|
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! |
|
Thread Tools |
5th Oct 2015, 12:05 pm | #1 |
Retired Dormant Member
Join Date: May 2011
Location: Johannesburg, South Africa.
Posts: 32
|
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...
|
5th Oct 2015, 12:33 pm | #2 |
Dekatron
Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
Posts: 7,061
|
Re: Zener diode replacements
Yes it works, though temperature coefficient is different, and the turn-on knee is a bit soggier.
|
5th Oct 2015, 7:43 pm | #3 |
Pentode
Join Date: Jan 2014
Location: Huddersfield, West Yorkshire, UK.
Posts: 129
|
Re: Zener diode replacements
better
TL431 R1 -100 kOhm R2 - 7.5 kOhm |
6th Oct 2015, 1:00 am | #4 |
Dekatron
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
Posts: 16,528
|
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!
__________________
....__________ ....|____||__|__\_____ .=.| _---\__|__|_---_|. .........O..Chris....O |
6th Oct 2015, 7:25 am | #5 |
Nonode
Join Date: Aug 2007
Location: South Bradford, West Yorkshire, UK.
Posts: 2,571
|
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.
Keith |
6th Oct 2015, 7:42 am | #6 |
Moderator
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,801
|
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.
David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done |
6th Oct 2015, 9:18 am | #7 |
Retired Dormant Member
Join Date: May 2011
Location: Johannesburg, South Africa.
Posts: 32
|
Re: Zener diode replacements
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.
|
6th Oct 2015, 3:45 pm | #8 |
Pentode
Join Date: Jan 2014
Location: Huddersfield, West Yorkshire, UK.
Posts: 129
|
Re: Zener diode replacements
|
6th Oct 2015, 5:52 pm | #9 |
Dekatron
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 13,953
|
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. |
7th Oct 2015, 7:55 am | #10 | |
Retired Dormant Member
Join Date: May 2011
Location: Johannesburg, South Africa.
Posts: 32
|
Re: Zener diode replacements
Quote:
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. |
|