WTA how to calculate the the battery life span for a circuit ?

[alpha91]

Hi, everyone, i am designing a simple circuit recently which using two AA battery to supply a 3V buzzer.
the configuration of the battery is 1.2V and 1900mAh.
so two batteries is 2.4V and 1900mAh right ?
can i know how to calculate how long it can last when connect to a 3V buzzer without resistor ?

 

[BradtheRad]

Suppose the buzzer draws 50 mA at 2.4 V. (Just an approximate figure.) A raw calculation says the batteries will last 1900 / 50,
...or 38 hrs.

This assumes the buzzer will operate on 2.4V in the first place. Furthermore the batteries will drop in voltage as they discharge. Only experimentation will tell you what the buzzer needs in terms of minimum supply V.

 

[kam1787]

depends on the type of battery too

some have a nearly flat voltage output until they near exhaustion, others have a somewhat linear voltage drop-off

 

[Audioguru]

You don't know and we don't know how much current the 3V buzzer draws at 2.4V. So you cannot calculate how long it will buzzzzz.

 

[alpha91]

so i need to calculate the ampere on the buzzer ?

 

[Audioguru]

You cannot calculate the current used by the buzzer. Its datasheet tells how much current it uses at a certain voltage or you can measure its current with a multimeter.

 

[alpha91]

Hi, i connect the multimeter in series before the buzzer. It is 1.72mAh. and since my battery is 1900mAh. so roughly it can last for around 1000 hours right?

 

[Audioguru]

The current is so low that I think your "buzzer" is a piezo beeper, not a mechanical buzzer.
Maybe your multimeter cannot measure the high frequency current pulses accurately.

The 1900mAh capacity of the battery is rated at a much higher current so the beep will last for more than 1000 hours, smaller AAA cells too.

But the voltage (and sound level) will drop as the battery cells run down.

 

[alpha91]

The current is so low that I think your "buzzer" is a piezo beeper, not a mechanical buzzer.
Maybe your multimeter cannot measure the high frequency current pulses accurately.

The 1900mAh capacity of the battery is rated at a much higher current so the beep will last for more than 1000 hours, smaller AAA cells too.

But the voltage (and sound level) will drop as the battery cells run down.

but DC current have frequency also?

 

[BradtheRad]

It's true a buzzer/beeper might draw pulses of current (rather than steady current).

Therefore I would use my analog meter to measure current. It will average the pulses, so you get a steady reading of the needle.

I find that my digital multimeter gives erratic readings when I try to measure pulsating DC. It does not average the pulses.

 

[Audioguru]

but DC current have frequency also?

A DC mechanical buzzer turns on and off and on and off over and over. Its DC is in pulses.
A piezo beeper has a transistor oscillator that has its DC current increasing and decreasing and increasing and decreasing over and over at the frequency it operates at.

 

[kam1787]

Hi, i connect the multimeter in series before the buzzer. It is 1.72mAh. and since my battery is 1900mAh. so roughly it can last for around 1000 hours right?

i think you mean 1.72mA

1,000 hours sounds possible

 

[alpha91]

It's true a buzzer/beeper might draw pulses of current (rather than steady current).

Therefore I would use my analog meter to measure current. It will average the pulses, so you get a steady reading of the needle.

I find that my digital multimeter gives erratic readings when I try to measure pulsating DC. It does not average the pulses.

A DC mechanical buzzer turns on and off and on and off over and over. Its DC is in pulses.
A piezo beeper has a transistor oscillator that has its DC current increasing and decreasing and increasing and decreasing over and over at the frequency it operates at.

i see... so that's mean the reading i get is not accurate? how to measure a DC current pulse? need analog meter?

i think you mean 1.72mA

1,000 hours sounds possible

ya ya.. sorry. it is 1.72mA. thanks for correction.