OK... who can help me with this (I hope) simple problem.

I need to run 2 devices from a 5V DC power adaper.
Device A uses 5V DC - device B uses 3V DC. Device B will run for 8 hours on 2 AAA batteries (3 V in series).

What size resistor do I need to drop voltage down on the power supply to 3V for device B?

Thank you
Tom

 

it is not recommended to drop voltage like that unless you are
absolutelly sure there will be no problems (varying load).
The best method is to use voltage regulator. As a cheap and handy
alternative, I would rather use string of three diodes instead of resistor.
Your post doesn't tell me much about current draw because there
are different batteries in that form and capacities vary but 1Amp
diodes like 1N4001-1N4007 should be sufficient.

 

Thanks for the quick reply.

The average AAA battery has about 750 ma. If it ran 8 hours that would mean that it was drawing 94 ma.

Can you share a voltage reg solution with part number and diagram?

How would I tap off 3 diodes?
All 3 in series and take voltage off the 3rd?

Thanks
Tom

 

If you go to digikey.com and search for voltage regulator you should find tons of possible parts. The data sheets will usually give a recommended circuit.

 

Thank you.
How would I use the 3 diodes? in series with the device?

 

please dont double post

 

Panic mode has the easiest and best idea. If you use a regulator you might next some extra components.
I sensed some confusion on how to hook up the diodes, so see the attached diagram. The cathode is the end that has a ring on one end of the diode.

Attached Images

diode_171.jpg (5.3 KB, 220 views)

__________________
The great thing about electronics is unlimited ways to do the job. The only limit is one\'s imagination. I generally think my way is best.
Show me a different way. I have an open mind.

 

I got my answer and it works... thank you all for your help