Re: Solid state relays

--- In CAD_CAM_EDM_DRO@egroups.com, Joe Vicars <jvicars@c...> wrote:

> Is it true that a ssr will only switch AC? I read somewhere

that if

> you try to switch DC it will turn on but that when you remove the DC
> control signal the relay won't switch off.
> Do I remember right?

Under most circumstances, yes they only switch AC. The "guts" are
usually a triac and opto-isolator setup. Triacs are like SCR's that
turn on and stay on as long as current is flowing. Since an AC
waveform goes through zero every 8 milliseconds (at 60hz) the device
will turn off as long as gate current has been removed prior to that
point. If you pulse an SCR just after it passes through zero it will
turn on and stay on until the next zero crossing. You get half wave
rectification (positive portion of the wave only). Put another SCR
in reverse with the first, and it will conduct during the negative
portion of the wave. Presto, fullwave rectification/switching. A
Triac is two SCR's back-to-back with a common drive circuit. Since
DC never has a zero crossing it will come on and stay on forever.
You can interrupt the current flow with a series switch but then who
needs the ssr?. DC is better switched with something like a descrete
power transistor or a FET.