Author Topic: UK GPO 746 line polarity confusion  (Read 1817 times)

Offline royalbox

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Re: UK GPO 746 line polarity confusion
« Reply #30 on: January 16, 2017, 08:04:11 AM »
Received the replacement diodes - 1N5344B (8.2V 5W). Paid 1.69 for 5 including postage. All working fine both ways (so far!), so thanks everyone for your help.

Barry.

Offline twocvbloke

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Re: UK GPO 746 line polarity confusion
« Reply #31 on: January 16, 2017, 11:19:14 AM »
Sounds like it's all sorted then, makes me wonder why those original diodes conked out, but I guess it's either age or a voltage spike in the line or something, Schottky diodes are usually pretty tough to kill...

Online Jack Ryan

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Re: UK GPO 746 line polarity confusion
« Reply #32 on: January 16, 2017, 11:46:00 AM »
I finally found a Transmitter Inset No 21A.

It contains a two transistor amplifier powered from a bridge rectifier so that either polarity works.

Two of the diodes in the bridge are zener diodes; these act as normal diodes in the bridge but the voltage across the amplifier is clamped at, in this case, 16.4 volts (two 8.2 volt zeners). The current when clamped is limited at the maximum loop supply current but would normally be less because of the loop resistance plus the IC resistance.

Jack