# Powering an MD player

#### TiZakit

Joined Apr 18, 2005
7
Firstly Hello, all.
Secondly, I am a n00b at this. *listens for groans*

Ok, So I want to power my MD player from the the house AC line. I've got a 12v power supply that I picked up somewhere and am trying to figure out how to get it down to 1.5v.

The MD player didn't come with a outlet for power, so I was just going to use alligator clips and clip them to the battery terminals on the inside of the player.

I've consulted a book and searched these forums a bit an still come out confuesed as to what I need to do.

Tom

#### David Bridgen

Joined Feb 10, 2005
278
Perhaps the simplest way is to wire a resistor and two diodes in series across your supply. This will give about 1.4V across the diodes, which should be okay.

In order to keep the power dissipated in the resistor to a minimum it would help if you used a lower voltage supply to start with, and, to select a suitable value for the resistor it would be useful if you knew the average current the player takes. Can you measure it?

#### TiZakit

Joined Apr 18, 2005
7
I know the current cannot be more than a single AA battery produces.
as for measureing what it actually draws, I don't think I can place probs where they need to go to figure it out, but incase I am wrong, please enlighten me as to where to place them.

#### David Bridgen

Joined Feb 10, 2005
278
Connect an ammeter (multimeter or dmm) in series with the cell and the player.

You can probably insert two thin bare wires, separated by a piece of paper, between one end of the cell and its holder.

#### TiZakit

Joined Apr 18, 2005
7
I took two wires and connected them to the unit, then connected the other end to the battery and took the reading in the middle of that...

Problem: when I try to read DCmA it just maxes out (its an analog meter)...

#### David Bridgen

Joined Feb 10, 2005
278
What current range was that?

#### TiZakit

Joined Apr 18, 2005
7
Originally posted by David Bridgen@Apr 18 2005, 08:45 PM
What current range was that?
[post=7047]Quoted post[/post]​
.5 or 50 it maxes
500 it doesn't read at all.

as A side note:
Thanks for taking the time to help a newb like me.

#### David Bridgen

Joined Feb 10, 2005
278
Ok. Strange it doesn't register on the 500mA range.

Working with what you have (the 12V supply) connect a 100 ohm 2W resistor in series with two diodes and use the voltage aross the diodes to power your player.

#### TiZakit

Joined Apr 18, 2005
7
Ok, thanks.

I'll have to go out and get stuff tomorrow, as I just found out that Im gonna need more heavy duty stuff. (so I just saw a smoldering resister, I should have checked..)

Edit:
reading the book on the supply it says that it actually produced 13.8 volts at 1.75 amps

so wattage is current * voltage right?
so 24 watts?

why doesn't this make sense? Im trying to find out what I need things to be rated at. Or am I just looking at this alll wrong?

#### David Bridgen

Joined Feb 10, 2005
278
Originally posted by TiZakit@Apr 19 2005, 03:46 AM
reading the book on the supply it says that it actually produced 13.8 volts at 1.75 amps

so wattage is current * voltage right?
so 24 watts?

why doesn't this make sense? Im trying to find out what I need things to be rated at. Or am I just looking at this alll wrong?
[post=7056]Quoted post[/post]​
24 watts is the maximum power it can deliver, which is much the same as saying that 1.75A is the maximum current it can deliver.

It will always give 13.8 volts out so long as the current taken is no more than 1.75A but the actual current is dependent upon the load you connect.
Less load = less current and vice versa.

For your applicaion with the player, just ignore the current rating. All it means in practise is that you will be drawing much less current than it can supply.

#### TiZakit

Joined Apr 18, 2005
7
Ok, that makes sense...

I went out and bought a dmm... much easier.

I actually take the time to measure the voltage comming out of the psu... its 16.5V
I hooked up the resister and the diodes anyways to see what might happen...

Voltages taken at:
PSU: 16.5
After resister: 16.5
After diodes: 16.5

so.. why isn't my resister.. resisting?

#### David Bridgen

Joined Feb 10, 2005
278
[attachmentid=578]This is how to connect it and measure the voltage.

#### TiZakit

Joined Apr 18, 2005
7
Originally posted by David Bridgen@Apr 19 2005, 10:40 PM
[attachmentid=578]This is how to connect it and measure the voltage.

[post=7098]Quoted post[/post]​
Thanks for dealing with me.. as I've never actually tried to build something from a need. just things out of circuits that were made by other people.. and I copied them.

1.92 volts

I really don't want to send that to the player... or will that .42 not make so much of a difference?
I would expect that in this situation.. better to be under than over.

#### David Bridgen

Joined Feb 10, 2005
278
Originally posted by TiZakit@Apr 20 2005, 05:00 AM
1.92 volts

I really don't want to send that to the player... or will that .42 not make so much of a difference?
I would expect that in this situation.. better to be under than over.
[post=7100]Quoted post[/post]​
Yes, to apply a little less would be better.

I expect you are measuring this without the player connected.
If the current through the diodes were reduced the voltage would go down a little.
I think you should be safe to connect it briefly to the player and measure the voltage when it is working.
If it is still too high you can increase the value of the resistor.