Minimal size 5V/100mA power supply

[ivanatora]

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...

 

[John Fields]

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...

 

[ivanatora]

Why not switch to LCD?

Are there tiny LCDs available? Like the size of one-digit 7seg
indication? I've seen but never used two-rows 40 signs (or something
like that). I've seen also on some catalogue 2 digits + a sign LCD,
but I think it is too big for my form-factor

 

[default]

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...

Run on 3 volts with no regulator from alkaline button cells or AAA.
If you need regulation use it for the sensor alone - lower voltage
almost always equals less power. Or use some of that pic power and
monitor the supply voltage and correct the sensor output for low
supply.

Try the duty cycle idea, that should cut the power in half. Do you
have only one single digit 7 segment display?

I experimented a bit with some leds. I needed an all light readable
indicator from next to no power - set the controller to output a PWM
signal to charge a 10 volt cap (small inductor flyback booster with
only 6 parts) (~10 ma 3 V) then dumped that in one flash that I gated
on and off with the controller. That should also work with a display
because of persistence of vision. You won't have a continuous monitor
of temperature just a burst of light at (long) intervals to save
power.

 

[John Fields]

Are there tiny LCDs available?

---
Yes, but they can be tricky to find.

Best bet is probably to Google "static LCD glass" ;

I found:

http://www.altadox.com/lcd/products/lcd_glasses_standard.htm
http://www.altadox.com/lcd/products/lcd_customized.htm

and:

http://www.azdisplays.com/index.php?id=standard_glass_panel_lcds

There must be many more.

 

[John Larkin]

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...

100 mA is a huge amount of current for an LED. Are you using an older,
low-efficiency part?

Duty-cycle modulation will drop current and apparent brightness
together.

LEDs keep getting better. Single led's are now bright at 1 mA. We
tried some new Osram surface-mount parts and, in room light, a couple
of the colors lit visibly, in room light, a 2 microamps.

John

 

[Bob Monsen]

LEDs keep getting better. Single led's are now bright at 1 mA. We
tried some new Osram surface-mount parts and, in room light, a couple
of the colors lit visibly, in room light, a 2 microamps.

That is really astonishing... Which colors?

Regards,
Bob Monsen

 

[Bob Monsen]

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

 

[Jamie]

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...

How about using a LCD display instead?


--
"I'd rather have a bottle in front of me than a frontal lobotomy"

"Daily Thought:

SOME PEOPLE ARE LIKE SLINKIES. NOT REALLY GOOD FOR ANYTHING BUT
THEY BRING A SMILE TO YOUR FACE WHEN PUSHED DOWN THE STAIRS.
http://webpages.charter.net/jamie_5"

 

[ivanatora]

The power consumation is so high (100mA), becouse the 7segment
indication is not a single LED, but 8 LEDs (including the decimal
point). That makes 12.5 mA current by each LED. Also I've put small
resistors - only 200 ohms for better reading, but I will change them
with 440 ohms.
@default: I didn't understood your power supply device. Can you attach
some schematics?
I've checked that page http://www.altadox.com/lcd/products/lcd_glasses_standard.htm
but the smallest LCD is still too big - 21.08x18.95x2.80
The LED I'm currently using (SA36-11SRWA) is 14x5x8.
The matchbox is roughly 30x50x15

Regards, Ivan.

 

[Don McKenzie]

On Mar 9, 12:40 am, Jamie

or maybe even an oled
depends on your price bracket, but you can get small OLEDs:
http://www.dontronics-shop.com/Micro-OLED-p-1-c-371.html

or even an OLED with a parallax propeller micro on board:
http://www.dontronics-shop.com/4d-Micro-OLED-96-PROP-0.96-Propeller-Powered-OLED-module-p-16687.html
It will fit your case, and be a lot less power than the LED displays.

Cheers Don...


--
Don McKenzie

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E-Mail Contact Page: http://www.dontronics.com/email

Intelligent 2.83" AMOLED with touch screen for micros:
http://www.dontronics-shop.com/product.php?productid=16699