[Hallicrafters] Silicon rectifiers

[Roger Halstead (K8RI)]

I'm sending this to the reflector as a note of interest on the term SCR and 
some really old equipment that truly qualified as "boat anchors".

>Sorry about the incorrect statement on the rectifiers.
>Don't know why I said SCR!

Don't worry about the term <:-))
Seeing SCR mentioned in reference to a rig of that era just  kind of 
surprised me.

At that time SCRs known as Silicon Controlled Rectifiers, or sometimes as 
Thyristors were just coming into use for power control.  They were a fun and 
cantankerous thing to work with. Much like a triode they had a third wire 
that could turn them on. Not off, just on. To turn them off required a 
reverse voltage which of course happened every half cycle.  Soooo... If you 
controlled the timing of the "on pulse" you could get 0 through 180 degrees 
of control. Adding a second SCR in the opposite direction could give a full 
0 to 360 degrees, or they'd control just like a variac (In theory)

The darn things developed some bodacious spikes. Those spikes could travel 
quite a distance on your circuits. So it was not unusual to adjust the power 
on one system, such as an oven and have the power change on another. Where 
it really became interesting was on 3-phase. You'd adjust one phase up and 
one of the others might go down while the third... well, the third might go 
up or down, but it rarely was unaffected.

This little problem meant the controllers were rarely as simple as the 
diagrams indicated and industrial controllers could be quite elaborate.  It 
didn't take a lot of experimenting and we decided it was easier and cheaper 
to go with commercial systems.

OTOH I've seen SCRs about the size of a hockey puck that "as I recall" could 
control up to a 1000 amps (or more) at 440V AC.  The silicon *slice*, or 
wafer inside was over an inch in diameter.

Now if you have been around for a really longgggg time, SCR meant Saturable 
Core Reactor.
these were nothing more than an iron core inductor that had a control 
winding.  It only took a small DC current to saturate the core and when that 
happened it became very much like an air core inductor with much less 
reactance.

These SCRs did the work of the old, *original* SCR or saturable core 
reactor.  The two "hockey pucks" took the place of an old SCR that would 
have weighed as much as several tons.

73

Roger Halstead (K8RI, EN73 & ARRL Life Member)
N833R, World's Oldest Debonair (S# CD-2)
www.rogerhalstead.com

>73

>At any rate I still appreciate the help and the link to the data sheet.

>I am certainly not an expert or a tech  Hi!
>I just like to tinker with old radios and learn what I can, and this group 
>is a great place to learn.

Bob, KG4NVX
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