[GreenKeys] Why high loop voltages?

[Brooke Clarke]

Hi Bob:

What's the DC resistance of the two parallel coils?

Have Fun,

Brooke Clarke
http://www.prc68.com

Bob McConnell wrote:
> Brooke Clarke wrote:
>> Hi Joe:
>>
>> The high voltage is needed so that a resistance can be added into the loop.
>> The time constant of an L R circuit is L/R and so by adding resistance you 
>> reduce the time constant (speed it up).
>>
>> If you do a DC test on the selector magnets you'll find you need a small 
>> voltage and 60 ma to reliability activate the magnet.  But the time constant 
>> will be L/R where R is some very small number so the time constant will be much 
>> longer than the symbol time and so is too slow to work.
> 
> If you want to try some calculations on that L/R circuit, IIRC when the 
> two selector magnets in a model 28 printer were connected in parallel 
> for 60 ma operation, the total inductance was 1 Henry. That's right, the 
> whole thing. No milli or micro fractions here. They were half a Henry 
> each, so the series connection for 20 ma resulted in 1/4 Henry.
> 
> Bob McConnell
> N2SPP
> 
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