Zener diode clipper

[Al (astral highway)]

Originally Posted by G8HQP Dave

... You always have to think about... the source and the clipping device, and the current which the latter will cause in the former.

Erm, yes, but I acknowledged the current issue in post no #1! I never said it was a great idea. Self-evidently, it was clutching at straws out of frustration, so gimme a break! I do highly value your contributions and they're spot on, just please don't knock me for being so unguarded about my fledgeling idea.

I'm sure many forum members have designed themselves into a corner at some point or another?! If not, hurray, I'm the first one!!

[McMurdo]

Not at all, you're in eminent company

[Al (astral highway)]

Lucky find! This was in my parts box all along. I can't remember why I bought it in the first place, at least 5 years ago now. I think these now cost around £150 quid.

Anyway, this should do the trick. It's 450VDC rated, comfortably ok. It will have to sit off the board because of its size, but that's a small price to pay to prevent the fireworks that would have ensued if I stuck with the existing one!

[electronwilly]

Hi Astral Highway, To me there are two possible solutions to your problem. First you need to up the working DC voltage of your capacitor. Lets say you need to up the dc working voltage of the capacitor by 30vdc. Stick a 30v zener from the negative on the capacitor to ground. This will up the dc working voltage by 30 volts dc. Second use an ac clipper. Before the bridge (AC) put two zeners in series anode to anode across the last ac supply, so you have cathode to cathode across the ac. The drop will be the voltage rating of one zener +.68volts ac. Before the zener put in a current limitig resistor. Hope this helps. Electronwilly.

[Tim]

First you need to up the working DC voltage of your capacitor.

That's a handy trick! How does it work?

[Al (astral highway)]

Originally Posted by electronwilly

Before the zener put in a current limiting resistor.

Electronwilly, thank you for your enthusiasm, but have you actually read this thread?

1)We all know what a zener diode clipper is and how it works - that's not what this thread is asking

2) See #1. There was never any question of putting a series resistor before the clipper, for the reasons I stated at the start

3) I don't quite understand from the information you give how you are configuring your zener to raise the working voltage of a smoothing capacitor - that's not possible? Can you draw a diagram please? But in any case, please see the lovely shiny 1100uF, 450VDC item I photographed and posted before yours...

[mhennessy]

Originally Posted by Tim

That's a handy trick! How does it work?

It doesn't

[G8HQP Dave]

Actually, it might work but only in a very poor and approximate way. It uses the leakage current of the electrolytic (perhaps augmented by a parallel resistor - like a bleeder) to bias the zener into its active region. Two big snags:
1. ripple current - which will far exceed the standing current for a reservoir cap: the trick might work for a smoothing cap
2. effective ESR - the zener dynamic impedance will be in series with the cap ESR

Not a good idea!

[kalee20]

It won't work.

When charging, the capacitor will see less than the applied voltage, due to the 30V Zener. So far so good. And any voltage rise will immediately be slugged by the combination - again, so far so good - it will 'look' like the original capacitor.

But, as soon as charging stops, and you try to discharge the combination, the Zener will stop conducting. You will then have a massive downward voltage step, till the Zener conducts in the other direction (with approx. 0.6V drop).

I don't think anyone would like a filter system with 30.6V steps in it... any takers?

[mhennessy]

Exactly - and that's 30.6V on top of whatever ripple voltage you had before. In series with the zener impedance, as noted by Dave. Not to mention that you have a capacitor capable of passing incredibly high peak currents connected via a semiconductor fuse!

BTW, it's not the leakage current of the current that goes via the zener; it's the charging current, which will be some 4-5 times higher than the DC current. So, the zener will be taking quite a "hit", however you look at it...

Another BTW: I have a Philips bench DVM (PM2521) that has 20V zener diodes connected to the primary of the mains transformer. It's a 220V transformer, and these are added for 240V operation. Typical Philips

that has 20V zener diodes connected to the primary

In series I hope.

[Al (astral highway)]

Originally Posted by mhennessy

I have a Philips bench DVM ... that has 20V zener diodes connected to the primary of the mains transformer. It's a 220V transformer, and these are added for 240V operation. Typical Philips

Oh, my reservoir cap is now sorted with a suitable upgrade, but Zeners could still be the answer to my problem, if regulation is still an issue. Not sure but I think on days when DC overvoltage is nearly +10 I will still have a critical bias voltage issue for a big linear regulator (MOSFET) whose bias is preset by a multi-turn potentiometer.

Why not just replicate what's good enough for Philips with my 230:115 xformer by chucking a string of Zeners on the primary side?

[mhennessy]

Obviously, the Philips DVM only takes a tiny current - the zeners are little ones, the size of an 1N4148. No idea what your load current is...

Obviously your replacement capacitor has solved your immediate problem, but I still think the SCR pre-regulator that I suggested a few years back is the best idea so far. It's been used in countless power supplies that I've seen.

[Al (astral highway)]

Originally Posted by mhennessy

. No idea what your load current is...

Just under 3A average on the circuit. Two transformers between that and our mains, as per post #1, not sure how lossy the second TX is - it gets slightly warm under load - but the first is hugely capable

Originally Posted by mhennessy

.Obviously your replacement capacitor has solved your immediate problem, but I still think the SCR pre-regulator that I suggested a few years back is the best idea so far. It's been used in countless power supplies that I've seen.

This looks very promising, thank you, and a low component count if I understand it correctly. I could squeeze this in quite easily. Can you recommend or reference a design, please?

Cheers fella,

Al

[PJL]

I have a 118V 500W CVT here that you can have FOC if that helps.

[Al (astral highway)]

Thank you! You have PM

[mhennessy]

Originally Posted by astral highway

This looks very promising, thank you, and a low component count if I understand it correctly. I could squeeze this in quite easily. Can you recommend or reference a design, please?

Take a look at the Weir 423 for ideas - the service manual is posted in this thread: https://www.vintage-radio.net/forum/...ad.php?t=56278

Cheers,

Mark

[Alistair D]

Here is a very simple power supply arrangement. From ETI circuits book 1976.

Al

[Al (astral highway)]

Mark: Thank you - very good material there.

Al: The bucket circuit is spot on and I'll build an off-board prototype ASAP using the huge SCR very kindly donated by PJL (#35) and report back.