Cig. Lighter Cell Phone Chargers

Apparently these adapters take 12 v DC and [maybe] invert it to ac, step down the voltage to 4.2 v and rectify it. The circuit has about three transistors, three diodes, three capacitors, several resistors and a tiny LED. There isn't the first hint of an inductor or coil anywhere.

You can buy these new at 99 Cent Store.

Question 1: Why do 120 v ac power supplies have heavy coils when a coil isn't necessary even for dc?

Question 2: Which resistor needs to be replaced to get 7.2 v dc? DeWalt is crazy if they think I'm going to pay $50.00 for a #$%^~! batter charger

Bret Cahill

Reply to
BretCahill
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a power supply transformer is nessessary to provide isolation from the ac mains. this is for your safety.

a drop in charger is pretty conveineint. but if you want to build your own start by thinking of it as a voltage limited current source rather then a voltage source. this idea is to charge the ni-cads at a current low enough to not cause exessive heating in the cells. this current should then reduce to a "trickle" or shut off when the terminal voltage reaches 1.3 volts per cell (series battery) again to prevent overheating and possable rupture or explosion.

some chargers are of a fast charge type that are intended to charge the battery on 1 hour. these usually have a heat sensing element (like a thermistor). the assumption is that if the battery are functioning properly that when they reach a certain temp they are fully charged. it then tells the charger to switch to "trickle".

assuming the battery pack is 2.5 amper-hours to charge it in one hour would require in exess of 2.5 amps (remember that nothing is 100% efficient except the speed of rumors) to charge it in 10 hours would take .25 amps (250mA)

me? i use the drop in fast chargers (makita)

Reply to
Tim Perry

On 01 Aug 2004 04:31:33 GMT, BretCahill put forth the notion that...

You're better off to just buy the charger, when you consider the amount of time and trouble you'll go through trying to slap something together. If it overcharges the batteries, it'll either shorten their life, cause a meltdown, or worst case, blow them up. I've bought brand new chargers at the local swap meet for twenty bucks. BTW, since Black & Decker bought the DeWalt line, I've noticed a sharp decline in battery life, but that doesn't really surprise me.

Reply to
Checkmate

That's because it is probably a linear power supply vs. switchmode. The compromise is loss of efficiency and the resultant heat. If the total current involved is low enough, this is a good design decision. I suspect that is what makes it so simple.

Ken

Reply to
KWS

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