A Diac is a two-electrode bidirectional avalanche diode which can be switched from off-state to the on-state for either polarity of the applied voltage. Again the terminal designations are arbitrary since the Diac, like Triac, is also a bilateral device. The switching from off-state to on-state is achieved by simply exceeding the avalanche breakdown voltage in either direction.
Click Here. Diac-DS 32 Circuit & datasheet
Most DIACs have a three-layer structure with breakdown voltage around 30 V. DIACs have no gate electrode and are commonly used to trigger TRIACs. DIACs are bidirectional devices, their terminals are labeled MT1 (“Main Terminal”) and MT2.
How to test with power supply.,
we need is a variable D.C power supply from 0-50 volts. In this test, the DIACs’ voltage appeared across R1 at about 30 volts shows connected with parallel voltmeter. The voltage should steadily increase across R as the voltage from the supply is turned up. Change the power supply from o to 50v step by step 2v.
DIAC relaxation oscillator
We can connect the test DIAC to form a relaxation oscillator and see the waves in the oscilloscope CRO. In the circuit above the capacitor will charge through the 100k resistor. When the charge voltage reaches the DIAC break over voltage the capacitor will quickly discharge through the DIAC until the voltage goes below the DIAC holding current, at which time the DIAC will turn off. At that point (around 15 volts) the capacitor will charge again and repeat the process.
HOW TO CHECK WITH DIGITAL MULTIMETER
Select digital multimeter to DIODE MODE:
STEP-1. Connect DMM probe to Diac.
DMM Red Test Lead to DIAC-MT1 (DMM Means Digital Multimeter)
DMM Negative Lead to MT2
DMM READING Result: OL, or 1
STEP-2. Change Polarity Of DMM:
Connect DMM probe to Diac.
DMM -VE Lead to DIAC-MT2
DMM Red Test Lead to MT1
DMM READING SHOWS OL, or 1
Verification: If you get reading in forward bias as 0000 or ‘OL’ or any small value and in reverse bias as 0000 (or) low values the DIA can be FAULTY and needs replacement.