Adding 2nd dc power input for a battery powered device.

Aug 07, 2006 4 Replies
M

I have a baby light and noise maker that is eating the three AA batteries like mad. I'm looking to add a wall wart power input to it but still keep the battery powered option.



Battery voltage is 5V and it drops out at 3 volts. Current draw is 200uA to



150mA at full load with the lights on and the songs playing

I'm currently thinking of using a dual shottky array that will drop about .25V at 150mA. I have several 9VDC wall warts waiting for something to do. I measured the smallest one and at 150mA the voltage is very close to 9Vdc. A LM7805 (to-220) will drop the voltage to 5V then the diode will drop it to



4.75 which would be near perfect.

The problem I have is that the regulator will be in a closed plastic battery compartment with no airflow and nothing to heatsink it to except the metal DC input terminal.



Therefore the regulator will dissipate 4V times 150mA (600mW) at full load.



will that get too hot in that closed compartment? Will it melt the plastic over long term use?


Any other low cost approaches?


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J

Hello Mook,

Maybe one of the little buck switcher chips? TI has a lot of them. You can yield efficiencies above 80% with some.

Regards, Joerg http://www.analogconsultants.com
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M

I thought about that but was trying to avoid a PCB.

I'm thinking cheap and dirty.

What do you think about a 12 ohm heatshrunk inline with the wallwart power.

that would drop the voltage down to about 7 volts at full power and reduce the power to about 300mW in the LM7805.

Too hot still?

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L

I know you have a 9V wall wart, but don't use it. Get a 5V one and your heat problem is gone. It will be much easier, and cheaper, to get 5V wall wart (from a junk store, old cell phone charger, or from a web surplus place) than it will be to do the hack you describe. A 5V wall adapter that supplies 150mA should csot you $3-5, or maybe $1 if your lucky at a junk store.

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J

Hello Mook,

You could probably do it "Sauerkraut" style. Just make sure the ground structure is ok.

I would not do that. Besides the heat problem there is also the hazard of plastics outgassing when stuff like surrounding PVC gets hot. Or worse if it starts a fire. At the big Duesseldorf airport fire many people where killed by fumes from burning plastic.

Regards, Joerg http://www.analogconsultants.com
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