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AC
Line Powered LED Strings
This shows 4 different ways to string white LEDs that are powered by a 120vac or
240vac power line. The circuit was designed by Ken Schultz.
AC Line Under/Over Voltage Alarm
Power lines, which deviate much beyond normal voltages can damage expensive
electronic equipment. The circuit below sounds an alarm whenever the line
voltage is higher or lower than normal. I set the alarm limits at about +-15%
from standard levels. The circuit rectifies and filters the power line signal.
I set the resistor values, so the DC voltage produced is close to 1% of the RMS
value of the line. Thus, a 120vac line would yield about 1.2v DC. That
voltage is fed to a pair of voltage comparators. The outputs of the
comparators decide if the sampled voltage is above, below or acceptable.
AC Power Line Hi/Lo Voltage Tester
If you wish to test a line-powered device under both a 15% high and a 15% low voltage
condition, you can use the circuit below. The circuit uses an 18v transformer with
a 3 amp rating. A double pole, double throw toggle switch then switches in the
transformer voltage in either a buck or a boost mode. In the buck mode, the
transformer voltage.
Active AC Power Line Indicator
The circuit below is designed to detect when a power cable is active (hot). When the
flat sensor end of the device is held near an active AC line, the bright green LED on
the top of the housing will glow. It is powered by a 3v CR2032 lithium coin cell.
It has no on/off switch and will function for years before the battery has to be
replaced. The circuit could be contained in a small plastic housing with a
diameter of about one inch.
CHARGE
COUPLED BI-DIRECTIONAL POWER MOSFET RELAY
The circuit uses an inexpensive C-MOS inverter package and a few small capacitors to
drive two power MOS transistors from a 12v to 15v supply. Since the coupling capacitor
values used to drive the FETs are small, the leakage current from the power line into
the control circuit is a tiny 4uA. Only about 1.5mA of DC is needed to turn on and off
400 watts of AC or DC power to a load.
Published in Electronic Design, Nov. 8, 1990.
Line Powered White LED Array
There has been a lot of excitement lately in the lighting industry. Some companies
are beginning to manufacture general lighting fixtures, which replace incandescent and
fluorescent lamps, using LED lights. When proper heat management is designed into
the light fixtures, these LED lights can produce more light with less electricity and
last much longer than other devices. They can also operate well in cold
temperatures. Some of these new lighting fixture designs use a large number of
white LEDs, wired into long series wired string arrays. The circuit below is
designed to provide a regulated DC voltage for these strings.
SOLID
STATE RELAY REQUIRES ONLY 50uA DRIVE CURRENT
This circuit demands a control current that is 100 times smaller than that needed by a
typical optically isolated solid state relays. It is ideal for battery-powered systems.
Using a combination of a high current TRIAC and a very sensitive low current SCR, the
circuit can control about 600 watts of power to load while providing full isolation and
transient protection.
