Electronics > Projects, Designs, and Technical Stuff
Replacing Zener with vreg?
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ogden:

--- Quote from: TERRA Operative on November 09, 2018, 06:14:28 pm ---Well I reckon the TL431B (0.5% version) sounds the way to go.  :-+

I'll just have to figure out the resistor values now and see how it goes.

--- End quote ---

Any progress?
TERRA Operative:
Soon, was waiting on the other parts to arrive and I've just laid out the PCB.
I'll just have to have a second look to tweak the PCB layout then I can see if it works..

I'll have to figure out how to determine the resistors needed, and dial them in in-circuit to nail the values.
ArthurDent:
As imo and Andreas have said, that original zener was chosen for a very good reason. A couple of precision calibration D.C. supplies I have use a 1N829A or a 1N3499 as their reference and are still working within spec after about 40 years. To maintain the accuracy of the voltage of these temperature compensated diodes, the supply voltage to the zener is supplied by a constant current source so they were immune from power line variations. Dave has a video where he got one of these standards on eBay and checked a few voltages and found it was still within the original specs. Your circuit uses a resistor from a regulated 12VDC supply which pretty much is a current regulator although I believe your reference doesn't require the ppm accuracy of the voltage standards. The actual voltage of the reference zener isn’t that critical and is generally compensated for later in the supply circuitry. Below is a description of how one of my references works.

To maintain the original accuracy and stability of your meter I would not try to modify the circuit, especially where the original part is still available from some suppliers. I’d be careful of eBay sellers in China and try to make sure you get a genuine part.
TERRA Operative:
Can you point me to any suppliers of the original part? (HP part number 1902-0692 which cross references to the JEDEC part number DT790107A)?

I'm having trouble finding it anywhere....
TERRA Operative:
So, in my desperate digging for diodes, I'm starting to think imo was on the right track.

It seems that the part I am looking for was a 'HP selected part' and the voltage tolerances of 1N828/829 diodes cover 6.3v.
I'm thinking HP were characterising batches of diodes and were selecting those that conformed to the 6.3V 1% requirements.

Problem is, I'm not quite rich enough to go buy a batch of those diodes at $10-18 a pop just to find one that is the correct value and not use the rest....


I need to make a friend at Mouser/Digikey who can go test a few diodes and just send me the good ones.  ;D
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