Re: gEDA-user: SSRs vs. discrete triacs

[DJ Delorie <dj@xxxxxxxxxxx>]

> I need to switch <1A @ 24VAC loads,

Been there.  http://www.delorie.com/house/furnace/

I use an opto-triac and an alternistor.  Alternistors handle the two
polarities with different silicon, which helps with inductive loads,
which is what I've got.  No additional discretes are required, except
for a 38 ohm current limiting resistor between the opto and the
alternistor.

> Triac-based SSRs with zero-cross circuits are more expensive.

Warning: check the trigger voltage for the zero crossing logic.  One
that I looked at was 30v, which is very high relative to a 24vac cycle
(about 60 degrees).  In my case, I'm using an MCU to syncronize the
triggers.  Also, if the on/off cycle is very slow (in my case, minutes
between changes), you may not need a ZC switch.

> But comparing this to building my own out of an
> opto-triac (for isolation) and a discrete power triac, I can build it
> for less even than the cheap MOSFET SSRs, while taking up only
> slightly more board space.

Check out http://www.delorie.com/house/furnace/pcb2/ which uses
smaller than usual optos.  I haven't found any smaller than that,
though.

> Doesn't integration pretty much always result in cost savings?

Not when demand is low, which raises the per-unit manufacturing costs.


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