Why no triacs for AC power off?

[leadbelly]

The project this concerns is a subwoofer amp which I want to power on/off from the remote amp trigger signal of my AV receiver.

Just about all the DIY designs I have seen use a relay for any kind of AC line switching. Why is that? I would have thought that the cheaper and more reliable way to go would be an optoisolator + triac combination. Is this avoided because leakage through the triac causes some unwanted effect?

[KISS]

I'll take a stab at that. I do use a triac to switch a stereo system from a funky timer circuit. No ill effects.

Relays do not provide isolation.

Your triac circuit has to have a large heatsink, snubber, zero crossing trigger recomended. It also has to have high surge capacity and the big modules are EXPENSIVE. There is a voltage drop across it that's higher than a contactor.

Look here:

http://www.omega.com/pptst/SSRL240_660.html

[Elvee]

Hi

If your sub amp has a regular power supply (50/60Hz Xformer +bridge + cap), you need to take some precautions: zero crossing switching has to be avoided, because mains transformers are designed for steady state operation and if a full first cycle is applied, the core will saturate; if a big toroidal transformer is used, the resulting surge current can be huge.
Another precaution is to use a continuous trigger rather than a pulse: the transformer is loaded only during the peaks of the voltage waveform, and the remaining time, the triac will only see the magnetizing inductance of the primary, with the risk of the triac turning off due to insufficient holding current.
Static relays for inductive loads are specifically designed to take these problems into account, and their cost is only moderately higher than regular types.
LV

[leadbelly]

Thanks for that, that is exactly the kind of information I was looking for.

So, in shopping for solid state relays, they list different features: random firing vs zero crossing; zero crossing triac vs zero crossing back-to-back SCR. Does one of these features make it more suitable for inductive loads?

[Elvee]

Let's take an example from a well-known manufacturer: Carlo Gavazzi (part of Omron):
http://www.gavazzionline.com/pdf/ssr.pdf
In their case, the suitable products carry the suffix -PS (peak switching).
Other manufacturers have similar offerings, with their own denominations.
LV