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 Post subject: "Stretching" a zener diode!
PostPosted: Jan Wed 20, 2016 3:57 am 
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Joined: Nov Fri 10, 2006 12:24 am
Posts: 2370
Location: Thornhill, Ontario, Canada
Couldn't think of the right title!
Anyway, if you are using a zener diode as a reference voltage in a prototyping project and you don't quite know what zener voltage you need, you can "stretch" it with a few forward biased Si diodes in series... this provides about 0.7 volt adjustment steps.
I am building a large, fixed-bias Class-AB2 P-P audio amplifier. Plate current changes by a factor of nearly 2.5 from zero to full power, so a cathode bias resistor will not work (the fixed-bias must stay fixed with increase in drive.) I could have used a separate, regulated fixed-bias supply, of course, but instead I used an 2N3055 NPN power transistor in the common P-P cathode line (collector to cathodes, emitter to B-), with the emitter-collector voltage regulated by a zener diode between base and collector (get it round the right way!) BTW, you need a decent heat sink - the 2N3055 dissipates nearly 7 watts at full drive. The nominal o/p tube fixed-bias from the spec sheet is 17 VDC. The closest zener I could find was nominally 16 VDC (and it actually gave only 15.5 VDC when "zenered" in the above configuration.) I added three forward-biased 1N4002 diodes in series with the zener for a possible +2.1 VDC adjustment using a jumper. This does not seem to increase the AC impedance of the zener by a significant amount (a forward-biased Si diode has a very low AC impedance, but I've not measured this.) Now I can adjust the fixed tube bias by 2 VDC, or so, in 0.7 volt steps, to optimize the operating point... the last tests showed all three diodes in circuit gave good results (I think the bias is now 17.5 VDC).
Depending on the application, I don't see any reason why up to half-a-dozen or more such diodes should not be used with a zener this way.
Hope this is useful.
Cheers,
Roger
PS. 1N4002's are only 100 Volt PIV... do not confuse with the more useful 1N4007 (1000 volt PIV). I find myself with several hundred "nearly free" 1N4002's (from a club auction) that I have little use for, except as above and for low-voltage logic and rectifier use, so I don't even save them from my bread-board and air-circuit experiments!

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Roger Jones,
Thornhill, Ontario
Ontario Vintage Radio Assoc. http://www.ovra.ca


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 Post subject: Re: "Stretching" a zener diode!
PostPosted: Jan Fri 22, 2016 9:19 pm 
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Joined: May Tue 30, 2006 4:46 pm
Posts: 9949
Location: Santa Rosa, CA
Low voltage zeners are hard to make (<4V). A Moto engineer once told me that such zeners were actually stacked rectifier chips.

Rich


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 Post subject: Re: "Stretching" a zener diode!
PostPosted: Feb Wed 24, 2016 2:54 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 4975
Location: Perrysburg, OH, U.S.A.
Actually, the low voltage types ARE Zener diodes and have a rather "soft knee". The higher voltage "Zeners" are actually avalanche diodes.

John

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“Never attribute to malice that which can be adequately explained by stupidity.”
― R. A. Heinlein


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