Re: Regulating Voltage with LM78XX


"Don" <dwbauer@xxxxxxxxxxxxx> wrote in message
news:frMZg.33013$vi3.188@xxxxxxxxxxxxxxxxxxxxxxxxx
I have a project that requires 12V. I have some LM7815's and looked at the
Fairchild pdf *** and I need some help understanding the formula to
specify 12 Volt out. (maybe that can't be done)?
------------------
in 18V------o-- 1 -| |- 3 ---o--------o- output
| | LM78XX | | > ^ ^
| ------------------ | < | Vxx
--- | 2 --- R1 > | |
c1 --- 0.33uF | | Co --- 0.1uF | | |
| | | | | | |
| Iq | | | | | v --- Io
|------------v--0-----------------o-----------|-----| |
| v
Vxx > /
Io = ----- +Iq RL </
R1 >
/<
/ |
GND

The only thing I know is R1 = 220 (I have these).
What is Iq/lq and Io/lo? You can see I am no good in math. I was a
programmer and If I know the formula and meanings of I? I could write a
program and let the computer do it. Most programmers are dumber than Dog
S!@# in math and anything not(simple minded) me, anyway. KISS is my motto.

While I'm here, this circuit is supposed to switch a 12V relay after being
on for 2 hrs, for 120V on/off and I was wondering how to use the 120V
instead of buying a transformer like 120/18V or whatever? to drive the
relay.
Any Help is appreciated;
Thanks, Don Bauer
If you must use that regulator, the only way to get 12 volt from the 15 volt
regulator is by further dropping the output with some series diodes.
So 15 - 12 leaves 3 volts, and each diode drops .6 to .7 volt so four series
diodes will drop 2.8 volts leaving you an output of 12.2 volt.
If your circuit has a stable current draw you can simply insert a resistor
in place of the diodes to drop the extra 3 volt. So say if the circuit
you're feeding draws .5 amp to drop 3 volts you will divide 3 by the .5 to
get you a 1.5 ohm resistor which consumes 3 * .5 = 1.5 watt so you would use
at least a 2 watt resistor there.


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