The proximity switch can work for a wide range of power, from 8v to 18v DC, D3 protects reverse power supply connections, and U1 regulates the supply to +5v , -5v is derived from U2 555 oscillator which serves dual purpose.

Circuit Operation

Part of – InfraRed LED Flasher for Optical Switch

The infra red diode D2 detector gets the reflected light from LED and some ambient light, The forward voltage drop of D2 will vary with the amount of light falling on it. Ambient light causes a DC component and the pulsing light from D1 causes an AC component.

Proximity Switch - Driver Supply

The capacitor C6 blocks DC and only transfers AC pulses if any to opamp amplifier U3A whose gain is set by R18, D9 rectifies the pulses to DC and this DC voltage is used by opamp comparator U3B which drives Q1 through Q2 for an open collector output for relays. LED D7 turns on when relay Output is high.

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This is an Application note i used to give with my ssr – app012.pdf

Diagram of SSR 5000 DC 3-30 Control / 230V AC N.O. @ 3A.

This is a Thyristor Based Normally Open 230V Switch that can be turned on / off at a fast rate. This switches at Zero Crossover . Use 5V DC for Best Results.

PCB SSR Wiring

  • No moving parts hence no wear and tear.
  • Dissipates Heat when in On Condition 5 Watts @ 3 Amps.
  • Mount with adequate Air Ventilation for 3 Amps.
  • Dissipation is 1.4 Watts per Amp thru Load.
  • Input to Output is optically isolated 1kV.
  • This one is DC 3-30 Control AC 230V N.O. Output.
  • Off state snubber Leakage is around 1 mA.

Notes –

The Termination’s of High Current Lines going to Load must be very tight and crimped. Loose contacts will Spark and cause Fire.

Fast Blow Fuse is to be used. Use a 2 Amps Fuse with Holder in the Hot Line. This diagram applies to all AC SSR with DC Control

A Center tap 50Hz Step Down Transformer with two diodes is used to get a train of Positive Sine Pulses at 100Hz which is applied to Q6 base via R51.

2N2646 is a unijunction transistor(UJT) in a TO-18 metal package. The control voltage or voltage proportional to error is fed to R56-Q3, The UJT drives a pulse transformer which provides isolation from the Load which the SCR bridge is controlling.

2N2646 based Voltage controlled SCR Pulser

This phase angle control gives a near Linear closed loop control for a SCR bridge which may be used in Electroplating or a Preregulator of a big power supply.

High Energy Circuits

MOSFET and IGBT based systems are now more in use than Thyristor – Triac – SCR based designs in power electronics. Thyristors are more robust than many power devices.

In this circuit we tackle the error indicated in the earlier Current Source. The LM336-2.5V eliminates the tiny error of the regulated supply and resistors. Thereby increasing Precision to a higher degree.

Precision Op-Amp Current Source

The opamp mirrors the stable 2.5V across P3 + R13. With P3 Bourns 10 Turn Trimpot you can trim the current for calibration. Q1 BC557B having a Beta – hfe of 200 is used. But a higher gain or a FET here may reduce error further, that may be needed if you are going for 16 Bit or more resolution. Then even opamp needs to change.

Mini RTD Pt-100 Three Wire Transmitter

Suppose you build it with the best Opamp, FET etc., but place it close to a Warm transformer, Regulator chip or even a Cooling Fan, you will see the lower digits of a 5-1/2 DMM spinning fast.