> On Apr 20, 2025, at 1:55 PM, Chuck Guzis via cctalk <[email protected]>
> wrote:
>
> On 4/20/25 09:18, Martin Bishop via cctalk wrote:
>> Why 5 mA current loop ?
>
> If one asks "why current loop?", the answer is obvious. Superb induced
> noise immunity I think, is the big one. If you're a musician on stage
> with a bunch of coupled electronic gear, bear in mind that EMI due to
> the lighting currents themselves is an issue. It's the reason that
> stage mics use a shielded pair.
>
> It's also why CL is used in industrial settings.
>
> From a pre-war 19th and 20th century setting, the reason is that not
> only does one gain noise immunity over long (miles long) runs, but that
> the required current for activation can simply be adjusted at one end by
> varying the voltage.
It also works well with inductive loads, which is why you find it with Teletype
machines. Early ones (the model 15 for example) used a 60 mA loop current; the
model 33 uses 20 mA. In either case, one would typically use a rather high
supply voltage with a correspondingly large series resistor to set the loop
current. If the resistance is substantially greater than the inductive
impedance of the receiver solenoid, that works well and the resulting waveforms
are not badly distorted.
paul
