In the UK the standard capacitor is 1.8uF and maximum ringing load is 1000
ohms (REN = 4) based on old legacy system. The actual ringing voltage is
'spiky' at a PRF of 25 Hz. I'll have a look at its actual waveform tonight
and report back, but it certainly is not that spiky and made up of 1000 Hz
components over here.
1.8uF has Xl = 3500 ohms at 25Hz so on a 1k load will be somewhat lossy but
bearing in mind a cable of 0.5mm diameter trwin conductors has about 200
ohms / km DC resistance, there could be over 1k of resistance anyway on a
long underground line, so the ringing margin needs to be quite high. The
old bells had inductive coils so this probably resonated with the 1.8uF
cancelling out a fair bit of the 'loss' due to reactance. I have a modern
electronic phone that rings with only 10V on it. It wet weather once, water
leaking into a phone connection in the garage shorted this capacitor (we
have a three wire system remember) and the phone spontaneously rang from the
DC present on the line, waking me up in the middle of the night :-(
I notice a lot of (legal) phone accessories in this country now only use two
wires, so it looks as if the idea of a single DC blocking capacitor in the
junction box with a 3 wire local connection to each phone is going away.
The 4th wire used within phone installations here is redundant, but was used
once upon a time as an Earth lead for signalling on party line working. It
was all a big mess in those days.
No wonder manufacturers of phone accessories tear their hair out over
international compatibility.
Don't we get onto some strange subjects on this reflector !
Andy 'JNT
A quick reality check, think through the reactance of a DC blocking
capacitor at 20 Hz and 1000 Hz, when you rectify the AC through the
capacitor and power a small relay from the result you are not
working with
much of the 20 Hz component.
Larry
VA3LK
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