[GreenKeys] Re: Rheostat

[Peter Gottlieb]

Wow I must be a space cadet - or getting old - as I completely missed 
the part of the sentence after "EMP."

My logic in using the transistor is that it serves both as a constant 
current device as well as the loop switch.  When I had just the keyboard 
in series with the magnet all I used as, in fact, just a simple resistor 
to set the 60 ma.  But now that I need a switch... why not "neaten up" 
the loop regulation against line voltage variations (105 to 130 here) at 
the same time.  Once I get the whole thing together I will post the 
diagram.  I think it does everything necessary easily, cheaply, and 
hopefully reliably.

Peter



Don Robert House wrote:
> "If necessity is the mother of invention, then simplicity is the 
> father of design"  --- Henry Drefus
>
> Don
> K9TTY
>
> On 9 Apr 2007, at 11:11 PM, WA5CAB at cs.com wrote:
>
> And the fourth scheme is to replace the fragile transistor with a 25K 
> 25W EMP
> and spike proof rheostat.  And never have to worry about it again.  First
> rule of fail-safe design - never use an active component when a 
> passive one will
> serve, work as well, and produce no more heat.
>
> In a message dated 4/9/2007 10:00:34 PM Central Standard Time,
> jhhaynes at earthlink.net writes:
>> You have to snub to protect the transistor.
>>
>> One scheme is an R-C series circuit, where you choose the R and C
>> so that it snubs adequately and dissipates the energy in the resistor.
>>
>> Another scheme is to run the R-C series circuit from colletor to
>> base of the switching transistor, so the inductive spike keeps the
>> transistor conducting until it dies out.
>>
>> And another scheme is to use a Zener diode or a MOV device to
>> absorb the transient.
>>
>
> Robert & Susan Downs - Houston
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