Wall warts and rechargable batteries?
Jeff Dege
Posts: 85
Over in the Project forum, I posted about my Binary Clock.
If I'm going to put it in a box, I need to put power in the box. Or provide a way to plug power in.
Up 'til now, my battery-powered Stamp projects have run off of 9V batteries, either through the Stamp's on-board voltage regulator or a separate LM7805. But 9Vs are Smile-poor storage devices, and voltage regulators throw away half of their energy.
So for the clock, I'm considering using an ST619LB and a pair of NiMH AAs. The AAs should be able to provide 2500 mA-Hours, as opposed to a 9v's 150, and if I read the datasheet correctly, the ST619 should be able to deliver up to 40mAH through most of their discharge cycle. My clock's current draw is dependent upon the intensity of the LEDs. At their dimmest it pulls 20 mA.
Has anyone here used the ST619 with a Stamp?
But that gets me to my question.
For a lot of battery-operated devices, five days of operating time on a battery charge is plenty. If you're not using them, they're turned off. But a clock? It's running around the clock
So I'm considered running it off a wall-wart. Which should be simple enough. Either find one with a 2.0-3.6V output and use the ST619, or one with a 7-25V output and use an LM7805. Make sure not to skimp on the filtering caps, and Bob's your uncle.
But what about both?
Could I have a design that powered off the wall-wart if it was plugged in, but off the batteries if it was not?
How difficult would it be to add the circuitry to recharge the batteries in place?
Would there be a risk of over-charging?
Pointers to circuits and other information on these issues would be appreciated.
Post Edited (Jeff Dege) : 3/10/2006 5:21:37 PM GMT
If I'm going to put it in a box, I need to put power in the box. Or provide a way to plug power in.
Up 'til now, my battery-powered Stamp projects have run off of 9V batteries, either through the Stamp's on-board voltage regulator or a separate LM7805. But 9Vs are Smile-poor storage devices, and voltage regulators throw away half of their energy.
So for the clock, I'm considering using an ST619LB and a pair of NiMH AAs. The AAs should be able to provide 2500 mA-Hours, as opposed to a 9v's 150, and if I read the datasheet correctly, the ST619 should be able to deliver up to 40mAH through most of their discharge cycle. My clock's current draw is dependent upon the intensity of the LEDs. At their dimmest it pulls 20 mA.
Has anyone here used the ST619 with a Stamp?
But that gets me to my question.
For a lot of battery-operated devices, five days of operating time on a battery charge is plenty. If you're not using them, they're turned off. But a clock? It's running around the clock
So I'm considered running it off a wall-wart. Which should be simple enough. Either find one with a 2.0-3.6V output and use the ST619, or one with a 7-25V output and use an LM7805. Make sure not to skimp on the filtering caps, and Bob's your uncle.
But what about both?
Could I have a design that powered off the wall-wart if it was plugged in, but off the batteries if it was not?
How difficult would it be to add the circuitry to recharge the batteries in place?
Would there be a risk of over-charging?
Pointers to circuits and other information on these issues would be appreciated.
Post Edited (Jeff Dege) : 3/10/2006 5:21:37 PM GMT
Comments
Just some things to think about...
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Truly Understand the Fundamentals and the Path will be so much easier...
The only trick is to ensure that the draw stays below what the ST619 can provide (40 mA, at 2.0V).
So it would be ideal to supply well regulated
DC that is in the ball park for regular operation and high enough to trickle charge your battery.
Battery operation really could be straight through as it doesn't vary like AC. In other words, you really don't need a regulator between a battery and the ICs to create a clean power source.
Of course, this nice like explanation gets to be jumbled when you start to look at the components you have to pull together.
Personally, I would come from a higher voltage and work down. Maybe I would have one 7805 set up to provide power and an adjustible regulator set to optimally charge the battery pack. When disconnected, the battery pack is on its own. [noparse][[/noparse]besure to consider diodes to protect from back flows. And remember that they drop the voltage by 0.7 volts or so].
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"When all think alike, no one is thinking very much.' - Walter Lippmann (1889-1974)
······································································ Warm regards,····· G. Herzog [noparse][[/noparse]·黃鶴 ]·in Taiwan
No. But I'd need the charge pump to run a 5V Basic Stamp off of a pair of 1.2V batteries...