# Power supply question

#### McManCSU

Joined Jun 14, 2005
10
So here's the plan, I have my MP3 player that runs off of a single AA battery. I am currently using it at work everyday so I am going to be going through batteries pretty quickly (even if they are rechargable). I decided I would make my own "battery" where I will use a 1.5V power supply (simple AC-DC converter) and just use a dowel rod to fit the battery shape and add on terminals that connect to the power supply. 1st off, this will work right?

Next, so I cant seem to find a power supply/cell phone charger/etc that has the right specs, so I thought, wouldn't it be fun to just make the damn thing. So say I do, what would be too much ripple voltage? 1mV? Less?

Also, what would be the best approach so that I could possibly add on a switch to change the output voltage? Whetstone, OPAMPs, what? Thanks for the help!

#### David Bridgen

Joined Feb 10, 2005
278
Originally posted by McManCSU@Jun 15 2005, 04:09 PM
..... I decided I would make my own "battery" where I will use a 1.5V power supply (simple AC-DC converter) and just use a dowel rod to fit the battery shape and add on terminals that connect to the power supply. 1st off, this will work right?
Yes, it will work fine.

Next, so I cant seem to find a power supply/cell phone charger/etc that has the right specs, so I thought, wouldn't it be fun to just make the damn thing. So say I do, what would be too much ripple voltage? 1mV? Less?
Also, what would be the best approach so that I could possibly add on a switch to change the output voltage?
I found two adapters on the web, both switchable. One is for mains input and costs a heart-stopping $15.75. Available from here if you want to pay that much. The other plugs into a car cigarette lighter socket so is probably of less interest to you. At$7.95, it's called the OTOMEK Model AT-100
Regulated DC-DC Automotive Adapter, and is available here.

Or you could buy a more affordable mains adapter with or without a switchable output and add an additional regulator stage if you can't find one which goes as low as 1.5V.

#### McManCSU

Joined Jun 14, 2005
10
Ya I was hoping to spend no more than $5 for it... Just something cheap. How about the Q's for if I just designed it? #### David Bridgen Joined Feb 10, 2005 278 Originally posted by McManCSU@Jun 15 2005, 11:02 PM Ya I was hoping to spend no more than$5 for it... Just something cheap. How about the Q's for if I just designed it?
[post=8543]Quoted post[/post]​
The "Q's" ?
I don't understand what you mean.

#### McManCSU

Joined Jun 14, 2005
10
Q's = Questions.

#### mozikluv

Joined Jan 22, 2004
1,437
hi

a variable zener circuit can give you the 1.5v that you need. power source will come from a multiple output (3v to 12v) plug in the wall ac/dc. the circuit that i have provided can handle a current draw of 500ma maximum. i don't know how much your cd player current draw.

#### McManCSU

Joined Jun 14, 2005
10
Could you briefly explain why i would need the transistors and all? Is it because its an amplifier circuit to get the .5A? If I used, say a 3V DC power supply, couldnt I just use a resistor to knock down the V? (Providing its a high enough wattage rating) I cant imagine my MP3 player would draw more than 20mA since its a single AA battery.

#### mozikluv

Joined Jan 22, 2004
1,437
Originally posted by McManCSU@Jun 20 2005, 08:25 PM
Could you briefly explain why i would need the transistors and all? Is it because its an amplifier circuit to get the .5A? If I used, say a 3V DC power supply, couldnt I just use a resistor to knock down the V? (Providing its a high enough wattage rating) I cant imagine my MP3 player would draw more than 20mA since its a single AA battery.
[post=8642]Quoted post[/post]​

hi

those transistors are configured to act like a zener diode. you get your desired voltage by turning R3, by the way you can connect the trimmer resistor center tap either on the ground side or supply side.

there's another approach to you problem, if you can procured a 2.2v zener diode, you can use that and drop the voltage further by using an 1N4001 w/c has a forward voltage drop of about 0.7v. you connect this in series to your output. hence you get an output voltage of 1.5v

#### mozikluv

Joined Jan 22, 2004
1,437
hi,

here's the circuit diagram of the voltage divider and the zener regulator. if you are going to use the voltage divider, voltage regulation will be poor due to mains fluctuation and component tolerance. while zener regulator will have a steady voltage output. your only problem here is procuring the 2.2v zener.

the voltage divider is computed to hve a current limit of about 70ma

#### McManCSU

Joined Jun 14, 2005
10
Not to be dumb, but I still dont see why the voltage divider output would not be constant. Assuming the waveform of the output of the power supply is a fairly steady DC voltage (I havent checked), I dont see why if you just cut it in half with the resistor it would not 'steady'.
Now, in the other example, would the reason you would have a 'more steady' output is because of the Cap's holding the charge and the diodes which limit drop on the output? I am trying to remember my circuits course I had a few years ago is all
Thanks

#### mozikluv

Joined Jan 22, 2004
1,437
Originally posted by McManCSU@Jun 24 2005, 08:01 AM
Not to be dumb, but I still dont see why the voltage divider output would not be constant. Assuming the waveform of the output of the power supply is a fairly steady DC voltage (I havent checked), I dont see why if you just cut it in half with the resistor it would not 'steady'.
Now, in the other example, would the reason you would have a 'more steady' output is because of the Cap's holding the charge and the diodes which limit drop on the output? I am trying to remember my circuits course I had a few years ago is all
Thanks
[post=8695]Quoted post[/post]​

you mains supply will have a +/-10% fluctuation and that will also have +/-10% effect on the step down secondary, & likewise the resistors have tolerances.

the zener will hold steady the output inspite the mains fluctuation. Correction on the zener part #, it should be 1N4616.(sorry for the typographical error)