How to Test a Diac

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testing a diac
Fig. 1 Sample DIAC test circuit.

How to Test a Diac

by Lewis Loflin

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. Two schematic symbols are shown above. 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 break down voltage in either direction.

The diac acts like an open-circuit until its switching or break over voltage is exceeded. At that point the diac conducts until its current reduces toward zero (below the level of the holding current of the device). The diac, because of its peculiar construction, does not switch sharply into a low voltage condition at a low current level like the SCR or triac. Instead, once it goes into conduction, the diac maintains an almost continuous negative resistance characteristic, that is, voltage decreases with the increase in current. This means that, unlike the SCR and the triac, the diac cannot be expected to maintain a low (on) voltage drop until its current falls below a holding current level.

Read more: http://www.circuitstoday.com/diac

The DIAC, or 'diode for alternating current', is a trigger diode that conducts current only after its breakdown voltage has been reached momentarily. When this occurs, diode enters the region of negative dynamic resistance, leading to a decrease in the voltage drop across the diode and, usually, a sharp increase in current through the diode. The diode remains "in conduction" until the current through it drops below a value characteristic for the device, called the holding current. Below this value, the diode switches back to its high-resistance (non-conducting) state. This behavior is bidirectional, meaning typically the same for both directions of current.

Most DIACs have a three-layer structure with breakdown voltage around 30 V. DIACs have no gate electrode and are commonly used to trigger, such as TRIACs. Some TRIACs contain a built-in DIAC in series with the TRIAC's "gate" terminal for this purpose.

DIACs are also called symmetrical trigger diodes due to the symmetry of their characteristic curve. Because DIACs are bidirectional devices, their terminals are labeled MT1 ("Main Terminal") and MT2. Ref. Wiki.

So much for theory, how does one check as DIAC? The only thing a volt-ohm meter will check is if the DIAC is shorted in the diode check mode. It will read "open" both ways otherwise. To really check a DIAC we need to apply a voltage and note where the DIAC conducts and turns off.

The Power Supply

The first thing we need is a variable D.C. power supply from 0-50 volts or more. (A SIDAC is a higher voltage version of a DIAC, but I don't have one to confirm this or test it.) In my industrial electricity class we do have a variable A.C power supply for 0-22 volts. If one is at home or has no access to a supply, one can be easily built.

Show above in Fig. 1 is a "home brew" test circuit. The "lamp dimmer" is simply a common dimmer circuit for room lighting available at any Lowe's or Home Depot. This should have a knob or slider control. This is used with a Radio Shack 120 to 24-volt transformer to form a variable A.C. supply. D1, D2, C1, C2 form a half-wave voltage doubler and filter. It wouldn't hurt to wire a 22k resistor across C1 and C2 to act as bleeder resistors to discharge the capacitors.

Perform the Test

If one has a 0-50 volt D.C. supply, all they need is U1 and R1. R1 is a 1000-2200 ohm 1-watt resistor.

Step 1: Set the voltage out to 0 volts as measured across TP1 and TP3. Connect a D.C. volt meter across TP2 (red lead) and TP3 (black lead). Slowly increase the voltage. In the case of my test DIACs voltage appeared across R1 at about 30 volts on TP1. The voltage should steadily increase across R1, don't go past 40-50 volts.

Step 2: Slowly turn the supply down and note the reading across R1. Between 25 and 30 volts in my case voltage suddenly dropped to zero.

Step 3: Reverse the DIAC, repeat steps 1 and 2. One should get the same results.

How to Test a Diac

The Power Supply

Perform the Test

Added July 31, 2011