[GreenKeys] Re: Voltage and Current

[Bob Camp]

Hi

With a boost mode switcher the input voltage and output voltage don't  
have to be equal. You can set up a "smart" switcher so it just boosts  
the voltage when you need to close the magnets. The rest of the time  
it generates what ever voltage you need to keep the magnets closed.  
With parallel magnets (60ma) that's not a lot of current.

Bob


On Jan 23, 2007, at 10:46 PM, Don Robert House wrote:

> 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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>
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