I forgot to fill in "For 82V, we'd use [values]", because I noticed that the values Philips used (953K and 59K)?make something like 150V instead of 116V.? I realize now that the junctions begin conducting near?6V, not (7.5 + 1.4), so we want about 120K and
9.1K .
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The TL431 still gets my vote.? (Using 150K and 4.7K .)
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From: hp_agilent_equipment@... on behalf of David Wise david_wise@... [hp_agilent_equipment]
Sent: Sunday, July 16, 2017 6:39 PM
To: hp_agilent_equipment@...
Subject: Re: [hp_agilent_equipment] Re: 141T reference replacement..a cure??
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The file 'Signalite-Hivac neon regulators-LM317.pdf' is available from this link:
in a thread called '120V, 400mW Zener Diodes?' on the UK Vintage Radio Repair and Restoration Discussion forum at
?.
I saved a copy on my hard drive.
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But the real gem is buried in the discussion.? The Philips PM3266 scope switched from a V116R2 to a zener/transistor circuit.
See the "Unit 7 - Focus" schematic on page 29.? Scans of the manual are out there.
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They used a BZX79-C7V5 zener, whose tempco is +4mV/K, and a BF422 darlington pair whose tempco is -4.2mV/K.? The transistor pair?is stacked on top of the zener, cancelling the tempco's, and the base is driven by a resistive voltage divider which multiplies
(Vz + Vbe) by (R1+R2)/R2.? For?82V, we'd use .? Any quiet low-current zener with +4.2mV/K tempco would do.
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However, the actual tempco of the zener is loosely controlled, +4 is just the typical value.? We're better off (and one component less) with a multiplied TL431.? This is in my opinion still the only approach that is simultaneously easy, cheap, and good.
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Since it's the ratio and not the individual value that's important, any pair of thermally-coupled resistors with similar tempco's would work.? Metal film, carbon film, as long as the value is stable over time.
Best Regards,
Dave Wise
