Lab 6: Diac and Triac Characteristics /
ELNC 1068 /
Mike May /
7/19/2009 /
Observations:
Laboratory Exercise #21
We can see from this exercise that the primary function of a DIAC is to phase shift a TRIAC. I do not have the reading of the voltage drop across the DIAC but knowing how a DIAC works I can tell what would happen.
I’m assuming that the DIAC we used was an IN5761. If so, the operating voltage would be 7 Volts and as long as the holding current (IH) does not go below what it is rated, the DIAC will continue to have 7 Volts dropped across it.
When the applied AC voltage has been lowered with the autotransformer the voltage drop across the DIAC remains constant because of the characteristics of the diode. With the DIAC being an AC intrinsic thyristor it essentially acts like a zener diode for AC. However, it can conduct on either half of an AC waveform because it is bi-directional unlike a zener diode which is only used in the reverse biased configuration.
Laboratory Exercise #23
Due to the triac essentially being two SCR’s connected in parallel with the gate leads tied together, the triac must be phase shifted to gain complete control of the AC wave.
In order to phase shift the triac a diac is used to trigger the gate of the triac during positive and negative alternations of the AC waveform.
The part of the circuit that controls the phase shifting is the resistors R1, R2, and capacitor C1. The size of R1 essentially determines the upper limit on the amount of charging current that can be delivered to the capacitor. With R2 being adjustable we can determine how long the charge time is for the capacitor and basically control when the triac is fired based on the characteristics of the diac.
By adjusting R2 what we would basically see is similar to the phase shifting of an SCR. The light bulb would dim and go bright depending on when the triac was fired thus determining how much power was delivered to the light bulb. The waveform shift would depend on how much resistance there was thus increasing the charge time of the capacitor resulting in a dim bulb for a high resistance. The triac would be fired late and only a portion of the power would be delivered to the load.
Conclusions:
The diac is a thyristor which is essentially a four layer diode that conducts when the voltage across its terminals exceeds the breakover potential. What we found is that a diac is basically a voltage sensitive AC switch that can conduct current in either direction when properly activated.
Its applications are limited when used by itself but works great in triac circuits for triggering because it is bi-directional. It can be used to function as a zener diode, however, for AC circuits where voltage regulation is necessary.
The triac is basically a diac with a gate terminal. It can be turned on by a pulse of gate current but does not require the breakover voltage to initiate conduction like the diac. Unlike an SCR, the triac is designed to conduct on both halves of the AC waveform. Therefore, the output of a triac is alternating current instead of direct current.
The triac has more than one application. Like the SCR, triacs can be used to control average power to a load by the method of phase control which was utilized in this lab. The triac can be triggered such that the AC power is supplied to the load for a controlled portion of each half cycle of the wave. During each positive half cycle the triac is OFF for a certain interval, and then it is triggered ON and conducts current through the load for the remaining portion of the positive half cycle. This is the same for negative cycles except the current reverses polarity.
The triac also has the ability to control a large amount of load current with a small amount of gate current which can be useful for high current loads such as motors and fans.