[HH] cheapest/simplest way to control a relay from a PC

[Jack Coats]

> My concern would be that it's actually an SCR internally,
> which doesn't work with DC. SCR's are (were?) commonly
> used for switching AC stuff (dimmers and motors and so on),
> but they won't work with a DC load.
>
>
> Fuzzy SCR lesson follows for those interested:
>
> SCR => Silicon-controlled rectifier
>
> One might think of an SCR as a "latch". When you turn it "on",
> it latches on, even if you remove the on signal, and it stays
> on until the voltage you're controlling goes negative. 120v AC
> goes negative at a 60hertz rate, so when you turn it off, it
> goes off when the 60hertz AC goes negative. So SCR's are OK
> to use with AC to design things like light dimmers and
> "relays" for 120v AC devices.
>
> They're a relatively cheap way to switch on AC.
> I think true solid state relays are more expensive.
> This is old school information I'm going off of,
> it's been a while since I worked with SCRs and similar.
> So, it might be that solid state relays have gotten cheaper
> than when I worked with them.
>
>

If my memory serves me correctly, SCR's basically need to be turned
off at 'zero crossing' point.  And yes, they are a rectifier.  They
can be turned on part way through a cycle, but off only at the 'end'
of a half wave.  Yes, they are AC only components, and work best on
restiveness loads, and inductive loads can blow them if they don't
have other 'protective' components.  This has been a long time since I
played with them.

A quick google shows at least some solid state relays allow for DC
loads ( http://www.onlinecomponents.com/omron-industrial-g3tc-iac15-ac-dc-120.html?p=38192527
for example).  This PDF
http://www.crydom.com/en/Tech/Tips/DC%20relays-technical%20guidelines.pdf
has some documentation on use (again, courtesy of Google).

SCR's used to be the DIYer standby for AC control, but SSR's seem to
have taken over for all but the lowest $ projects.

Enjoy.

Then again, I could be completely wrong.