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Silicon Controlled Rectifier (SCR) Circuits
Last Updated on: Saturday, August 30, 2014 10:28 AM

Circuits Designed by Dave Johnson, P.E. :
  • MINIATURE ISOLATED AC/DC POWER SUPPLY
    This circuit uses a novel approach to produce a fully isolated and regulated 5 volts @30ma from the 120vac power line. It uses two tiny SCRs that alternately discharge two capacitors through a miniature high frequency transformer. The voltage spikes produced through the transformer are rectified, filtered and regulated. A very common 8-ohm audio impedance matching transformer can be used for the transformer. Published in EDN, Feb. 17, 1992

  • LOW POWER 12,000-VOLT POWER SUPPLY
    If you need about 12,000 volts DC for an ion generator this circuit might be the ticket. It draws power from the 120vac power line but it uses a small 6KV camera flash trigger coil. The output signal is isolated from the power line. Although the circuit can only deliver about 5uA of current it can produce dangerous shocks, so be careful.
  • CIRCUIT PROTECTS FROM BATTERY POLARITY REVERSAL
    This simple circuit can protect a sensitive electronic circuit from an accidental connection of a battery with a reversed polarity.  The N-channel FET connects the electronic device to the battery only when the polarity is correct.  The circuit shown was designed for a device powered from a single 1.5 volts button cell battery.  However, the circuit will operate with higher voltages as well.

Links to electronic circuits, electronic schematics, designs for engineers, hobbyists, students & inventors:
124 Volt AC Lamp Dimmer Full Wave SCR -  (circuit / schematic design added 6/06)
125 VAC Lamp Dimmer (full wave SCR) -  The full wave phase control circuit below was found in a RCA power circuits book from 1969. The load is placed in series with the AC line and the four diodes provide a full wave rectified voltage to the anode of a SCR. Two small....(electronic schematic added 6/06) 
Circuit Forms Constant Current SCR -  02/01/01 EDN-Design Ideas EDN is migrating links. This link is not verified.  Search the "title" + EDN for new link.

Circuit Protects from Battery Polarity Reversal -  This simple circuit can protect a sensitive electronic circuit from an accidental connection of a battery with a reversed polarity.  The N-channel FET connects the electronic device to the battery only when the polarity is correct.  The circuit shown was designed for a device powered from a single 1.5 volts button cell battery.  However, the circuit will operate with higher voltages as well. …. [Designed by David A. Johnson]
Crow Bar #1 -  Depending on the setting or R1, when the voltage exceeds a certain amount, SCR1 triggers, which activates relay RY1 and opens the circuit.  S1 resets the SCR by interrupting power to it.  This is a simple crow-bar circuit for low amperage protection, up to 4 amps at 30V max. Adjust the SCR for a model that suits your needs…. [Tony van Roon's circuit]
Crow Bar #2 -  This circuit can be added to any power supply design that does not have additional circuit protection. When the voltage exceeds either 10V or 30V, depending on the voltage switch setting via the front panel, the zener diode ZD1 conducts and fires the gate of the SCR. This provides a short-circuit over the output voltage and in turn will blow the AC input fuse of the powersupply you wish to protect.…. [Tony van Roon's circuit]
Low Power 12,000 Volt Power Supply -  If you need about 12,000 volts DC for an ion generator this circuit might be the ticket. It draws power from the 120vac power line but it uses a small 6KV camera flash trigger coil. The output signal is isolated from the power line. Although the circuit can only deliver about 5uA of current it can produce dangerous shocks, so be careful.... [Designed by David A. Johnson]
Miniature Isolated AC/DC Power Supply -  This circuit uses a novel approach to produce a fully isolated and regulated 5 volts @30ma from the 120vac power line. It uses two tiny SCRs that alternately discharge two capacitors through a miniature high frequency transformer. The voltage spikes produced through the transformer are rectified, filtered and regulated. A very common 8-ohm audio impedance matching transformer can be used for the transformer. Published in EDN, Feb. 17, 1992 …. [Designed by David A. Johnson]
Nanosecond SCR Switch for Reliable High Current Pulse Generators & Modulators -  MicroNote 601 from Microsem....[App Note]
Schmidt trigger -  In Dutch.  Ik kan me voorstellen dat de lezer zijn wenkbrauwen fronst, een Schmitt trigger met transistoren? Die maak je toch met een OpAmp of met daarvoor speciale ontwikkelde IC's. Soms heb je toch het probleem dat je over een lage voedingsspanning beschikt,....(added 05/08)
SCR phase control yields solid-state switch -  03/15/01 EDN-Design Ideas....SCRs (silicon-controlled rectifiers]/ PDF contains multiple circuits scroll to find the one of interest, or thyristors, have higher current and voltage ratings, lower conduction losses, and more robustness than triacs. For these reasons, SCRs are better suited to high-power applications.... EDN is migrating links. This link is not verified.  Google the "title" EDN for new link.
SCR Voltage Regulator -  Though not as accurate as a linear or a high-frequency switching regulator, an SCR voltage regulator wastes little power and creates no RFI.  [Designed by Andrew R. Morris  (added 6/06)]
Silicon Controlled Rectifier Model Railroad Throttles & London Model Railroad Group SCR Throttle -  The following is a schematic drawing of a Silicon Controlled Rectifier type throttle for use on larger scale model railroads. Three versions of this throttle are shown on this page. They are not sophisticated designs but work well and are tough and reliable. [from Rob Paisley's Model Train web site]
Thermostat for 1KW Space Heater SCR controlled -  Here is a simple thermostat circuit that can be used to control a relay and supply power to a small space heater through the relay contacts. The relay contacts should be rated above the current requirements for the heater. Temperature changes are detected by a(added 6/06)
UPGA301A Pulse Circuit Analysis -  MicroNote 602 from Microsem  (app note added 08/08/08)

Silicon Controlled Rectifier (SCR) Circuits

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