Wall wart output 9 VDC - need 4.5 VDC or less...

Thread Starter

likes_shiny_things

Joined Sep 11, 2011
24
I want to use a wall wart to replace 3 AAs in a string of micro lights.

I have a wall wart that is rated at 3.7 volts, but it measures 9 volts with no load (it was originally for a cell phone).

How can I regulate that wall wart so it puts out 4.5 (or a little less if I want the lights to be not as bright)?

(I did a ton of searching and while there are many threads online that address similar issues, I found none to answer my question).

Thank you!
 

MaxHeadRoom

Joined Jul 18, 2013
23,368
Most likely the Wall Wart is a linear supply, they rely on the rated load current to bring them down to the rated level.
Wall Warts are cheap so you could just buy the correct one!
The other alternative is a LM317 set up for the voltage you need.
What is the current?
Max.
 

dl324

Joined Mar 30, 2015
12,927
I want to use a wall wart to replace 3 AAs in a string of micro lights.

I have a wall wart that is rated at 3.7 volts, but it measures 9 volts with no load (it was originally for a cell phone).

How can I regulate that wall wart so it puts out 4.5 (or a little less if I want the lights to be not as bright)?
How much of a load does the wall wart need to drop to 4.5V? What is the load from the lights?
 

Thread Starter

likes_shiny_things

Joined Sep 11, 2011
24
What is the current?
The wall wart says 0.35 A

If you need to drop the voltage further use one or more diodes (1N4004 or similar) in series for 0.6V drop per diode.
Thank you for the diode suggestion.

How much of a load does the wall wart need to drop to 4.5V? What is the load from the lights?
The lights draw 66 mA. I got the wall wart to go to 6.9 V with a 100Ω load - that's the lowest value resistor I have.
 

Thread Starter

likes_shiny_things

Joined Sep 11, 2011
24
Three rectifier diodes (1N4001) should drop the voltage by about 2.5V.
So, are you suggesting the three diodes (in series) in addition to the 100Ω resistor in parallel with the load? If yes, what's the minimum rating for the resistor?

I appreciate everyone's responses.
 

AnalogKid

Joined Aug 1, 2013
9,290
Since you don't know the exact circuit or components inside the wall wart, or more importantly, the transformer characteristics under light load, depending on the wart's unknown output impedance to form a voltage attenuator with the load is risky and not stable. As the transformer core heats up during operation, the voltage across the load will change.

Nothing will give you a more predictable output than a series pass regulator. I suggest a 7805 with a small power diode (1N4002, etc.) in series with the output. This will give you a relatively stable 4.3 V -ish output.

ak
 

dendad

Joined Feb 20, 2016
3,887
Why not just get a 5V plugpack, often for free from an old phone, or for not much at a Salvos store or almost any OpShop...
Then, add a series diode, 1N4007, and it will be 4.3V or there abouts.
 
Top