Hello Vincent, and thank you for your question.
I'll do my best to answer it as completely as possible here, but for more detailed explanations you can access anytime, Crydom provides thorough explanations of what solid state relays are, how they're used, the selection of which types to use, how to determine heatsinking requirements, etc., etc., under the "Tech Library" and "Service and Support" heading on our Crydom Website home page, www.crydom.com Links to the Service and Tech Library are:
http://www.crydom.com/en/Service/Training.shtml
http://www.crydom.com/en/Tech/Tips.shtml
So to your questions...
#1 and #2 pertain to the 2 general types of SSR's. Unlike mechanical relays, solid state relays are specificly constructed to switch either AC loads or DC loads. They cannot switch both. SSR's that switch AC loads typically use dual power SCR's, (Silicon Controlled Rectifiers), connected inversely and in parallel, with each SCR switching one side of the AC voltage. DC switching SSR's utilize Bi-polar power transistors or more commonly power MOSFET devices. Again, more details on this subject can be obtained through the links provided.
#3, #4, and #5 refer to the switching characteristics available with solid state relays. The terms "Non-zero crossing" and "Random turn-on" are interchangeable and mean exactly the same thing. Since solid state relays are of course, "solid state", and can react in micro seconds if desired unlike mechanical relays, the exact switching point in the AC waveform where the output turns on can be easily designed into the SSR. The main advantage of using a zero-crossing turn on SSR is when the load being switched tends to be of a "high inrush" type, usually having a capacitive characteristic or a tungsten lamp load where the "cold resistance" of a tungsten filament is much lower than its "hot resistance" and would result in a very large current surge for the first few AC cycles. The turn-on at the zero voltage crossing point lessens that surge substantially. The zero-cross effect means that if the input signal is applied at any point during the AC output wave other than very close to the zero voltage point of that wave, the output will "wait" to switch on until the AC wave reaches the following zero point. The "random switching" type SSR means that the output switches on within micro-seconds of the application of the input regardless of where the AC waveform is at that point. Random turn-on type SSR's are typically recommended for inductive load applications such as motors, coils, etc, due to factors involving the current and voltage phase shift involed, ( a much deeper subject than can be covered here), along with applications that use the SSR to vary the power levels of AC loads through "phase angle control" such as lamp dimming or resistive heater control.
The last part of your question regarding a need to "instantly" turn-off your AC output when a low level DC input is received can be covered in two parts.
First, the turn-OFF of either a zero-crossing or random turn-on SSR are both identical. Since the output uses SCR's, even if an input signal commands a turn-on the SCR that is conducting at that time will NOT stop conducting until the load current is reduced to zero, (for a resistive load that point will be at the zero-voltage point of the AC wave). Therefore, the turn-off time for an AC output SSR can be almost as long as 1/2 the AC cycle time, (for 50 hz that would be 10msec), or as short as a few micro seconds if the turn-off was received just at the zero-cross point.
The second part of your last question involves your need to turn-off the load when a DC signal is received. This requires a "normally closed" SSR function (output always on without an input signal, output will switch off with an input signal, sometimes referred to as a form "B" switch). Just about 99.99% of the SSR'[s manufactured in the world are of the normally open type. Fortunately, Crydom covers that 00.01% with a solid state series of relays that includes some normally closed (for "B") versions. Please look through the datasheet for our S1 series (link below) where you will see that for most of the variations listed a "-B" suffix can be added that designates a normally closed version.
http://www.crydom.com/en/Products/Catalog/s_1.pdf
I hope this "small" dissertation has helped you in your understanding of the Crydom solid state relays.
Thank you again,
Pete B.
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