[GreenKeys] constant current

[WB6BLV]

Hi Ralph,

You may note from my original post that Irv Hoff did NOT design this cct; he
(and us his followers) used the venerable 6V6 as the active element in the
loop.  It is much simpler, having but a single 1K wirewound pot (aside from
the necessary filament supply).

As far as I know, no bipolar transistor is either full on or full off.  My
experience has been that there is some forward resistance wihen the device
is 'on' as well as some leakage when the device is 'off'.  In some
situations either of these can be substantial.  Though I am not completely
familiar with all the various solid state devices, I believe I have read
that of them the power mosfet can have a fairly  low 'on' resistance which
accounts for it frequent use in solid state relays.

There are indeed some transistors that will handle high voltages at loop
current levels.  I have a series of Burroughs boards that drive 'jumbo'
nixies, and they use the same package as the ancient 2N404.

As far as this circuit goes, it is my understanding that the transistor is
acting basically as a variable resistance to compensate for the change in
loop resistance as various devices are switched in or out of the loop

Guys (oops, sorry Roberta) if I am in error, please educate me; though not a
spring-chicken my gray matter is still a bit removed from being cement so I
think it can still absorb new knowledge from time to time.  I make no claims
to be the world's foremost authority.

John
WB6BLV at inreach.com