Using old power modem power supply

[mongoq]

I have a battery powered device.
I'm sick of changing the 3 AA batterys every second day.
I have some old modem power supplys which supply 6V.

It smells like "voltage divider". You put the device in series with a resistor and connect them to the modem power supply.

The formula should be:

Rvoltagedivider=(Upowersupply - Udevice)/I

I should be the maximum supported current of the device. It doesn't have it printed on, so I took the specification of the power supply (200mA), hoping it will neither be too much or not enough for the device

I calculated the formula, (6-4,5)/200*10^-3=7,5

Will a 10 Ohms Resistor be right ?

 

[Nigel Goodwin]

Simple resistors only work if everything is constant, most electronic devices don't draw constant current, so it's generally not a good idea.

What is the actual device?, and what current does it take?,

 

[Oznog]

The resistor won't regulate the voltage though. You can use the resistive divider when the current is unchanging and well known, like a heater, light bulb, rechargeable battery, or LED. Also, unless the power supply has a regulator in it, the source voltage will vary quite a bit.

You need a voltage regulator. 4.5v would be unusual so you probably want an adjustable one like a 317.