Rotatone Zener Diode question - UK Vintage Radio Repair and Restoration Discussion Forum

els1967 · 14th Mar 2017, 5:40 pm

I've fitted a few of these now but one thing I just don't get is this: if the ctt. polarity rectifier is fitted the instructions also say to fit a Zener diode to limit the voltage. Looking at the diagram I can't really see how the diode will have any affect as it is fitted after the Rotatone? Wouldn't it need to be fitted before the Rotatone to act as a voltage clamp?

Dave Moll · 14th Mar 2017, 8:25 pm

As far as I can see, the 24V zener is ensuring that no more than 24V is supplied to the Rotatone and the 5.1V one is ensuring that the speech circuit cannot drop the voltage below about 5V, thus keeping the Rotatone within its operatating range.

els1967 · 14th Mar 2017, 10:22 pm

The thing I'm struggling with is how the z.d. would stop the voltage dropping below 5v. Isn't it facing the wrong way in that it will allow a max of 5v through on the positive side which is after the positive on the Rotatone?

Dave Moll · 15th Mar 2017, 8:24 am

I think input is needed here from someone with a deeper knowledge of zener diodes than I have - not difficult, given that mine just about ends at them stopping the voltage across them exceeding the specified level.

TonyDuell · 15th Mar 2017, 9:44 am

I think the idea is that without the 5.1V zener (the wires to it being connected, not open, of course), if the transmitter and induction coil circuit have a sufficiently low resistane then they will draw enough current from the telephone line to pull the voltage across the Rotatone down to less than 5V, at which point the Rotatone doesn't work properly.

With the 5.1V zener in the circuit, whatever the resistance of the transmitter and induction coil there will be at least 5.1V across that circuit (due to the zener) meaning the Rotatone can work.

14th Mar 2017, 5:40 pm	#1
els1967 Pentode Join Date: Aug 2016 Location: Dunstable, Bedfordshire, UK. Posts: 140	Rotatone Zener Diode question I've fitted a few of these now but one thing I just don't get is this: if the ctt. polarity rectifier is fitted the instructions also say to fit a Zener diode to limit the voltage. Looking at the diagram I can't really see how the diode will have any affect as it is fitted after the Rotatone? Wouldn't it need to be fitted before the Rotatone to act as a voltage clamp? Attached Thumbnails

14th Mar 2017, 8:25 pm	#2
Dave Moll Dekatron Join Date: Feb 2005 Location: West Cumbria (CA13), UK Posts: 6,127	Re: Rotatone Zener Diode question As far as I can see, the 24V zener is ensuring that no more than 24V is supplied to the Rotatone and the 5.1V one is ensuring that the speech circuit cannot drop the voltage below about 5V, thus keeping the Rotatone within its operatating range. __________________ Mending is better than Ending (cf Brave New World by Aldous Huxley)

14th Mar 2017, 10:22 pm	#3
els1967 Pentode Join Date: Aug 2016 Location: Dunstable, Bedfordshire, UK. Posts: 140	Re: Rotatone Zener Diode question The thing I'm struggling with is how the z.d. would stop the voltage dropping below 5v. Isn't it facing the wrong way in that it will allow a max of 5v through on the positive side which is after the positive on the Rotatone?

15th Mar 2017, 8:24 am	#4
Dave Moll Dekatron Join Date: Feb 2005 Location: West Cumbria (CA13), UK Posts: 6,127	Re: Rotatone Zener Diode question I think input is needed here from someone with a deeper knowledge of zener diodes than I have - not difficult, given that mine just about ends at them stopping the voltage across them exceeding the specified level. __________________ Mending is better than Ending (cf Brave New World by Aldous Huxley)

15th Mar 2017, 9:44 am	#5
TonyDuell Dekatron Join Date: Jun 2015 Location: Biggin Hill, London, UK. Posts: 5,208	Re: Rotatone Zener Diode question I think the idea is that without the 5.1V zener (the wires to it being connected, not open, of course), if the transmitter and induction coil circuit have a sufficiently low resistane then they will draw enough current from the telephone line to pull the voltage across the Rotatone down to less than 5V, at which point the Rotatone doesn't work properly. With the 5.1V zener in the circuit, whatever the resistance of the transmitter and induction coil there will be at least 5.1V across that circuit (due to the zener) meaning the Rotatone can work.