4

4768B–INDCO–10/05

T2117

4.

Triac Firing Current (Pulse)

This depends on the triac requirement. It can be limited by the gate series resistance which is

calculated as follows:

where:

V

G

= Gate voltage

I

I

p

= Average gate current

t

p

= Firing pulse width

T

= Mains period duration

5.

Firing Pulse Width t

p

This depends on the latching current of the triac and its load current. The firing pulse width is

determined by the zero-crossing detection which can be influenced by the synchronous resis-

tance, R

where

I

L

= Latching current of the triac

V

M

= Mains supply, effective

P

= Load power

The total current consumption is influenced by the firing pulse width which can be calculated as

follows:

R

Gmax

7.5 V

V

Gmax

–

I

Gmax

------------------------------------

36

Ω

–

≈

I

P

I

Gmax

T

--------------

t

p

×

=

t

p

2

ω

--- arc. sin

I

L

V

M

×

P2

-------------------

⎝⎠

⎛⎞

=

R

sync

V

M

2 sin

ω

t

p

2

----

×

⎝⎠

⎛⎞ 0.6 V

–

3.5

10

-5

A

×

------------------------------------------------------------------

49 k

Ω

–

=