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clock radio project from battery backup to fully battery operated
- Thread starter mohamed
- Start date
Generic advice:
Buy a battery operated clock.
Modify only an existing clock if you have no alternative.
Some clocks rely on main as a method of keeping time. By removing this and using batteries, you may introduce excessive drift in your now battery operated clock. To top it off, many of these clocks use LEDs for the display which will eat up your batteries pretty quickly.
Id this a generic questions, of do you have a specific goal?
Buy a battery operated clock.
Modify only an existing clock if you have no alternative.
Some clocks rely on main as a method of keeping time. By removing this and using batteries, you may introduce excessive drift in your now battery operated clock. To top it off, many of these clocks use LEDs for the display which will eat up your batteries pretty quickly.
Id this a generic questions, of do you have a specific goal?
sorry for the late reply.. I wanted to thank you very much for your help. I get it.. I understand about the LED lights and the battery drainage.. but how about 2 or 3, 9V batteries. Shouldn't that hold the clock for about at least a month? That'll do me. I'd be okay with that. And I don't understand the drift.. why would there be drift using batteries as opposed to A/C?
For a battery operated clock to last a length of time you need:
-To determine the current draw of the device.
-Find a suitable battery or pack of batteries to supply the current for the desired length of time.
For sake of argument. Let's calculate the current draw at 10mA per segment on a clock, and be really generous and say that on average, only 3 segments will be lit per number. The numbers are usually multiplexed and scanned, so only one number is on at a time. So 30mA for the numbers, plus another 10mA for the colon. We will also slap on maybe another 10-15mA for the time keeping circuit. I have been incredibly generous with these numbers... so on average the LED digits for the face will consume 40-50mA at 1.3V which is 65mW or power. The time keeping circuit (lets be very generous again and say it's 3.3V) will draw 50mW.
So every hour, your clock will consume at least 115milliwatts of power assuming perfect efficiency of the circuit.
Average disposable 9V cells have a 3600mWH capacity (Zinc-carbon).
So... each 9V cell can provide you with roughly 31 Hours of life if the circuit was perfect and designed for battery use.
LED based clocks are not designed for this though, so they are a lot more power hungry. You would be lucky to get a day out of a 9V.
The drift I am referring to is based on how many cheap mains powered clocks operate. Instead of relying on an accurate internal oscillator to keep time, they keep time using the frequency of mains which is pretty close to 60Hz. by counting these cycles, the clock keeps time. The backup battery will usually power a small circuit to keep the clock in rough synch with a cheap oscillator. By relying on this cheap oscillator, you will notice that over time, your clock will not be accurate and will slowly drift. This is not a guaranteed problem, as some clocks use better parts than others, but this will be common as there is no need to put a very accurate oscillator into a clock that can keep time with a different method.
...and if you really really wanted to use 9V batteries. You would need to use at least 23 for the month assuming you can find a clock that is as generous as my numbers above.
Good luck.
-To determine the current draw of the device.
-Find a suitable battery or pack of batteries to supply the current for the desired length of time.
For sake of argument. Let's calculate the current draw at 10mA per segment on a clock, and be really generous and say that on average, only 3 segments will be lit per number. The numbers are usually multiplexed and scanned, so only one number is on at a time. So 30mA for the numbers, plus another 10mA for the colon. We will also slap on maybe another 10-15mA for the time keeping circuit. I have been incredibly generous with these numbers... so on average the LED digits for the face will consume 40-50mA at 1.3V which is 65mW or power. The time keeping circuit (lets be very generous again and say it's 3.3V) will draw 50mW.
So every hour, your clock will consume at least 115milliwatts of power assuming perfect efficiency of the circuit.
Average disposable 9V cells have a 3600mWH capacity (Zinc-carbon).
So... each 9V cell can provide you with roughly 31 Hours of life if the circuit was perfect and designed for battery use.
LED based clocks are not designed for this though, so they are a lot more power hungry. You would be lucky to get a day out of a 9V.
The drift I am referring to is based on how many cheap mains powered clocks operate. Instead of relying on an accurate internal oscillator to keep time, they keep time using the frequency of mains which is pretty close to 60Hz. by counting these cycles, the clock keeps time. The backup battery will usually power a small circuit to keep the clock in rough synch with a cheap oscillator. By relying on this cheap oscillator, you will notice that over time, your clock will not be accurate and will slowly drift. This is not a guaranteed problem, as some clocks use better parts than others, but this will be common as there is no need to put a very accurate oscillator into a clock that can keep time with a different method.
...and if you really really wanted to use 9V batteries. You would need to use at least 23 for the month assuming you can find a clock that is as generous as my numbers above.
Good luck.
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