designed by David A. Johnson, P.E.
May 14, 2007
A while back a guy by the name of Ken Schultz sent me a simple drawing of how he connected a string of 30 LEDs, to make a nice under the counter accent light, powered by 120vac. He wired the strings in two series sections of 15 LED each, but wired in opposite directions. He then used just one capacitor to limit the AC current through the two stings. I looked at the circuit and decided that it seemed quite reasonable. The only change I decided to make was to add a metal oxide resistor in series with the capacitor, to act as fuse and to limit the peak current, should there be a voltage spike on the AC line. With the two strings of 15, the current is first pumped through one series string, then as the AC line polarity changes, it flows through the second string. Since the capacitor acts as a constant current source, you can use other string numbers. The capacitor value shown keeps the current limited to about 20ma for the LEDs. In Europe and Australia, where the line frequency is 50Hz, you may see a noticeable strobing of the LEDs, if there is substantial separation between the two different polarity strings.
Soktha from France () sent me his version of this circuit, using two strings of 14, for a total of 28 white LEDs. He mounted the LEDs onto a wood stick. When powered up from a 240vac 50Hz source, the devices provided a nice white light.
Click on Schematic below to view PDF version of this Circuit
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.
Click on Drawing Below to view PDF version of Schematic
AC Power Control CircuitsFET Circuits
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.
Click on Drawing Below to view PDF version of Schematic
AC Power Control Circuits
ISOLATED AC CURRENT MONITOR
This circuit uses a small AC current transformer from Magnetek to produce an isolated voltage proportional to the AC current in the primary winding. The transformer contains a single turn primary with a low 0.001-ohm resistance. It can easily handle 30 amps of AC current and provides at least 500vac of isolation. With the components shown, the output AC voltage is scaled so 1 amp of current produces 100mv of AC voltage.
Click on Drawing Below to view PDF version of Schematic
AC to AC ConvertersAC to DC ConvertersCurrent Monitors
+12V/-12V Power supply unit
The power supply unit is using the ready-made. It inputs AC100 V and it is possible to take out 12 V of the two DCs which are independent as the output.
I considered and made the use (for the experiment) in the future the one with the a little big output current capacity. The positive power supply is the 1.2 amperes and the negative power supply is the 0.9 amperes.
I used ERA22FWA of the POWER SOURCE company as the power supply unit.
I am installing the power supply switch and the output terminal in the surface,
considering the operability.
I opened some holes for the ventilation at the top and the bottom of the case.
The size of the case is the height: 56 mm, the width: 150 mm, the depth: 190 mm.
(It contains the umbo)
The ammeter used the 1 ampere.
The right is for +12 V and the left is for -12 V. I put the mark later so as not to forget.
I used the small one as the power supply switch. It is because the switch isn't carelessly touched.
I used the a little thick wire for the side of the output.