Re: Options for driving triac
- From: Spehro Pefhany <speffSNIP@xxxxxxxxxxxxxxxxxxxxxxx>
- Date: Tue, 26 Jul 2005 10:18:25 -0400
On Tue, 26 Jul 2005 21:31:32 +0800, the renowned "bruce varley"
<bxvarley@xxxxxxxxxxxxxxx> wrote:
>Hi, I need to drive triacs from CMOS, the reliable gate current is somewhat
>above what the gates can source. There are two obvious options using
>bipolars:
>
>OPTION 1. Emitter follower, base to gate output, collector to +Vcc, emitter
>via a suitable resistor to triac gate.
>
>OPTION 2. Q1 (NPN) as a switch, with resistor divider from gate output to
>base, emitter to gnd, collector resistor divider driving a PNP current
>source from Vcc down to the triac. Current limiting resistor between PNP
>collector and triac gate. Esentially a complementary switch pair.
>
>It intuitively feels to me that the second option may give better
>'protection' to the sensitive CMOS, but in theory Option 1, which uses a lot
>less bits, should be OK. I do require a solution that doesn't significantly
>degrade MTBF (stated vaguely, I know).
>
>Is there any objective reason why one would go option 2? TIA
>
You'd be better off sucking current out of the gate.
If you tie Vdd to MT1 and use an NPN (collector to gate through
resistor, emitter to Vss, base resistor to input...
Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
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