.

[Rob Mercure]

Hey Steve,

Thanks. I wasn't considering using the phone w/o the internal battery - instead was just considering if I could make a "QAD" backup supply with a few batteries and a diode - I've got a small holder that takes three cells and the diode doesn't take much room - was going to feed it through the PS jack. And, yeah, I could probably buy the adapter for $20 but the phones only sell for about $35 around here and this is a temporary loan so if I can cobble up a truck adapter it seem to make more sense to me. But I was going to measure the unloaded voltage - as you suggested - just got to the keyboard first earlier this morning. The whole phone thing is an "out of the blue" sort of loan so I really haven't really thought about it that much. Hell, I'm not even sure if 3.7 VDC is a "standard" zener voltage or not - haven't looked yet and I'm not sure whether it's worth cobbling up an active regulator or just using zeners/resistors. Some part of me want to make a two stage voltage dropping/regulator system - perhaps the first stage to drop the voltage down to 9-10 volts and the second stage dropping it to the final 3.7 +/-. But I'm not even sure why my "instincts" prod me in this direction - I guess I'm just not used to making low voltage regulated supplies.

Anyway, thanks for the help and for any further suggestions.

Rob

[Steve A.]

Rob:

I guess Nokia isn't as picky as Motorola, but my Raz'r will only recognize the chargers designed for it. (I have mini-USB adaptors and chargers that work for a half dozen other devices but not the Motorola.)

If you are looking to extend the battery life of your Trac phone, I'd recommend that you get an automotive power pack (a sealed battery with a 12v lighter socket and maybe even a 120v outlet). If you don't get one with the 120 volt outlet you can always plug in a small inverter...

Not exactly the most compact arrangement but it'll keep the Trac phone charged up while driving, and when the Trac phone is gone you'll still have a cool rig for everything else... LOL

True story time- about 25 years when I got my first work van, I decided to put all of my rechargeable tools on a board behind the driver seat, with voltage regulators I had put together myself, guesstimating the voltage that each device would prefer. (These included a cordless soldering iron, a cordless pump, a cordless vaccuum, a cordless drill...)

I guess my guesstimates weren't even close because after 6 months every single one of my rechargeable devices was dead from charging at the wrong voltage or current or whatever!

After that experience I figured it would have been better if I had just used an inverter and plugged the ac adaptors into an inverter- but back then inverters were very expensive!

And since then, the knowledge and technology of rechargeable batteries has advanced a lot so you could undoubtedly design circuits better than mine.

P.P.S. After reading my post, I see that there is no reason on earth why you would need the automotive battery pack since you could just plug an inverter into your lighter socket... but I think that the battery packs are really cool! LOL

[Don Symes]

I'd be nervous about a QnD resistor/zener thing for the truck. In vehicle electrical systems there's this thing called a 'load dump' which happens every time you shut the engine off. IIRC, it makes some nasty, relatively high-voltage spikes, some of them negative.

[Rob Mercure]

Hey Don,

So, what do manufacturers of automotive and other such circuitry do to control "load dump?" Thermistors and VDRs and such across the line? While I'm glad you brought that to our attention it can't be too awful hard to design around as the Trac fone folks sell their own inexpensive "on the road" charger rigs and power stations.

Rob

[Don Moose]

A National LM2937 looks like a good choice - 500mA, reverse battery protection, load-dump protected - but the lowest available Vout (all fixed voltages) is 5V. Maybe a zener in series with that?

Their LP2960 is available in an adjustable version and its absolute max input range is -20 to +60. It's got reverse battery protection, but no mention of load dumps.

Then the LM2940 is good for load dumps and reverse battery, but only down to 5V.

Nothing jumped out at me on the ON Semi, Linear Tech or Maxim sites, but maybe I didn't look hard enough. You might also look at Philips.

It might be a good idea to start at digikey or mouser (or your favorite supplier) and see what brands they sell/stock, then hit those sites.

Anyway, hope this helps.