> From:         Tim ODriscoll[SMTP:tim@STOKEPOGESWAY.NET]
> Sent:         Friday, December 26, 2003 6:42 AM
> To:   PICLIST@MITVMA.MIT.EDU
> Subject:      [EE:] Triac driving problems

> Hi Everyone,

> I'm having a few problems getting my triac to turn on through an
> opto-triac. More specifically, the opto-triac is turning on and putting
> mains voltage at the triac's gate, but the triac won't start conducting.

> A schematic:

> +5vDC                                       240vAC (live)
> |-R1---LED1---|     |------R2-----|------------|
>               |     |             |
>              LED2  TR1           TR2
>               |     |            | |
> --------------|     |------------- -------LOAD-|
> Gnd                                           Neutral

> R1     200 Ohms
> R2     5.6K
> LED1   Red indicator LED
> LED2   MOC3041 opto-triac
> TR1    MOC3041 opto-triac
> TR2    BTA16-600 Triac (16A, 600V)

> My LOAD is a 60W table lamp I'm using for testing.
>
> I'm a bit nervous of experimenting too much with this because I've already
> welded my meter probes together and flipped the house electrics off
> yesterday, and if it wasn't for my (now black and sooty) safety goggles,
> I'd be very unhappy.

> TR2's data sheet says it needs a max 50mA of gate current to turn on,
> and 240/5600 is 42mA. From the datasheet's graphs, it looks like the
> minimum Igt is 16mA. I've tried it with a 9.6K resistor in there and a
> fresh triac, but still no light.

> I've read up as much as I could find on interfacing to triacs, and (to me)
> mine looks OK.

> Any ideas of what I'm doing wrong?

> Cheers,

> Tim

Assuming that the optocoupler triac is turning on and that the gate current
is above the required minimum, there is the issue of correct connection.
The gate current must be applied between the gate and MT1 (main
terminal 1). The triac is not symmetric with respect to its "ends". MT1
should be connected to neutral, a limiting resistor and the optocoupler
to the gate, and MT2 to the load. The other end of the resistor-opto
combination should go to MT2. This way, the gate current drops to
a low value once the triac turns on. If you derive the gate current from
the hot side of the line, there will be significant gate current throughout
the power cycle, heating the resistor. If you reverse MT1 and MT2, the
triac may not turn on.

John Power

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