Electronics Forum / Electronics / October 2004

For more efficiency than a linear you have no choice but to go switch-mode.
The National semi "simple switcher" series is easy to use and has low parts
count. You can probably expect about 85% efficiency, depending on load
though.

It would be more beneficial by far to put the design effort into reducing
the current requirements of your receiver than the power supply. This is
where you can make great gains. My Arlec cordless cheapie doorball receiver
lasts months on 2 AA cells.

Hello Russell,
just for fun I had a look at the 78L05 data sheet.
http://www.onsemi.com/pub/Collateral/MC78L00A-D.PDF
on page 2  
input bias current   typical 3.8 mA   and     max 6 mA
I am assuming that is the current consumed while the
device is not supplying the load and you want better
than that.     Wow!

Again just for fun I opened up a  mobile phone charger,
the type that plugs into a cigarette lighter in a car.
Inside is this IC
http://www.onsemi.com/pub/Collateral/MC34063A-D.PDF
The bottom of page 3  says     Total device  4 mA

I measured 13 mA input current to the charger on a 12 V
supply but it had a LED indicator as well. The LED is
probably using most of the input current.

The  reason I took an interest in this, was,  that you
can buy these cigarette lighter chargers, cheaply, from
Oatley Electronics. I received one of their e-mails yesterday.
http://www.oatleyelectronics.com/2dollarshop.html
http://www.oatleyelectronics.com/pdf/gcl3.pdf

The car charger that I opened up came from Dick Smith,
sold on special as a phone accessory in a box with leather
case and earplugs for varous phones. These accessory boxes
ranged in price from $10 to $1 to  even  eleven cents.  
The MC34063   IC itself costs around $5 dollars

What I am leading up to tell you is, try one of these el-cheapo
swich mode power supplies (which you will have modified to
give your desired output voltage according to the instructions
on the Oatley link)  and  see what the overall efficiency is with
your particular load at 4.5 or 5 volts.

Compare the efficiency of the 78L05 regulator using the
same load.  

Russell, I am a cheapskate when it comes to my
electronics hobby but I don't think you can beat that.
Oh! wait a sec!   I spoke to soon.  You could put a
self tapping screw into the lead link that connects
the cells together on your battery.
That would give you roughly 4 and a bit Volts.
Might be enough.  
You didn't say what the current consumption
of the receiver is. If it is small, maybe just tapping
the battery will be fine.

Regards,
John Crighton
Hornsby