Triacs, SSR, and Alternistors

[dknguyen]

I know what a triac is (including the sharing of silicon between the SCRs). But how is this different than an SSR? And what is an alternistor? I see the word mentioned, but Googling only turns up products and no explanations.

THe only thing I found on alternistors was that an alterinstor is ACTUALLY two SCRs in parallel (no sharing of silicon). But this was just a couple sentences off of an small forum, and I drew the conclusion myself that he meant they did not share silicon. So I'm not sure.

 

[adaminc]

I got this from another website

"An Alternistor is a chip that has been designed which effectively offers the same performance as two thyristors (SCR's wired inverse parallel i.e. back-to-back). This chip construction provides two electrically separate SCR structures, providing enhanced dv/dt characteristics while retaining the advantages of a single chip device."

 

[dknguyen]

Yeah, that's the only article I found. That same one. I also found a couple of references to a triac needing a snubbing circuit to drive inductive loads, while an alternistor does not...I am not sure why though.

 

[Papabravo]

I think an SSR is a Solid State Relay. It should be at least a four terminal device. On the input side is a circuit like an LED that can be driven from logic inputs. This LED type input provides isolation just like an opto isolator.

Unlike an opto-isolator the output is more like an analog switch, controlled by the LED, with a high resistance state which blocks current flow and a low resistance state which allows current to flow. The output can be designed to emulate normally open or normally closed contacts.

If there are more than four terminals then ecah pair of output "contacts" could be normally closed or normally open.

 

[dknguyen]

Yeah, I was using SSR as an abbreviation for solid state relay.

 

[zevon8]

Be aware that with SSR's there are conditions to be met for proper operation. Depending on the type of output, ie SCR, TRIAC, zero crossing devices for AC operation, etc, there may be restrictions to the type of load required for proper operation. Some devices will not operate without a minimum load, and others will actually show a high resistance, but not "open-circuit" ( as mentioned by Papabravo ) when off. There are also 3-phase devices and bridge devices, as well as FET based devices.

Often, unless it is for serviceability, or maybe ease of "agency approval" when connecting or controlling mains powered equipment, SSR's are not the cheapest way to do things. Convenient yes, cheap no.

For me, they fall under the old design maxxim of " cheap, fast, good" and you can only get to pick 2 of the 3 options, with the 3rd being what you won't get.

 

[dknguyen]

So an SSR is just any generic solid-state device used for switching? It could be triac, mosfet, or alternistor based and therefore different SSRs will have different load capabilities such as inductive or resisitive and AC or DC? That's what it seems like from what I have been finding on SSRs- they are very varied in operation and load capabilities and it just seems like they are just that- a non-mechanical relay regardless of the components they use (as long as they are solid-state components).

Now it seems the issue left is what is the difference between a triac and an alternistor, since so far all the information presented so far has come from the same obscure article.

 

[zevon8]

Alternistors appear to be just a pair of SCR's, as you have found out. Pretty much just seems to be a single packaging of what used to be 2 devices. You get all the benifits of 2 SCR's, isolation, improving dv/dt ratings for inductive loads, etc. I would have a look at the datasheets here, maybe it wil shed some more light:

**broken link removed**

This is pretty much the same as Triacs with diacs built in, it is all about reducing pakaging, simplifying construct and mechanical assembly. Very neat idea, but I don't think it is really anything "new", other than how it is made.

Also, try this website for info about SSR's, and thyristors: **broken link removed**

They make just about all of what you were asking about.