Re: REN Booster
It occurs to me that the only way to 'boost' the REN is by substituting locally generated ringing current for the incoming one from the exchange.
I don't think it is particularly difficult and it should be possible to build a suitable unit for considerably less than £150! In fact, if you've got a well filled junk box, it could cost next to nothing!
In the attached circuit, the local source is based on a mains transformer, bridge rectifier and smoothing capacitor, plus a surge limiter.
Providing a replacement 50Hz voltage won't provide the correct ring, so I'm suggesting switching a DC source in sympathy with the incoming ringing current.
Instead of a bell, the incoming line feeds a relay via a diode and the usual capacitor.
RLA is closes for half a cycle of incoming ring current, then opens for half a cycle when the diode is reverse biased. The contacts of this relay reverse the polarity of the local DC supply every half cycle to produce a roughly square wave AC output current at the same frequency as the incoming signal.
Another capacitor couples this local AC to the output whilst providing isolation from the DC when there is no incoming ringing current.
A mains transformer with a 25 - 30V secondary will provide an RMS equivalent output from about 63 - 75V. None of the other components are critical provided that they are adequately rated. The output current demand is low, so just about anything available will be suitable for smoothing - and output ripple isn't going to be a problem, anyway. The surge limiter can be anything handy - 100 to 500Ω, say.
Any handy DPDT relay should be ok, with an optional series resistor to mop up excess voltage.
Other things you might consider are replacing the mains derived DC with a suitable battery supply - 3 x PP3 in series, perhaps - to make the unit independent of mains power; snubbers across the relay contacts; an isolating transformer on the output to improve the waveform.
See what you think.
Terry
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