# Battery current problem

Discussion in 'Power Electronics' started by habafflof, Nov 4, 2012.

1. ### habafflof

6
0
Nov 4, 2012
Greetings...
I'm working on a project of putting an mp3 player into a TI-84 calculator. The player itself is a cheap pcb a size of a coin which plays mp3 files stored on an microSD card. The sound output from the player is amplified by an LM386N 8 pin audio amplifier and is played on an 18 OHM 0.2 W speaker. The whole thing is powered by 4 AAA batteries of the calculator. Under 6V the amplifier and the speaker drain 125 mA. the player drains about 25 mA.
However my problem is that when I connect them both to the battery pack they only drain about 75 mA instead of 150 mA and you can barely hear any music. Am I missing something? Why does the circuit all of a sudden starts draining half its normal current? I would greatly appreciate any help.

2. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,496
2,837
Jan 21, 2010
What voltage does the batter read when they're connected and drawing power?

Were you using the same type of battery pack when you were trying it out outside the calculator?

What are the batteries (not AAA cells I hope) -- Oh, they are I've just checked the specs on a good brand of AAA battery, and you'd probably expect about 3 hours from new to near death of the batteries. If they're rechargeable, probably half of that (or even less).

3. ### habafflof

6
0
Nov 4, 2012
tanks for the reply. The whole thing is currently on the prototype board. By AAA I did mean the AAA cells but the lifespan of the battery is not really a concern because if I get this to work then I can always get rechargeable batteries. When the circuit is drawing power the voltage drop across the battery is 5.93 V and 6.14 V while it's not.
The prototype board has an adjustable power supply built in and when I use it to power both circuits the current is also only about 75 mA while when I use a battery pack to power the amplifier it draws 150 mA and power supply to power the mp3 circuit it draws about 25 mA. So isn't suppose to draw 175 mA altogether when I join 2 circuit to be powered from 1 source?
Also since I tried using a power supply instead of the batteries and still got the same problem I think that eliminates the theory that the batteries aren't supplying enough current.

4. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,496
2,837
Jan 21, 2010
Is anything getting hot? Check especially anything around the LM386

5. ### habafflof

6
0
Nov 4, 2012
no, it's all in a reasonable range. The amplifier gets warm when draining 150 mA and I tested it by running it for about 20 minutes and it never got significantly hot so the overheating is not a problem.
When both circuits are using the same power source the voltage drop across the mp3 circuit is 5.83 V and the current draw is the same 25 mA as before However, the draw of the amplifier circuit is only about 20 mA which is too low to produce any significant gain in audio signal. What might've caused the decrease in current drain of amplifier once the 2 circuits are combined to use the same power source?

6. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,496
2,837
Jan 21, 2010
Can you show me the schematic of your amplifier?

7. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,496
2,837
Jan 21, 2010
Note that an LM386 should draw under 10mA with no input signal.

8. ### habafflof

6
0
Nov 4, 2012
Here is the whole schematic of my project. I'm tried to use as few components as possible because there isn't a lot of space in the calculator case but now I'm thinking maybe I'm missing a resistor or a capacitor somewhere...

9. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,496
2,837
Jan 21, 2010
Yeah. Look at the datasheet and redesign your amplifier to follow the designs they give you.

If you're lucky you haven't damaged the 386.

People have done far worse and I've been astounded that haven't destroyed things, so don't be too concerned about that just yet.

The first example of page 5 gives you the minimum parts count amplifier.

Is your MP3 player stereo? You'll need to combine the channels. You do this by connecting each channel to the input of the 386 via a separate resistor (50k should be fine, but anything from 1k up to 100k will possibly work)

10. ### habafflof

6
0
Nov 4, 2012
It's working! thanks for the help! I made the exact circuit as shown in the application notes from the data sheet and it worked perfectly.

11. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,496
2,837
Jan 21, 2010
Great!

(those datasheets are darn useful eh?)

What is the current draw now?

12. ### habafflof

6
0
Nov 4, 2012
It is about 150 mA as it is suppose to be. thanks again!