Zener B+ drop: I'm having a blond moment

[reubster]

I must be having a blond moment.
When using a zener diode to drop B+, the recommendations are to connect the zener between the HT CT and ground with the zener's cathode to ground.
Obviously this works as many people do it.

What I can't understand is that (to me) this appears to be a forward biased connection (hence a Vf not a no Vz drop)?

[cminor9]

If it's not working, flip it around. I can never remember the right way for those to go. The line means nothing to me. Is that cathode? Anode? I forget. Every time. I have learned that if it's not working, then it's hooked up wrong.

Reminds me of the "green side up" joke.

[Enzo]

Think of current flowing up from ground, through the transformer, through a rectifier and into a load, thus returning to ground. It can only flow the one direction because of those rectifiers. Now add a diode in series between ground and transformer. If you ground the anode end, then current flows forward through it, through the tranny and through the rectifier. No zener action. If you ground the cathode, then that current has to travel through that zener in reverse bias, so it must reach the zener voltage before it will conduct. And that snips out some voltage from the waveform.

The forward bias as you describe would only occur when current was flowing down through the diode, instead of up through it. That would only happen when the rectifiers on the far end of the transformer winding were already blocking current flow. SO for that half cycle wher forward conduction would want to occur, it can;t because ther is zero current.

The thing to do here is think in terms of current flow direction. Then look at the whole circuit. DOn't try to think in terrms of positive and negative ends of parts.

[Enzo]

Minor, I in my mind just think of "the line end" and I know which way current flows regardless anyway.

But what always kept anode and cathode straight for me was that the symbol for a diode is a triangle and a line. The triangle is shaped similar to the letter "A", A for ANode. SO the end represented by the triangle part of the symbol is the anode. Hence the line end is the not-anode, or cathode.

[reubster]

Thanks
That makes sense for the CT HT FWR.
So with a non CT HT and using a bridge rectifier, we put the zener between the nominated zero side of the rectifier and ground.
I can see Enzo's explanation works for the "positive" side bridge rectifier diodes.
The "negative" side bridge rectifier diodes will block for this condition, so current cannot flow "up" from ground, therefore it must flow "down" to ground which gives us reverse bias (Vz)
Therefore, does it mean that for a non CT HT with a bridge rectifier, the orientation of the zener is not important?

[tubeswell]

Thanks
Therefore, does it mean that for a non CT HT with a bridge rectifier, the orientation of the zener is not important?

No. it is important. Its the same orientation. Banded end to ground.