I have a KA7809 voltage regulator. I have a couple of buffering caps from
the Vin to ground (1uF and .22 uF) and from Vout (.01uF).
Seems pretty straightforward, yes?
The problem:
if I use an 18V DC 400 mA wall wart as my Vin I read 7.5V at the Vout.
if I use a 23V 1.8A AC transformer through a bridge I read 8.2V at the Vout.
Ive tried 2 different makes of regulator with the same results. What would
account for this strange drop in Vout?
1. Inadequate smoothing (too much ripple) on the wall wart output
for the load, causing the 7809 to go out of regulation when it
runs out of headroom.
Fix: if your load current is less than 400mA, place an electrolytic
cap large enough to keep the wall-wart's output ripple valleys to
11V from the input terminal of the regulator to ground. If the
load current is higher than 400mA, get a new wall wart.
2. Too large a load for the wall wart to support, causing it to fall
below 11V, the 7809's typical dropout voltage.
Fix: Get a new wall wart.
3. If not filtered, (or not filtered enough) dropout when the voltage
out of the XFMR/bridge combo falls below 11V.
Fix: Figure out how large a filter cap you need from:
IdT
C = -----
dV
Where C = the required capacitance in farads
I = the average output current in amperes
dt= the period of the ripple frequency in seconds
dV = the permissible ripple voltage in volts
For a full wave rectifier, dT will be 1/120 = 0.0083s and dV will be
the output voltage from the bridge minus 11 volts. The output voltage
from the bridge will probably be something like the 23 volt RMS out of
the transfromer multiplied by 1.414 to get the peak voltage, then from
that, 1.4 volts subtracted for the diode drops in the bridge. That
comes to
Vout = (VRMS * sqrt2) - 2Vf = (23 * 1.414) - 1.4 ~ 31V
Subtracting 11V from that to get the permissible ripple voltage gets
us 20V for dV so, with the exception of the load current, here ya go:
IdT I * 0.0083s
C = ----- = ------------- = ???
dV 20V
Once you get the capacitance, don't forget to take into consideration
the tolerance of the capacitor, its voltage rating, and its allowable
ripple current rating at the ambient temperature in which it'll be
running.
Just to be on the safe side of everything, increase the capacitance
you get by about 50%, get a cap rated at 50VDC, and make sure the
ripple current rating is about twice your load current rating.
BTW, it wouldn't hurt to change the output cap to 0.1µF, and connect
it and the input caps as close to the regulator package as you can.