[GreenKeys] Current limiting

[Ken Schwieker]

To cut back on the need for all the voltage drop and adjustment in a 
single rheostat it may be better to have most of the voltage 
drop/power dissipation occur in a sufficiently size fixed resistor 
ahead of a smaller rheostat. The advantages of this method is the 
smaller wattage rheostat and that there would be no way that the 
rheostat could be inadvertently adjusted to supply very high current 
to the selector magnets with possible damaging results. Using 
Duncan's example a fixed 1750 ohm resistor would dissipate about 7 
watts connected to a 500 ohm rheostat that would dissipate about 2 
watts max in a 60 ma loop and allow for adjustments of 3 to 4 
additional selector magnets in that loop.  Another example is a 1500 
ohm fixed resistor that would dissipate about 6 watts and a 1000 ohm 
rheostat that would dissipate about 4 watts max. Not a big deal but 
just some options for a little extra safety factor.

Ken S

At 08:08 PM 1/19/2009, you wrote:
>With a 135V supply, you need 2250 ohms of resistance (R=E/I = 135/.06= 2250
>ohms) to limit the current to 60 ma.  The selector magnet resistance is
>only in the order of 100 ohms or so (varies with different machines), so
>most of the voltage drop is across the current limiting resistor.  So a
>2500 ohm rheostat is usually used.
>
>Assuming that the whole 135V is dropped across the 2.5K, the power
>dissipation would be P=I^2*R = (.06)^2 * 2500 = 9 watts.  The DC output of
>the supply will sag some under load and the resistance will be less than
>2500 ohms, so a ten watt variable is reasonable.
>
>A ferroresonant transformer is a voltage regulator.   It would help
>minimize changes in the loop current from changes in the line voltage.  But
>most "modern"  loop supplies are unregulated.
>
>--Duncan
>
>
>
> > [Original Message]
> > From: Bob McConnell <rmcconne at lightlink.com>
> > To: Larry Tighe <larryradio at worldnet.att.net>
> > Cc: <greenKeys at mailman.qth.net>
> > Date: 19-Jan-09 19:11:09
> > Subject: Re: [GreenKeys] Current limiting
> >
> > Larry Tighe wrote:
> > > Howdy Folks,
> > >
> > > what limits the current in the WECo power supply under the TTY machine
>to 60
> > > ma.?  Is it the ferro resonant?  This p.s. is very quiet and I don't
>have a
> > > print on the box....model 12 power supply I think.
> > >
> > > lar
> > > Larry
> > > www.antiquetelephone.com
> >
> > Hi Larry,
> >
> > The circuits I was familiar with all used a big honking rheostat in the
> > loop. Usually the loop supplies were about 135V DC with very low
> > impedance. The rheostat was adjusted to 60 ma each time we added or
> > removed a machine from the loop. I believe they were 5 watts each, but
> > they may have been more.
> >
> > Bob McConnell
> > Ex-RM2 USN - NFIT, NLM
> > N2SPP
> > _______________________________________________
> > GreenKeys mailing list
> > GreenKeys at mailman.qth.net
> > http://mailman.qth.net/mailman/listinfo/greenkeys
>
>
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