Re: Minimal size 5V/100mA power supply

"ivanatora" <ivanatora@xxxxxxxxx> wrote in message news:39845075-dfdf-45e8-9607-b2420d60a711@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Hello,
I'm designing a matchbox-termometer. It will consist of a 7-segment
indication, PIC16F88 (18 pin) and TC77 - SPI temperature sensor
(smd).
The hard part is to find some sort of a power supply (battery) that
will fit into the matchbox. I need 3-5V and about 100mA to drive the
LED indication. I tried with 3 AG6 batteries (coin and button type)
but it barely outputs 20-30mA. The whole device is powered only during
the temperature reading and displaying - about 10-15 secs, so it won't
be draining battery power in idle state.
Currently I'm using a 9V battery + 7805, but that combination is
bigger than the board itself :)

Can you come with some cunning sort of power supply?

Now I'm wondering if I make the LED indication blinking about 100
times per second (dynamic indication) with duty cycle of 0.5 (half
time ON, half time OFF) will the power consumption shring in half? If
I can make the 7segment indication draw 50mA that is more suitable,
becouse I will put bigger resistors (like 400 ohms instead of 200
ohms) and the consumation will shring further into the capabilities of
the 3 AG6 batteries...


I got out an old kingbright 7 segment display with 4 digits. It offers a multiplexed output using a common anode. To get reasonable brightness, it required 10mA per segment, meaning the most current it would need at one time is 70mA.

However, when you multiplex it, you end up with brightness which is roughly proportional to the on duty cycle. So, being on 1/4 of the time means you it might need more current per segment.

Because of this, I don't think you'll be able to use a coin or button cell, since they have too much internal resistance. You could go with an AA alkaline cell and a boost SMPS, or multiple AA cells. A boost switcher would give you a regulated output, and waste less than a 9V and linear regulator. There are also small rechargeable cells. Check this out:

http://www.powerstream.com/BatteryFAQ.html

A 1/4 AAA is 10.5 by 14 mm. Not too bad.

Regards,
Bob Monsen

.

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