[GreenKeys] Current limiting

[Duncan M. Brown]

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
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