Calculating Power Consumption

[kfchoong]

Hi,
While building a kit using PIC16F876A, I realized that the power consumption is very high. A new 9V battery will last about 24 hours. Knowing that the power consumption is dependent on the circuit components and design, I have attached the schematic diagram in PDF.

Could anyone please tell me how I can calculate the power consumption?

Thanks,
kfchoong

[AlexR]

I think you will find that most of the power is taken by LCD backlight.
If you run the cct off a battery turn off the backlight.
Also having the power LED on all the time does not help matters. To get maximum life out of your battery pull that out as well.

[kfchoong]

Thanks for the reply. During one of my test, I removed no only the LCD's backlight, but the entire LCD. Even after doing so, the power consumption is still very high. It probably only added a few more hours to the battery life.

I'm new to circuit, not sure if the variable resistor used to control the LCD brightness is causing the problem. Depending on the resistance set, doesn't it cause some sort of short circuit?

[S_lannan]

ok.
here's my rule number one of batteries.

9V VOLTS BATTERIES ARE CRAP.

do not ever use them. period.
using a step up converter with a set of 2xaa cells would be waay better.

a single led will use more power than a lcd display.

as far as having any longevity out of a 9v battery don't expect nothing.
They are landfill demons, they have no capacity and are expensive.
I cannot stress this enough.

[kfchoong]

Thanks for the advice, I will give it a try. But will changing the battery from 9v to AA makes such a big difference? Or is there something wrong fundamentally that causes the battery to only last a day?

Is there a way in which I can measure the mA consume by the circuit?

Thanks again

[hgmjr]

Originally Posted by kfchoong

Thanks for the advice, I will give it a try. But will changing the battery from 9v to AA makes such a big difference? Or is there something wrong fundamentally that causes the battery to only last a day?

Is there a way in which I can measure the mA consume by the circuit?

Thanks again

If you can obtain a Digital Multimeter you can measure the current with that.

hgmjr

[gurudutt pai]

Your basic application seems to be a LCD clock or a programmable timer.
I have downloaded your diag. let me study it & will post the bugs in the same.
I have been working with low consumption systems for years. The simplest method to start with is to know the min. curr consumpton of each device which will be powered up by the battery. Take the total consumption , & divide the battery capacity ( mAH ) by cir. consumption to get the life of battery. If the life is unacceptable you have no choice but use a higher capac. battery or change the devices used where avail. in a lo curr. type.
e.g if you are using a pwer hungry LS or similar gate , try using Cmos or
HC mos devices , for normal 555 use C555 .

[kfchoong]

How do I measure the current with a Digital Multimeter? Must I measure each component individually?

Also, if powered by a 9v battery, how long should I expect the device to work? Articles from different website seem to indicate that the PIC16F876A consumes very little power and could last for months on battery power. Is this true normally or are they stating the best case scenario?

BTW, I have a link to the document, showing what the kit does and how it works. Maybe it will provide a clearer picture of the device.

http://www.cytron.com.my/attachment/.../PR12%20v4.pdf

[beenthere]

The best indicator for current draw would be on the wall adapter. The visible plate gives the output voltage and the current. It's too small to see in the photograph in the PDF file, though.

[gurudutt pai]

You are wasting battery capacity in 9 v to 5 v dc regulator. So i advise you to use 3 x 1.5 v pencil cells or 4 x 1.2 nicads to get around 4.5 v without the need for regulator. The higher mAH cap will also work in your favour.
Also keep the back light & leds off after a set time interval till the display function is activated by user pressing a button. This will have to be added to your cir. Use the pic with as low osc freq as possible to reduce active power consumption. Otherwise , as your basic RTC function is in a micropwer chip
the pic can also be powered into standby mode till user presses the wakeup button as noted above. Also the lcd bias resistor should be disconneced in low per mode to maximise battery life. For leds use hi efficiency types with a suitably higher val resis to reduce active cur.
These modifications will keep your battery alive for several days or more depending on your implementation.