newbie battery question

[pepperell]

When I use a 12v dc power adapter connected to a 7805 regulator to provide a voltage, my circuit works fine.

BUT let's say instead of that, I have 3 regular AA batteries to provide a voltage. The voltage across the batteries starts out at around 4.5 V when they are first connected, but the voltage drops something like 0.02V every second or so, until the voltage is so low that my circuit doesn't work anymore.

The circuit I am refering to is nothing more than a PIC16F83. I have also seen the same results when using an LM386.

What could be causing this? Am I not using the batteries in the right way?

 

[akg]

the voltage drop depends on the load on the battery . how much is that ?

 

[audioguru]

Are you using Name-Brand new alkaline battery cells, or cheap carbon-zinc ones that have been on a boat for over 5 years?

 

[pepperell]

Actually, the 9V is some cheapo brand. But the AAs were duracell.

The LM386 datasheet says it draws 4mA, but when I test the current being drawn from the 9V, it is >100mA

 

[audioguru]

The LM386 datasheet says it draws 4mA, but when I test the current being drawn from the 9V, it is >100mA

At full output into an 8 ohm speaker a 9V supply's current is about 100mA.
Maybe it is oscillating. Did you use a small pcb and the capacitor in series with a resistor to ground on its output?
Is its output coupling capacitor shorted or installed backwards?
Attach its schematic.

 

[pepperell]

The circuit I have built matches your schematic very closely. When I get home from work, I will draw up what I have and attach it.

I am doing t his all on a breadboard.

Is 100mA too much from a battery?

Related question: What should the current typically be for V_In to the LM386 (The battery is still being drained quickly whether or not the input is connected though)

 

[audioguru]

I am doing t his all on a breadboard.

Most amplifier circuits oscillate when made on a breadboard. I use Veroboard (stripboard) with short tracks and solder the parts to it.

Is 100mA too much from a battery?

A 9V battery can supply a peak current of 100mA to an amplifier but 100mA continuously will cause its voltage to drop to 7.2V in about 2 hours and be dead in about 3 hours. With an amplifier playing music pretty loud a 9V battery should last for about 20 to 30 hours.

What should the current typically be for V_In to the LM386 (The battery is still being drained quickly whether or not the input is connected though)

The audio input to the volume control shouldn't have any DC current and should be capacitor-coupled if the source has a DC voltage. The supply current should be 4mA to 10mA without an input signal.
Yours is probably oscillating at a very high frequency because the wiring on your breadboard is too long causing resistance and inductance. It also has capacitance between its tracks which also causes amplifiers to oscillate.

 

[pepperell]

And an oscillating amp could cause the battery to drop under 4V in a couple minutes?

 

[Nigel Goodwin]

And an oscillating amp could cause the battery to drop under 4V in a couple minutes?

If it's a cheapo PP3 type, quite easily - even good quality PP3's are pretty poor (just too small to provide much current).

 

[poopeater]

I'm confused. In the first post, you talk about powering the circuit with either a 12V DC adapter, or three AA cells (4.5V). Why has the discussion turned to 9V batteries?

The three AA batteries may be too low to drive the circuit after the voltage drops a small amount. The amplifier can take more voltage, so my recommendation would be to add a fourth AA battery to your circuit. The AA batteries should drive the circuit, and will last much longer than any 9V you could get.

It also sounds like you are using poor quality, used, or old AA batteries. Use either new alkalines or some flavor of rechargeable. I recommend NiMH. They come in capacities up to 2700mAh nowadays.

 

[pepperell]

I was originally saying that if I used a regulated power supply, the circuit worked fine, but if I instead used batteries, they would drain to the point of the circuit failling in a couple minutes. The current being drawn from the battery is approx 10mA when the LM386 is idling.