SILICON CONTROLLED RECTIFIERs (SCR)

SECTION ELEVEN

               SILICON CONTROLLED RECTIFIERs (SCR)

The Silicon Controlled Rectifier (SCR) is a semiconductor device that is a member of a family of control devices known as Thyristors. It is a 3-leaded device and when a small current enters the Gate, the thyristor turns on AND STAYS ON.  It only conducts current between Anode and Cathode in one direction and it is mainly only used in DC circuits. When it is used with AC, it will only conduct for a maximum of half the cycle.

 

To understand how an SCR "latches" when the gate is provided with a small current, we can replace it with two transistors as shown in diagram B above. When the ON button is pressed, the BC547 transistor turns on. This turns ON the BC557 and it takes over from the action of the switch. To turn the circuit off, the OFF button removes the voltage from the base of the BC547.

Testing an SCR

An SCR can be tested with some multimeters but a minimum current Anode-to- Cathode is needed to keep the device turned on. Some multimeters do not provide this amount of current and the SCR Tester circuit above is the best way to test these devices.

Shorted SCRs can usually be detected with an ohmmeter check (SCRs usually fail shorted rather than open).

Measure the anode-to-cathode resistance in both the forward and reverse direct ion; a good SCR should measure near infinity in both directions.

Small and medium-size SCRs can also be gat ed ON with an ohmmeter (on a digital meter use the Diode Check Function). Forward bias the SCR with the ohmmeter by connecting the black ( - ) lead to the anode and the red ( + ) lead to the cathode (because the + of the battery is connected to the negative lead, in most analogue multimeters). Momentarily touch the gate lead to the anode while the probes are still touching both leads; this will provide a small positive turn-on voltage to the gate and the cathode-to-anode resistance reading will drop to a low value. Even after removing the gate voltage, the SCR will stay conducting. Disconnecting the meter leads from the anode or cathode will cause the SCR to revert to its non-conducting state.

When making the above test, the meter impedance acts as the SCR load. On larger SCRs, it may not latch ON because the test current is not above the SCR holding current.

Using the SCR Tester

Connect an SCR and press Switch2. The lamp should not illuminate. If it illuminates, the SCR is around the wrong way or it is faulty.

Keep Switch 2 PRESSED. Press Sw1 very briefly. The lamp or motor will turn ON and remain ON. Release Sw2 and press it again. The Lamp or motor will be OFF.

TRIACs

A triac is a bidirectional, three-terminal dual, back-to-back thyristor (SCR) switch. This device will conduct current in both directions when a small current is constantly applied to the Gate.

If the gate is given a small, brief, current during any instant of a cycle, it will remain triggered during the completion of the cycle until the current though the Main Terminals drops to zero.

This means it will conduct both the positive and negative half-cycles of an AC waveform. If it is tuned on (with a brief pulse) half-way up the positive waveform, it will remain on until the wave rises and finally reaches zero. If it is then turned on (with a brief pulse) part-way on the negative wave, the result will be pulses of energy and the end result will be about 50% of the full-energy delivered at a rate of 100 times per second for a 50HZ supply.

TRIACs are particularly suited for AC power control applications such as motor speed control, light dimmers, temperature control and many others

Using the TRIAC Tester

Connect a TRIAC and press Switch2. The lamp should not illuminate. If it illuminates, the TRIAC is faulty. Keep Switch 2 PRESSED. Press Sw1 very briefly. The lamp or motor will turn ON and remain ON. If the lamp does not turn on, reverse the TRIAC as the current into the gate must produce a slight voltage between Gate and Main Terminal 1.

Release Sw2 and press it again. The Lamp or motor will be OFF

MICA WASHERS AND INSULATORS

Plastic insulating sheets (washers) between a transistor and heatsink are most often made from mica but some are plastic and these get damaged over a period of time, turn dark and become cracked. The plastic eventually becomes carbonized and conducts current and can affect the operation of the appliance. You can see the difference between a mica sheet (washer) and plastic by looking where it extends from under the transistor. Replace all plastic insulators as they eventually fail.

SPARK GAPS

Some TV's and monitors with a CRT (picture tube), have spark gaps either on the socket at the end of the tube or on the chassis.

These can consist of two wires inside a plastic holder or a glass tube or special resistive device.

The purpose of a spark gap is to take any flash-over (from inside the tube), to earth.

This prevents damage to the rest of the circuit.

However if the tube constantly flashes over, a carbon track builds up between the wires and effectively reduces the screen voltage. This can cause brightness and/or focus problems. Removing the spark-gap will restore the voltage.

These are not available as a spare component and it's best to get one from a discarded chassis.

CO-AX CABLES

Co-Ax cables can produce very high losses and it seems impossible that a few metres of cable will reduce the signal.  The author has had a 3 metre cable reduce the signal to "snow" so be aware that this can occur. Faults can also come from a splitter and/or balun as well as dirty plugs and sockets. This can result in very loud bangs in the sound on digital reception.

TESTING EARTH LEAKAGE DETECTORS or

Residual Current Devices or

Ground Fault Circuit Interrupters or GFCI

An Earth Leakage Detector or Sensor is a circuit designed to continuously monitor the imbalance in the current in a pair of load carrying conductors.

These two conductors are normally the Active and Neutral.  Should the imbalance current reach 30mA the sensor will "trip" and remove the voltage (and current) from the line being monitored.

Some detectors will trip at 15mA.

You cannot alter the sensitivity of the device however there are a number of faults in these devices that can be fixed.

In some devices the cont act pressure for the 10Amp or 15 Amp contacts is very weak and they arc and produce an open circuit. The result is this:  When you press the rest button, power is not restored to the output. Clean the contacts with a small file and bend the metal strips to the contacts so they make a very strong contact.

The other fault is the trip mechanism.

The magnetism from the coil does not allow the pin to move and "trip" the contacts. It may be due to a small metal filing or the pin not moving freely enough.

All good Earth Leakage Detectors have a TEST BUTTON. This connects a resistor between the active line and earth so that 15mA or 30mA flows.

The detector should trip immediately. Make sure the trigger mechanism trips when the test button is pressed.

None of the electronics in the detector can be replaced however you can test the mechanical operation and the pressure on the contacts when the unit is removed from the power. Do not work on the device when it is connected to the mains.