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

[mook Johnson]

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?

 

[Joerg]

Hello Mook,

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?

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

 

[Mook Johnson]

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?

 

[Joerg]

Hello Mook,

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

I'm thinking cheap and dirty.

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

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

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.