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Reduce Voltage without resisters

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resisterHater

New Member
Hi Guys,

Sorry for the ignorant question, but here goes. I have some great lithium ion batteries form old cell phones I'm no longer usinng. I'd like to use one to power my MP3 player which is forever consuming my rechargable NmH tripple A battery (which is actually only 1.2 volts).

It's my understanding that reducing the voltage from my 3.8 volt LI battery by placing a resister between it and the MP3 player would simply consume the 'extra' power and leave me with the 1.8 volts (the power of a new socalled 1.5 volt tripple A battery) I seak. It seems that simply wasting the power rather defiets the purpose.

Firstly, is the above statement about resisters simply wasting power true?

Secondly, is there some other reasonable way to reduce the voltage without using resisters that is less wasteful?

Thank you very much for enlightening me.
 

audioguru

Well-Known Member
Most Helpful Member
If you use a resistor to drop the voltage then your MP3 player might blow up, or not work because its voltage will depend on how hard it works (its current). You need a voltage regulator IC to regulate the 1.5V. It still wastes the extra voltage as heat.

But many voltage regulator ICs won't work when the lithium battery voltage drops to only 3V.

A lithium battery might catch on fire if you charge it wrong.
 

Speakerguy

Active Member
You want a buck regulator. Given your questions about resistors, it's probably beyond you to design one from scratch.
 

kchriste

New Member
Forum Supporter
Another thing. What capacity (Mah) does the NiMH battery have? What capacity (Mah) does the lithium ion battery have?
Also, now your compact MP3 player becomes bulky with a larger battery and extra DIY circuitry hanging off it. How big is your pocket?
 

RCinFLA

Well-Known Member
Your 1.5 vdc battery powered MP3 play actually has a switcher boost power supply that runs most of the internal IC's from 3.5v to 5 vdc. These boost switchers will operate down to about 0.75 vdc which can suck an alkaline battery dry yielding also every bit of energy from it.

Higher end players like iPod use a Li-Ion directly.

I am guessing your MP3 player did not come with rechargable NiMH battery but you figured how to replace the non-rechargable Alkaline battery with a rechargable NiMH, using an external charger.
 

bountyhunter

Well-Known Member
Hi Guys,

Sorry for the ignorant question, but here goes. I have some great lithium ion batteries form old cell phones I'm no longer usinng. I'd like to use one to power my MP3 player which is forever consuming my rechargable NmH tripple A battery (which is actually only 1.2 volts).
Does this player have a DC input jack on it? What voltage does it say next to the jack?

You can certainly knock the 3.8V Li down to 1.25V easy enough using just about any linear regulator on earth, but as you said: it does just burn the excess power up in heat. The bandgap voltage in linear regs is about 1.24V, so if you can get an adjustable version and wire it to put out the reference voltage. You will need to know how much current the player draws to select the regulator. national semiconductor and Linear Technology make tons of these type of regulators, they are basically foolproof to use.

There are some buck regulator IC's that are pretty easy to use, but they are more complicated and use more parts.
 
Last edited:

bountyhunter

Well-Known Member
Hi Guys,

Sorry for the ignorant question, but here goes. I have some great lithium ion batteries form old cell phones I'm no longer usinng. I'd like to use one to power my MP3 player which is forever consuming my rechargable NmH tripple A battery (which is actually only 1.2 volts).

It's my understanding that reducing the voltage from my 3.8 volt LI battery by placing a resister between it and the MP3 player would simply consume the 'extra' power and leave me with the 1.8 volts (the power of a new socalled 1.5 volt tripple A battery) I seak. It seems that simply wasting the power rather defiets the purpose.

Firstly, is the above statement about resisters simply wasting power true?

Secondly, is there some other reasonable way to reduce the voltage without using resisters that is less wasteful?

Thank you very much for enlightening me.

Another option you may not have considered: buy a couple of extra AAA NIMH batteries and carry them in your pocket. That would probably be easier and take up less space. harbor Freight tools sells them cheap like four for $6.
 
Last edited:

Hero999

Banned
Your 1.5 vdc battery powered MP3 play actually has a switcher boost power supply that runs most of the internal IC's from 3.5v to 5 vdc.
Yes, if you tap the output of the internal switching reulgator, you might be able to bypass it and run directly from the Li cell.

The switching regulator might even work fromany voltage below 4.5V meaning you can directly power it from the LI cell anyway.
 

Attemp2Learn

New Member
Any applications I have had that require a voltage reduction I have simply used 1N4001 diodes in series. When the current draw of the load is low, I have found this to be a simple solution.

If your load / device is drawing 300mA+ id go with an IC that can do the job without wasting too much power.

This is a simple one for low current applications and may be a quick dirty method that works for you.

If im wrong in this, somebody, by all means, please chime in :)
 
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