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How to power a small device with a bigger battery? Answered



Im building this portable rechargeable guitar amp for myself and somewhere along this project, this problem got in the way...

I have a small guitar amplifier thats powered by 2 AAA 1.5v batteries.
And i also have a 6v 1.4AH rechargeable battery which i will be using to power the guitar amplifier.

I took some current draw readings from the amplifier using my multimeter, i used the regular AAA batteries for this test and these were the results:

-Device turned on with nothing plugged in, no headphones, aux etc. Has a reading of about 22 mA of current running through it.

-Device with everything hooked up at full blast with everything running consumes about 155 mA running through it.

 
Now:
*The guitar amp at its lowest load or just turned on without using it or raising the volume knob, i calculated the device's resistance being at about 136 ohms (@22mA current draw) or 0.066 watts!
*At its peak performance i calculate the total resistance to be 19.35 ohms (@155mA current draw) or 0.464 watts!

So,
With a 20 ohm resistor we should be fine running this thing full blast with everything running on it with the 6v battery being that it will provide aprox 2.92v to the device and a current of about 150ma which is perfect!

But as it turns out, the device will need less power when applying small loads and as the device needs less power it will decrease current but according to my calculations it will increase voltage to 5.14v to the device as the current drops. I dont get it! It only needs 2 AAA batteries give or take about 2.90v. I am worried about burning the device with the 6v battery.

So my question is the following:

Since we have a fluxuation of current from 22mA to 155mA will a 20 Ohm resistor do? Or will it damage the circuit being that will put about 5.14v into the device when it is at rest or has minimum load.

Tell me what i need maybe a voltage regulator or a transistor perhaps? I wouldnt know what type of either to get if i ended up needing one of those so any advice would be of great help!

Thanks in advance!!


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rickharris

Best Answer 4 years ago

Don't use a resistor.

You need to build a 3 volt voltage regulator. There are lots of circuits for variable voltage regulators on the web.

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mobman713rickharris

Answer 4 years ago

Thanks a lot, i will look into building a 3 volt voltage regulator. Afterwards i will run some tests to make sure i did everything correctly. But i have only one question, why cant i use a resistor?

I am not challenging your knowledge i just really want to know. What will it do?

Thanks in advance

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rickharrismobman713

Answer 4 years ago

The amount of voltage the resistor drops is dependent on the current flowing through it. If the current changes then the voltage dropped changes.

V=I x R

What you need is a stable supply as provided by a regulator.

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mobman713rickharris

Answer 4 years ago

That is what i feared using only a resistor. I have already started gathering some materials i have laying around to build regulator. Thank you so very much for your information.

May you have a wonderful day!