Diac

Diac are electronic components with two ports and bidirectional diode are called. The distance between the terminals is only conductive when the voltage exceeds the breakdown voltage of them. The term diac is an acronym that stands for English diode for alternating current ( dt diode for alternating current). In addition diac exist under trade names such SIDAC, english Silicon Diode for Alternating Current depending on the type having higher ignition voltages.

Construction

Diac are multi-layer semiconductor. There are versions with PNP transitions (three- layer diode) PNPN junctions ( four-layer diode ) and pnpnp transitions ( five-layer diode). Structurally, it corresponds to an anti- parallel connection of two thyristors, as shown in the adjacent equivalent circuit similar to a triac. In contrast to this, the control terminal is absent.

The two terminals are referred to as anode 1 and the anode 2, since there is no polarity may be indicated by the symmetrical structure.

A precursor component in the form of a four-layer diode, which is no longer manufactured, the Shockley diode.

Function

The diac can cycle through its bi-directional construction and AC voltages. When the voltage exceeds a certain threshold voltage at the terminals (A1 and A2 ), it turns on and routes the PN be conductive. This type of ignition that is also possible with triacs ( should be in these, however, avoided ), also called overhead ignition.

Only when the current through the diac is below a certain value, the holding current, the resistance increases again rapidly. This process is referred to as deletion of the diac.

By variations in the crystal lattice, and other manufacturing tolerances, the two switching voltages in positive and negative directions are not exactly the same. The difference is called the deviation from symmetry.

Diac have a similar function as spark gaps, which are low even when a certain ignition voltage. However, they differ in the normal voltage ranges. While diac be used at low voltages up to a few 100 V, spark gaps are used in the field of high voltage such as the Marx generator to several 100 kV.

Characteristics

The following values ​​are only guidelines, which should give an idea of the approximate orders of magnitude.

  • Ignition voltage, VBO in the diagram as indicated: 20 ... 200 V ( typical value 35 V with strong manufacturing tolerance )
  • Symmetry deviation: ± 4 V
  • Resistance in the high-impedance state: a few milliohms
  • Resistance in the low-resistance state: some Ω
  • Maximum power dissipation of 300 mW

Use

Diac are mainly used in firing circuits of triacs to generate a trigger pulse with a very steep slope. In practice, they are used eg in the control electronics of energy saving lamps or dimmers. In Phase Angle with vertical control diac be used as a switch for an adjustable threshold. In the Royer converters of energy saving lamps they are intended to provide a pulse for the safe starting of the converter, as Royer converter Anschwingschwierigkeiten have if your supply voltage to rise slowly when turning.

Diac also be used to protect against strong noise (crackle protection) in telephony, such as in older telephones with a dial where the diac is switched in parallel with the handset to set - produced by the mechanically -generated pulse dialing steep -edged Pulse - may to prevent over to the acoustic pain threshold beyond loud clicks in the telephone receiver.

Since the current -voltage characteristic is very similar as that of the glow lamp (which has only one higher firing voltage) can, just a relaxation oscillator, the oscillator can be realized with a diac.

Other uses as electronic AC switches are also possible, but in practice hardly find application as better options exist.

Type overview

There are few common diac types, therefore, no effort here a type overview will be given.

  • BR100 -03: DO-35 package; Manufacturer Philips / NXP
  • DB3 DB3TG, DB4, D30, ER900: DO- 35 package; Manufacturer ST
  • DB3A, DB3B: DO- 35 package; Manufacturer Taitron
  • SMDB3: SOT- 23 package; Manufacturer ST
  • NTE6407, NTE6408: DO- 35 package; Manufacturer NTE
  • NTE6411, NTE6412: DO- 35 package; Manufacturer NTE
  • TMMDB3, TMMDB3TG: Minimelf housing; Manufacturer ST
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