K0900E70 Datasheet, PDF (177/224 Pages) Teccor Electronics – Thyristor Product Catalog

Application Notes
AN1007
tally that general purpose AC circuits will generate minimum
electromagnetic interference (EMI) if energized at zero voltage.
The ideal AC circuit switch, therefore, consists of a contact which
closes at the instant when voltage across it is zero and opens at
the instant when current through it is zero. This has become
known as “zero-voltage switching.”
For applications that require synchronized zero-crossing turn-on,
the illustration in Figure AN1007.11 shows a circuit which incor-
porates an optocoupler with a built-in zero-crossing detector
Rin
1
Vcc
2
3
Zero
Crossing
Circuit
6
22
5
4
G
MT2 100
MT1
Hot
120 V ac
0.1 µF
Neutral
Load
Figure AN1007.11 Optocoupled Circuit with Zero-crossing Turn-on
(Triac or Alternistor)
Also, this circuit includes a dv/dt snubber network connected
across the power triac. This typical circuit illustrates switching the
hot line; however, the load may be connected to either the hot or
neutral line. Also, note that the series gate resistor is low in value
(22 Ω), which is possible on a 120 V line and above, since zero-
crossing turn-on is ensured in any initial half cycle.
Summary of Zero Crossing Turn-on Circuits
Zero voltage crossing turn-on opto-drivers are designed to limit
turn-on voltage to less than 20 V. This reduces the amount of RFI
and EMI generated when the thyristor switches on. Because of
this zero turn-on, these devices cannot be used to phase control
loads. Therefore, speed control of a motor and dimming of a
lamp cannot be accomplished with zero turn-on opto-couplers.
Since the voltage is limited to 20 V or less, the series gate resis-
tor that limits the gate drive current has to be much lower with a
zero crossing opto-driver. With typical inhibit voltage of 5 V, an
alternistor triac gate could require a 160 mA at -30 °C (5 V/
0.16 A = 31 Ω gate resistor). If the load has a high inrush current,
then drive the gate of the triac with as much current as reliably
possible but stay under the I_TSM rating of the opto-driver. By using
22 Ω for the gate resistor, a current of at least 227 mA is supplied
with only 5 V, but limited to 909 mA if the voltage goes to 20 V. As
shown in Figure AN1007.12, Figure AN1007.13, and Figure
AN1007.14, a 22 Ω gate resistor is a good choice for various
zero crossing controllers.
Rin
1
6
Input
5
2
Zero
4
Crossing
3 _Circuit
22
G
Load could be here
instead of lower location
MT2
MT1
Hot
100 Ω
120/240 V ac
Triac or
Alternistor
0.1µf
Load
Neutral
Figure AN1007.12 Zero Crossing Turn-on Opto Triac Driver
Non-sensitive Gate SCRs
Rin ₁
6
G
Input
5
2
4
Zero
3
Crossing
Circuit
22
100
K
A
G
A
K
Load
120/240 V ac
0.1µF
Load could be here
instead of lower location
Figure AN1007.13 Zero Crossing Turn-on Non-sensitive SCR Driver
Sensitive Gate SCRs
1K
Rin ₁
6
Input
5
2
4
Zero
3
Crossing
Circuit
22
1K
*
G
KA
AG K
*
Load
100
120/240 V ac
0.1 µF
* Gate Diodes to Have
Same PIV as SCRs
Load could be here
instead of lower location
Figure AN1007.14 Zero Crossing Turn-on Opto-sensitive Gate SCR
Driver
©2002 Teccor Electronics
Thyristor Product Catalog
AN1007 - 5
http://www.teccor.com
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