Replacing Zener with vreg? - Page 1
Forum section: electronics / projects

author: TERRA Operative
author_rank: Super Contributor
timestamp: November 09, 2018, 04:39:52 pm
content: I'm currently in the process of recreating an unobtanium option board for my HP LCZ meter and I am stuck on one part.In the top left corner of the schematic below, there is a 6.3V 1%, PD=0.4W, TC=0.001% zener diode.I'm having trouble sourcing a zener of this type, is it possible that I could stick a vreg here instead?I think it shhhoouulldd be relatively simple to drop in a vreg set to 6.3V, but I just want to get someone more knowledgeable to make sure there's not a trick hiding in there.

author: Whales
author_rank: Super Contributor
timestamp: November 09, 2018, 04:58:42 pm
content: QuoteThat sounds crazy grade. 0.001% = 1e-5. Is the opamp it connects to even that good (eg in terms of offset voltage, tempco)? I don't see any trimmers on the resistors either, so I can't imagine this needing that high of a spec.Typo? I could still be wrong, but I'm suspicious given the schematic has no further info.Sidenote: there are variable voltage references available. Cheapest is something like a TL431, but it doesn't meet the above tempco spec

author: TERRA Operative
author_rank: Super Contributor
timestamp: November 09, 2018, 05:03:22 pm
content: Yeah, I was wondering about that... The part is CR1 in the pic below.The AD7530LN is good for 2ppm of FSR/oC according to the datasheet.I haven't looked at the TL074CN, but here's the datasheet:http://www.ti.com/lit/ds/symlink/tl074.pdf

author: iMo
author_rank: Super Contributor
timestamp: November 09, 2018, 05:06:09 pm
content: It could be an 1N828/829 there, for example (0.001-0.0005%/C).I think any regulator with tempco <50ppm/C will do..

author: TERRA Operative
author_rank: Super Contributor
timestamp: November 09, 2018, 05:11:42 pm
content: Quote from: imo on November 09, 2018, 05:06:09 pmThat looks like a 6.2V zener, would that be ok to use instead of a 6.3V?It looks like the HP part number (1902-0692) cross references to the JEDEC part number DT790107A but I can't find anything on that number online beyond HP cross references.

author: SiliconWizard
author_rank: Super Contributor
timestamp: November 09, 2018, 05:13:29 pm
content: If you want to replace the zener diode and not change the inverting arrangement, you will have to use a negative regulator (just making sure)I'm guessing that U6.4/U6.5 are mutually exclusive open/close?Seems like U5.2 is buffering either -6.3V or +6.3V depending on U6 position.I was pondering whether the zener diode didn't serve any kind of temperature compensation purpose, but at first sight, given that the zener has pretty tight specs (1%, TC = 0.001%), it's probably not, and a reference voltage with a precision regulator should do the trick.Maybe someone experienced with this kind of instrument has another take on this arrangement, but I'm tempted to believe that this was chosen just because it was probably one of the most stable yet cost-effective solution at the time to get a stable reference voltage.If you're willing to modify the board a bit more, I personally would be tempted to "invert" the topology: using a positive regulator (because it may be much easier to find an appropriate positive voltage regulator than a negative one). Make sure you modify the connections to U6.4/U6.5 s accordingly so you end up with the same polarity as before.Also take care of the feedback resistors you use if you select an adjustable regulator, as you may end up with a worse temperature stability than with the zener setup.

author: iMo
author_rank: Super Contributor
timestamp: November 09, 2018, 05:15:29 pm
content: The 1N828 or 1N829 are not exactly 6.3V.All those zeners are +/-0.3V. They are expensive because of the tempco.PS: there are 1N828-1 or 1N829-1 with 1% tolerance.

author: SiliconWizard
author_rank: Super Contributor
timestamp: November 09, 2018, 05:16:17 pm
content: Quote from: Whales on November 09, 2018, 04:58:42 pmYeah, that doesn't seem to make sense to have such specs for the zener diode and then buffering it with an opamp (unknown specs?), the worst being the inverted position were it would travel an inverting amplifier with two 20k resistors and then the opamp buffer. I'm really wondering what kind of precision and stability you end up with.

author: ogden
author_rank: Super Contributor
timestamp: November 09, 2018, 05:19:01 pm
content: Yes, obsolete zeners could be replaced with shunt voltage references. You can try two series (5V+1.25V) LT1634 which have required 10ppm TC. Thou I would try mentioned (trimmed to 6.3V) TL431 shunt reference first and see how it does, maybe do some cold-spray test (using "canned air") as well. TL431 is in your junk bin for sure, it's very popular in isolated switching supplies.

author: ogden
author_rank: Super Contributor
timestamp: November 09, 2018, 05:23:50 pm
content: Quote from: SiliconWizard on November 09, 2018, 05:16:17 pmIt makes a lot of sense to BUFFER voltage reference/zener using opamp.

author: TERRA Operative
author_rank: Super Contributor
timestamp: November 09, 2018, 05:29:40 pm
content: Yeah, I'm not sure why they went with such a precision part. Maybe it was just a jellybean part that was in their internal catalogue.The circuit is designed to set a DC offset across a capacitor under test. The inputs go through the AD7530LN DAC that set the reference voltage then the transistors amplify and output to the front panel terminals.There is two ranges, 0v - +/-9.99V (10mV resolution) and +/- 10.0V - +/- 40.0V (100mV resolution).I've included the two pages from the manual that explain how it works.I have TL431's here so they are a viable option.

author: SiliconWizard
author_rank: Super Contributor
timestamp: November 09, 2018, 05:31:01 pm
content: Quote from: ogden on November 09, 2018, 05:23:50 pmOf course you have to buffer it. What it does to absolute precision (which is not what they seem to be after here) and temperature stability is another matter. We can suppose they selected the opamps carefully and mounted them appropriately. Still, the "inverting position" poses even more questions here IMO.

author: iMo
author_rank: Super Contributor
timestamp: November 09, 2018, 05:31:50 pm
content: Quote from: ogden on November 09, 2018, 05:19:01 pmLM431 claims 50ppm/C - that fits well..

author: SiliconWizard
author_rank: Super Contributor
timestamp: November 09, 2018, 05:36:58 pm
content: Quote from: TERRA Operative on November 09, 2018, 05:29:40 pmMaybe they just use it as a reference in other parts of the design?Again when you look at the whole chain, I kind of doubt you get such a temp. stability in the end anyway?

author: exe
author_rank: Unknown
timestamp: November 09, 2018, 06:05:05 pm
content: Quote from: SiliconWizard on November 09, 2018, 05:13:29 pmOr a shunt regulator.Final voltage can be trimmed, if needed. So, it doesn't need to be precisely 6.3V. But replacing resistor and adjusting for zero tempco can be a challenge.For me it's unclear how precise and stable the circuit should be. If it allows some deviation, I'd probably just put lm329 or even a simple zener. May be trimming even not needed if it can be re-calibrated to the new range.

author: ogden
author_rank: Super Contributor
timestamp: November 09, 2018, 06:10:51 pm
content: Quote from: TERRA Operative on November 09, 2018, 05:29:40 pmCould be so that they just had loads of that zener in stock and copy-pasted it. Anyway stability oftest signal amplitudebias generatormost likely will not impact measurements. I would just put TL431 here and see how it goes.Quote from: SiliconWizard on November 09, 2018, 05:31:01 pmOpamp is TL074CN, it's offset drift is 18 uV/C - way better than required. Also opamp inverter could be quite temperature-stable if low-TC resistor array used.

author: TERRA Operative
author_rank: Super Contributor
timestamp: November 09, 2018, 06:14:28 pm
content: 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.

author: GigaJoe
author_rank: Frequent Contributor
timestamp: November 09, 2018, 07:58:31 pm
content: so zener feed reference input for a DAC that produce a voltage output.means the long term precision for measurement defined by this particular zener diode.I would suggest to replace it by LM399 + voltage divider, just that , R1-6 we disconnect and we can use to feed LM399, and voltage divider,make sure about heater:https://www.eevblog.com/forum/projects/negative-m399-voltage-reference/resistors for voltage divider 5ppm PTF56, should work.as switch U6.5 connecting positive and negative , producing a precision meander , i guess, to feed input of opamp, so current are minimal and do not need buffer for resistor divider+LM399 ....

author: ogden
author_rank: Super Contributor
timestamp: November 09, 2018, 08:06:14 pm
content: Quote from: GigaJoe on November 09, 2018, 07:58:31 pmWell, it's not evenmeasurement, but bias voltage applied to DUT. Most LCR/LCZ meters does not eve have bias option, yet they are fine.QuoteI would not. LM399 is huge overkill for bias voltage gen.

author: GigaJoe
author_rank: Frequent Contributor
timestamp: November 09, 2018, 09:13:23 pm
content: "bias" - ?precision zener , 10bit DAC - ? for bias ?I'm skeptical, I rather say it a precision meander, generated by switch and then feed ADC to measure

author: ogden
author_rank: Super Contributor
timestamp: November 09, 2018, 09:26:00 pm
content: Quote from: GigaJoe on November 09, 2018, 09:13:23 pmYou can be believer or not, but this is what it is. Get service guide from HP site, check schematics, page 303. Pay close attention to name of the module. You can check file name of the attachment in the OP as well. DC bias specs (two ranges): +/- 40V, 0.1V step, +/- 10V 0.01V. Quite a bits needed for that.

author: exe
author_rank: Unknown
timestamp: November 09, 2018, 09:28:01 pm
content: Quote from: GigaJoe on November 09, 2018, 09:13:23 pmWell, that's what in the original schematic. I still feel no shame suggesting lm329 as it is cheap (from TI).But I agree, it's unlikely DC bias circuitry needs precision.

author: GigaJoe
author_rank: Frequent Contributor
timestamp: November 09, 2018, 09:41:18 pm
content: well, ok, probably a reason of famous HP bulletproof design.lm329 - still large tempco, LM369 - also old one ,how about some modern 10V + divider ?

author: ogden
author_rank: Super Contributor
timestamp: November 09, 2018, 09:48:04 pm
content: Quote from: GigaJoe on November 09, 2018, 09:41:18 pmJust noticed R7 (2K) trimmer named "FS adj". Modern 5V with adjustment of particular trimmer could be option as well. If trimpot range does not reach 40V with 5V reference, then R1-8 could be increased.

author: Andreas
author_rank: Super Contributor
timestamp: November 10, 2018, 05:18:16 am
content: Quote from: TERRA Operative on November 09, 2018, 04:39:52 pmHello,before LM399 the 1N829A zeners were state of the art for up to 6.5 digit instruments.These zeners are selected for T.C. (zero T.C. current) and usually also for noise.Typical you have around 2-3 uVpp with these so you cannot replace them by a standardbandgap (like TL431) which has minimum a factor 5 more noise.I also guess that R1 (resistor array) is specially paired/selected (with low T.C.)together with the zener to get accurate output.So it will be difficult to use a standard voltage divider (with 50-100ppm/K T.C.)without degrading the cirquit.1N829A are still available (e.g. at Farnell) even if they have now much higher priceas they used to be at the time when they were state of the art.Eventually you will have to adapt the 1K resistor to get optimumzero T.C. current for a different device.So before replacing the temperature compensated zener witha different device I would carefully check if I can maintain the specsof the instrument with a different (low noise, low drift) voltage regulator.With best regardsAndreas

author: ogden
author_rank: Super Contributor
timestamp: November 27, 2018, 09:19:04 am
content: Quote from: TERRA Operative on November 09, 2018, 06:14:28 pmAny progress?

author: TERRA Operative
author_rank: Super Contributor
timestamp: November 27, 2018, 09:50:04 am
content: 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.

author: ArthurDent
author_rank: Super Contributor
timestamp: November 27, 2018, 01:59:28 pm
content: 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.

author: TERRA Operative
author_rank: Super Contributor
timestamp: November 27, 2018, 02:32:31 pm
content: 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....

author: TERRA Operative
author_rank: Super Contributor
timestamp: November 28, 2018, 06:15:55 am
content: 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.

author: ogden
author_rank: Super Contributor
timestamp: November 28, 2018, 06:23:58 am
content: Quote from: TERRA Operative on November 28, 2018, 06:15:55 amYou don't need to - because R7 trimpot is already your friend

author: TERRA Operative
author_rank: Super Contributor
timestamp: November 28, 2018, 02:41:08 pm
content: Alrighty, a bit of an update on the schematic.I updated it with a few corrections, which weren't completely clear from a cursory glance at the schematic. Most notably the + and - 6.3v rails from the zener at the top right corner and a few labels here and there.Might help make things clearer maybe.Also, re. the zener and R7, the different voltage (if it isn't 6.3V) isn't going to cause problems with the 'vref' input in U3 (AD7530LN)?

author: ArthurDent
author_rank: Super Contributor
timestamp: November 28, 2018, 02:53:35 pm
content: Quote from: ArthurDent on November 27, 2018, 01:59:28 pmAs I said in my previous post, the actual voltage isn't that critical. The LM399 and LTZ1000 references HP has used in their most accurate and stable multimeters have a range of voltages and that isn't a concern because these are compensated for in the circuitry. As pointed out in your schematic, R7 is a gain adjustment to correct for the minor differences in reference voltages. You will gain nothing by trying to select this zener.

author: TERRA Operative
author_rank: Super Contributor
timestamp: November 28, 2018, 03:11:02 pm
content: Cool. I'll get myself a 1N828/829 and stick it in.Sorry if I sound stubborn or something, it's not that (well I am stubborn, but not right now.), just that I am still learning this stuff so I'm still figuring out where I can and can't tweak numbers and values.

author: ArthurDent
author_rank: Super Contributor
timestamp: November 28, 2018, 04:11:53 pm
content: Here is a little more info on the circuit you want to recreate as I understand it. First, there is an error on the first 001 option schematic you show. The 100K pot marked R1 should be marked R8. Second, this 001 option only allows you to use an internal bias supply instead of using an external precision supply. This eliminates the need to use a separate supply to bias the DUT with up to +/-40VDC. There are two ranges on the internal bias supply of 0 to +/-10VDC and 0 to +/-40 VDC selected by the U6.2/U6.3 switch and U6.4/U6.5 selects polarity. I also assume R1-1 through R1-6 is a low temperature coefficient precision resistance module. Here is the adjustment procedure that indicates that the actual Zener voltage isn’t too critical because there is an adjustment on the supply to get the exact voltage HP wanted.