Alternistor = trade name for traic?

"N_Cook"

** Nope.

** The term refers to a special type of triac that has a very high dv/dt rating - so high, that in most applications there is no need for a RC snubber.

With ordinary triacs, a fast rising voltage wave or a high frequency wave will cause the device to trigger itself on.

Not so with an alternistor.

.... Phil

They seem to be a special case version though:

Also, Wiki says: "A TRIAC which can only operate in quadrants I through III, and cannot be triggered in quadrant IV, has improved turn-off (commutation) characteristics. These devices are made specifically for improved commutation when controlling a highly-inductive load, such as a motor or solenoid, an application where normal TRIACs have problems due to high voltage/current angles"

I'd managed to miss that distinction along the way. It seems odd that RS, probably the largest industrial electronic components supplier in the UK , makes no mention of them alongside or within their triac listings.

--

Phil is correct.  Also called Snubberless Triacs by some manugacturers.  
Another thing I can add is that the construction of an alternistor is that  
of two inverse-parallel connected SCRs.  This arrangement has been long  
known to avoid the triggering problems associated with using a triac to  
control highly inductive loads, such as motors. 

Dave M

googling alternistor "600 volt" site:co.uk throws up no UK suppliers for 600V alternistors luckily no one ever uses a snubber in this use, usually just a triac, so replaced with an ordinary 600V triac and turns on and off ok, presumably maker had a stock of alternistors , so used them instead of a triac there

Depends whether you count Digikey as a UK supplier (they have UK pricing and supply to the UK) - they list them anyway.

there

Digikey seemed to list alternistors but not 600V ones. The failed one has no T1-gate resistance in the 10s to low hundreds of ohms, is that a (sometimes) characteristic of alternistors ? otherwise no gas venting through the epoxy fill or other signs of distress

--

http://uk.farnell.com and http://uk.rs-online.com both list 600V units.  
Search for either Alternistor or Snubberless Triac.  They will show up in  
the search results.  Just select the appropriate voltage and current rated  
device you need. 


Dave M

The alternistor is one I'd forgotten until it was just mentioned.

Is that the "magic ingredient" in electronic florescent starters?

The triac in those basically replaces the bi-metal contacts in the glow tube that break the current for a large back emf from the iron ballast.

Its a long shot - but check out the ST semiconductor range of triacs for electronic florescent starters, they have to withstand the back emf from the iron cored ballast, and presumably a pretty sharp dV/dT.

"Lee"

** The wiki quote is a bit misleading - cos there are two specs for dV/dt.

1. The regular spec referring to the "withstanding" condition where the triac is in the off state.
2. The other called " Commutating dV/dt" - referring to the condition where the triac has just stopped conduction because the load current has fallen below the holding threshold, about 15mA to 50mA for most devices. At this critical point in time, the device will retrigger if the voltage rises too fast. The back emf from an inductor can easily do so.

The commutating figure is generally much lower that the regular one, up to

100 times lower - eg the triac ( or SBS ) used in the MOC series of opto-couplers has a regular dV/dt of 10V/uS, falling to a mere 0.1V/uS while commutating off.

A simple RC snubber across the main triac normally solves the problem.

BTW: If properly designed, the triac/SBS in a MOC goes off as soon as the driven triac begins to conduct so the commutating dV/dt issue never arises.

... Phil