Battery Drain after circuit wired

[KypDurron]

Hello all,

Couple quick questions...this is an aside from the LED Bargraph question I asked earlier...I took everyone's advice and got the LM3914 and it worked like a charm...much better than the complex circuit I was trying to devise. My only problem now is that it seems to be draining my battery quite a bit...I am using an 12V N-type battery since it is small however it seems to be draining in my circuit quite fast (After 3 days of very sparse use it is already down to 8 V). It is not affecting the circuit yet as I got the LM3914 and the Bargraph to run well off of 3Volts, but just wondering if anyone has had similar experience with those type of batteries or could recommend a small battery with a high voltage. I plan to put this inside a rather small space.

I also hooked up two LEDs off the side of the same circuit operated by a switch...obviously when the Bargraph is fully lit there is not enough juice to power the LEDs, but just wondering if this may be causing any issues...should I have also put some resistors in there or should they be ok?

Any thoughts would be great. Thanks!!

 

[MacGyverS2000]

How much current is each LED pulling... what value did you use for their resistors?

 

[VEBill]

The biggest issue is likely the current draw of the LEDs themselves - you can confirm or refute this.

One trick is to use super bright LEDs, but run at very low current (less than 1mA, instead of 10mA or more). If you're using a BarGraph Display assembly, you may not have this option.

IIRC, that family of ICs offer the choice of having a bar graph (many LEDs illuminated), or having a single LED at a time illuminated (just one at a time). You might need to wire up the bottom LED as a visual reference point if just using one LED at a time (if you're using it in the dark and can't see the printed display).

Also, maybe change the power switch to a press and hold button. The user will tend to leave the device off unless he is actually looking at the display.

 

[KypDurron]

I have two sets of LEDs off the circuit...the first is an LED Bargraph driven by the LM3914 with an input variation from a POT...depending on how high the POT is turned on I measure anywhere from 40mA down to 9mA (I may be measuring wrong so let me know if these sound wacky).

I also have two regular LEDs...I have tried varying resistances in there...when I put a 150K resistance in there everything seemed to work fine but I got a large measurement for the current (around 30-40 mA).

Any advice would be great...I have a schematic that I drew in PSPICE but it is non operational (just pictoral) if anyone has time to take a look and let me know what changes they think I should make. Let me know. Thanks!

 

[BrianG]

Hi KypDurron, I have been following this thread for a while so I thought I would look up the data sheet for the LM3914. As you say, using a pot connected to the reference pin as shown on the data sheet should vary the brightness. 9 to 40 mA seems about right (are talking of all bars lit here?)since each LED bar driver transistor is a constant current source which drives each LED at 10 x the reference current.

When you say that the "two regular LEDS with 150K resistor measure about 30-40mA" are you talking about the overall current drain or just the two LEDS? If it's that second choice then you must be measuring it wrongly since the maximum current you could draw from 12v with a 150K series resistor is 80 uA.

 

[MacGyverS2000]

Man, had this whole replay typed up, then noticed BrianG beat me to the punch on all accounts

 

[IRstuff]

When you say "N-type" battery, are you referring to an MN-21? The standard "N" battery is only 1.5V.

The big problem is that per Duracell datasheet:

http://www.duracell.com/oem/Pdf/MN21.pdf

the MN-21 battery only has a capacity of about 40 mAHr on a good day

TTFN

 

[KypDurron]

Actually got this battery at Radio Shack...it is a Type 23 12v (23-144b) I have not had a chance to look into the battery specs but that is a good idea and I will do that when I get home tonight.

 

[IRstuff]

according to the Radio Shack catalog page:

http://www.radioshack.com/product.asp?catalog_name=CTLG&product_id=23-154

the part is equivalent to the MN-21

TTFN

 

[KypDurron]

Is there a better battery to use for this application? I was looking for something high voltage but small in size. Thanks.

 

[IRstuff]

If you can really run everything off 3V, I'd go with 3 AA or AAA batteries. The AAA has a capacity of about 1 Ahr, while the AA has a capacity of about 3 Ahr

TTFN