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From: Martin Brown <'''newspam'''@nonad.co.uk>
Newsgroups: uk.d-i-y,sci.electronics.design
Subject: Re: Current consumption of LCD kitchen timer?
Date: Mon, 12 May 2025 21:37:05 +0100
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On 12/05/2025 15:05, Pamela wrote:
> On 12:10  12 May 2025, Martin Brown said:
>> On 10/05/2025 14:37, Pamela wrote:
>>> I'm interested to know the current drawn by a kitchen LCD digital
>>> timer.
>>>
>>> (1) How much current does the timer draw when counting time?
>>>
>>> (2) How much current is drawn when the piezo buzzer is sounding?
>>> (Averaging out beeps and silent bits.)
>>>
>>> My guesses are 2mA and 25mA, respectively. Is that about right?
>>>
>>>
>>> I mean a timer similar to this one, running off a 1.5V battery.
>>> https://www.amazon.com/Digital-Kitchen-Timer/dp/B00GOPICNM
>>
>> Second one might be about right perhaps 10mA is more likely but I'd
>> be very surprised if the display operating and timing current was
>> more than 20uA (and it could easily be lower).
>>
>> Bare metal LCDs take almost no current at all (but have to be driven
>> with an AC signal). They are no different at the core to LCD digital
>> watches with an alarm function probably even using the same chips and
>> a slightly beefier piezo sounder.
> 
> If beeping consumes 10mA then, in theory, you would get 100 hours of
> beeping from single AA alkaline (1,000 mAh).
> 
> That's 6,000 one-minute "overruns", under optimal conditions. Let's say
> it's only 1,500 in real life conditions.
> 
> In conclusion, if overruning beeping is done twice a day, then the
> battery would last a couple of years.
> 
> That's seems slightly optimisitic because I usually change the battery
> about once a year or so when I don't let the beeping run on. (However, the
> abttery is changed largely because the display has started to dim.)

LED contrast is *very* sensitive to battery voltage - the chip is 
already using an antiphase pair of signals and possibly a voltage 
doubler to drive the LCDs. 3.3v to 5v being a typical drive voltage.

At such tiny currents the battery has essentially its shelf life and 
will supply 10-20uA almost forever but at gradually lower voltages. Once 
the voltage is below a certain point the clock still runs but you can 
only see the display by holding it at just the right angle.

One big annoyance with such LCD displays is that they really won't 
accept rechargeable cells because the fully charged voltage is too low 
to run the display with an acceptable contrast.

-- 
Martin Brown