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Hints, Tips and Solutions (Do NOT post requests for help here) If you have any useful general hints and tips for vintage technology repair and restoration, please share them here. PLEASE DO NOT POST REQUESTS FOR HELP HERE!

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Old 5th Oct 2015, 12:05 pm   #1
rswanborough
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Default Zener diode replacements

If I want a particular zener voltage and all I have is the scrap box then I use ordinary silicon diodes to make up the voltage required. A silicon diode has a voltage drop of about 0.5 volts (in effect it is a half volt zener), so a six volt zener can be emulated by 12 diodes in series. They can be mixed; for example, one of my jobs required a 15 volt zener and I had only a 12 volt zener, so I added 6 regular diodes for the extra three volts. Job done...
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Old 5th Oct 2015, 12:33 pm   #2
kalee20
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Default Re: Zener diode replacements

Yes it works, though temperature coefficient is different, and the turn-on knee is a bit soggier.
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Old 5th Oct 2015, 7:43 pm   #3
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Default Re: Zener diode replacements

better
TL431
R1 -100 kOhm
R2 - 7.5 kOhm
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Old 6th Oct 2015, 1:00 am   #4
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Default Re: Zener diode replacements

Going back to the 1.5V "zener" that sparked this all off I'll bet the tempco of the IC version is at least an order of magnitude "better" than the 2500ppm of the original. Though as a reference for an LTP tail current source that's probably not a problem!
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Old 6th Oct 2015, 7:25 am   #5
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Default Re: Zener diode replacements

The TL431 would not be suitable as a replacement for a 1.5V zener as the voltage on the ref pin is 2.5V meaning the minimum regulated voltage is 2.5V. I would also suspect the tempco of the 1.5V zener would be quite high.

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Old 6th Oct 2015, 7:42 am   #6
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Default Re: Zener diode replacements

As a long tailed pair current bias for the tail, there was a neat circuit in Wireless World in the early 70's which seems to have been forgotten. "Ring of two reference" It uses twice as many components but it's a neat trick.

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Old 6th Oct 2015, 9:18 am   #7
rswanborough
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Default Re: Zener diode replacements

Quote:
Originally Posted by kalee20 View Post
Yes it works, though temperature coefficient is different, and the turn-on knee is a bit soggier.
True, but I have found that it is fine for 99% of purposes. I check the 'pseudo-zener' voltage with a variable dc power supply. I connect the power across the stack and crank up up the voltage until the current on the ampmeter suddenly starts to rise. The voltage shown on the voltmeter is the pseudo-zener voltage.
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Old 6th Oct 2015, 3:45 pm   #8
k_yller
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Default Re: Zener diode replacements

Quote:
Originally Posted by KeithsTV View Post
The TL431 would not be suitable as a replacement for a 1.5V zener as the voltage on the ref pin is 2.5V meaning the minimum regulated voltage is 2.5V.
yes, sure TL431 will work for 2.5-36 V
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Old 6th Oct 2015, 5:52 pm   #9
G6Tanuki
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Default Re: Zener diode replacements

Historically, "low-voltage" Zeners have been a bit of an issue: you get down to a region where the Silicon band-gap voltage and the Zener-effect voltage are competing - and which one wins long-term is open to fun-and-games.

Forward-biased power-diodes are my choice for anything below 1.6V: in the past I've used a mix of Brush/Westinghouse S3AR12 Silicon and the rather-less-available Germanium GEX541 [if anyone has a good source of these please tell me!] for bias networks.

For simpler situations the Vbe-multiplier transistor-with-the-base-fed-from-a-potentiometer-between-emitter-and-collector is a good bet, specially if the transistor itself is closely thermally-associated with the rest of the circuit.

Roberts radios made extensive use of this using transistors usually labelled "T3" or "T7" which looked the same as the ubiquitous OC71 - opinion is that these transistors were indeed OC71-or-equivalents which had not made the grade for audio applications but which still had enough gain/DC-thermal-sensitivity to serve in bias-control duties.
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Old 7th Oct 2015, 7:55 am   #10
rswanborough
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Default Re: Zener diode replacements

Quote:
Originally Posted by G6Tanuki View Post
...Roberts radios made extensive use of this using transistors usually labelled "T3" or "T7" which looked the same as the ubiquitous OC71 - opinion is that these transistors were indeed OC71-or-equivalents which had not made the grade for audio applications but which still had enough gain/DC-thermal-sensitivity to serve in bias-control duties.
Ah, the OC71, every hobbyists germanium transistor for the 60s; and its clear case cousin, the OCP71, that I used in light sensitive applications. And then the OC44 with RF capability made transistor radios possible.

Germanium diodes, if you can find them, had a voltage drop of about 0.2 volts against the silicon diode's 0.5v. Useful for 'fine tuning' pseudo-zeners!

All killed by the silicon transistor. Still have a few, though.
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