A device frequently used in solid-state equipment such as lighting dimmers and variable speed controls is the silicon controlled rectifier (SCR). As shown in Fig. 1, it's a semiconductor device having three electrodes: an anode, a cathode, and a gate. An SCR's anode and cathode are similar to those of an ordinary semiconductor diode. (See Back To Basics - Part 1, April 1995 issue for detailed discussion of a diode.)

How does an SCR differ from a diode? Well, for one thing, it has the aforementioned gate electrode, which is its control point. (More about this later.) For another, it will not pass significant current, even when forward biased, unless the anode voltage equals or exceeds the forward breakover voltage. Once this breakover voltage is reached an SCR will switch ON and become highly conductive.

SCR characteristic curve

When an SCR is reverse biased and its gate diode is not connected, its voltage-current characteristic curve is as shown in Fig. 2. (See Back To Basics-Part 1, April 1995 issue for discussion of forward and reverse bias.) In this mode, an SCR operates like a regular zener or avalanche diode. (See Back To Basics-Part 2, May 1995 issue.) In other words, there is a small amount of current flow until avalanche is reached, after which the current increases dramatically. And, as is the case with a zener diode, this current can cause damage if thermal runaway begins.

When an SCR is forward biased, there's a small current, the forward blocking current. This current will stay relatively constant, at least until the forward blocking voltage is reached. At this point, which is called the forward avalanche region, the current will increase rapidly. Here, an SCR's resistance is very small. In fact, an SCR acts the same as a closed switch here, with the current limited only by any external load resistance. As such, a short in an SCR's load circuit will destroy the SCR if inadequate overload protection is provided.

Gate control

As mentioned earlier, an SCR works just like a mechanical switch: it's either ON or OFF. When the applied voltage on an SCR is below its forward breakover voltage ([V.sub.BRF]), the SCR fires (is ON). It will stay ON as long as the current stays above the holding current value; it will turn OFF when the voltage across it drops to a value too low to maintain the holding current.

How does an SCR's gate electrode come into play here? Well, when the gate is forward biased and current begins to flow in the gate-cathode junction, [V.sub.BRF] is reduced. The higher the forward bias, the less [V.sub.BRF] needed to get the SCR to conduct. This is shown in Fig. 3.

Once an SCR is turned ON by its gate current, this current loses control of the SCR's forward current. Even with its gate current completely removed, an SCR will stay ON until its anode voltage is removed. It also will stay ON until the anode voltage is reduced enough so that the current is not sufficient to maintain a proper holding current level.

SCR applications

Basically, an SCR is used as a DC switch because of its many advantages over mechanical DC switching. These include arcless switching, low forward voltage drop, rapid switching time, and no moving parts. An SCR can be used for AC switching, although two SCRs are needed.

Varying power to a load is perhaps an SCR's most prominent application. This is because of its ability to turn ON at different points in its conducting cycle; thus, its usefulness in varying the amount of power delivered to a load. This type of variable control is called phase control. Don't confuse the term "phase" as used here with that pertaining to power distribution systems. Here, "phase" refers to the time relationship between two events, in this case, between trigger pulse and the point in the conducting cycle at which the pulse occurs.

Testing an SCR

You can "rough" test SCRs using an ohmmeter and a test circuit, as shown in Fig. 4, and the following steps. If an SCR does not respond as indicated for each of these steps, it's defective and should be replaced.

Step 1. Set the ohmmeter on the "R x 100" scale. Connect the ohmmeter's negative lead to the SCR's cathode and its positive lead to the SCR's anode. The ohmmeter should read infinity. (Resistance will actually be over 250,000 ohms.)

Step 2. Close the switch. This will short circuit the gate to the anode. The ohmmeter should read almost zero ohms. (Resistance will actually be about 10 to 50 ohms; this range of readings will not register on the "R x 100" scale.) Open the switch and the ohmmeter should still read zero ohms.

Step 3. Reconnect the ohmmeters leads, positive lead to the SCR's cathode and its negative lead to the SCR's anode. The ohmmeter should read infinity. (Resistance will actually be over 250,000 ohms.)

Step 4. Close the switch. This will short circuit the gate to the anode. The resistance reading should remain high because the SCR is reverse-biased and, therefore, can't conduct.

Step 5. Open the switch. The resistance should remain high because the SCR is reversed-biased and has no gate current.