[GreenKeys] Re: Voltage and Current

[Don Robert House]

Bob,

48 volts should be the minimum used to operate selector magnets.   
Personally I prefer -130vdc 62.5 ma but you know that I suppose.

Don
K9TTY


On 23 Jan 2007, at 8:42 PM, Bob Camp wrote:

Hi

A typical constant current loop control transistor puts a fixed bias  
on the base (or gate) and resistance from the emitter (or source) to  
the other side of the bias. As the current in the resistor drops the  
current in the transistor increases.

These days you can do some interesting things with a switching  
regulator. Twelve volt supply constant current loops anybody?

Bob


On Jan 23, 2007, at 9:02 PM, WB6BLV wrote:

> 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
>
>
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