Use of GPO 2mf capacitor C1?

[PJL]

I picked up a big black flat 12x12x1cm capacitor stamped G.P.O. 2mf C1. It measures 2.5mf but not sure of voltage.

What would it have come from and I guess it's not the first capacitor the GPO used!

[Dave Moll]

I would imagine that it was the capacitor in series with a bell - to block the DC. With modern PST wiring this is superseded by the 1.8µF in the master socket.

[Tim]

Probably 250 volt DC

[AndiiT]

Originally Posted by Dave Moll

I would imagine that it was the capacitor in series with a bell - to block the DC. With modern PST wiring this is superseded by the 1.8µF in the master socket.

Hi,
Dave's guess is most probably correct, some Switchboards and early Bellsets used a 2uF (mF) capacitor in the bell circuit.

Remember that the "bell" capacitor (whether in the phone/switchboard/master socket etc.) is sometimes also employed as part of the Anti-sidetone balancing network.

Modern day equipment (2-wire phones, ADSL filters, dial up modems etc.) use a 1.8uF capacitor as Dave mentions.

Voltage wise 250V AC seems a reasonable guess, ringing current is 75 Volts AC at 25 Hertz.

Regards
Andrew

[russell_w_b]

Originally Posted by AndiiT

Voltage wise 250V AC seems a reasonable guess, ringing current is 75 Volts AC at 25 Hertz.

According to Atkinson, the standard condensers used in telephony are designed to withstand a working voltage of 200V DC, and must not break down with a test-voltage of 300V DC. A high value of insulation resistance is required, with a minimum value of 1000 Megohm - microfarads (product of the insulation resistance and capacitor value). A 2uF capacitor would therefore have an insulation resistance of 500 Megohms.

The capacitor in question was sometimes found on the earlier type 336 telephones. Mine has a cylindrical unit fitted, clearly marked as 200V DC working.

The 2uF capacitor would be the bell capacitor, with a combined 0.1uF capacitor across the carbon inst microphone to by-pass RF pick-up.

[russell_w_b]

Originally Posted by russell_w_b

The 2uF capacitor would be the bell capacitor, with a combined 0.1uF capacitor across the carbon inst microphone to by-pass RF pick-up.

Just had a peep inside my British Ericsson type N1071 Bakelite wall instrument, and it is fitted with a grey, square section dual capacitor (2uF and 0.1uF) and is clearly stamped: 200V DC WKG.

[KeithsTV]

I acquired this capacitor from a GPO engineer about 40 years ago. 3 caps 2uF, 0.5uF & 0.3uf in series rated at 200v. Sealed in a metal case 6.3cm x 5.5cm x 2.5cm and made by Sprague part number CA-355. No idea what it would have been used for but I guess it would have been a GPO custom part.

Keith