zener reference instability - Page 1
Forum section: electronics / beginners

author: nnills
author_rank: Contributor
timestamp: March 10, 2019, 08:23:46 pm
content: Hi all, I just built a zener driver/reference(much like for a LTZ1000) with a 5V zener and a LM358(on a breadboard). It is powered from a 12V plug pack so ignore the top part of the schematic(and the bnc...). I would expect some instability but it varies around 3mV in 2-3 minutes, and the ripple is around 5mV RMS! What could be the problem(of both DC and AC)?The cheap zenerThe 12V supplyThe LM358The breadboardlong-ish leadsConditions: Temp: 22C Vin: 12.225V VinAC: 130uVSchematic: PDF attachment

author: Kleinstein
author_rank: Super Contributor
timestamp: March 10, 2019, 08:58:48 pm
content: The filter cap at the input could be rather small, even for just a few mA cold be too small. from 12 V to 9 V is not much to spare for the 7809 and also the LM358 has not much reserves to get some 6 V from a 9 V supply. So maybe power the LM358 directly from the raw 12 V.The 100 nF cap at the output is not helping the OP and may lead to oscillation

author: alex-sh
author_rank: Regular Contributor
timestamp: March 11, 2019, 02:07:28 pm
content: Power LM358 directly from 12V. How noisy is your 12V PSU? You may need some additionally filteringLM358 - According to the datasheet - Single Supply: 3V to 32V

author: nnills
author_rank: Contributor
timestamp: March 11, 2019, 02:25:10 pm
content: Firstly, The AC/ripple is now negligible(<1uV). Secondly, on the breadboard there is a 100u cap. But the DC instability is still quite horrible(~3mV).

author: nnills
author_rank: Contributor
timestamp: March 11, 2019, 02:49:46 pm
content: QuoteIt is. I have a plug pack directly connected to the breadboard

author: iMo
author_rank: Super Contributor
timestamp: March 11, 2019, 03:06:39 pm
content: The LTspice simulation oscillates with or without the capacitors.Amplitude about 20mVpp.PS: it works better with opamp's inputs reversed (??). Almost none dependency on Vcc (9..12V) and the temperature see below.

author: nnills
author_rank: Contributor
timestamp: March 11, 2019, 03:57:12 pm
content: QuoteWas this a joke? On my breadboard it doesn't work, gives 2.170V instead of 5.578V.

author: iMo
author_rank: Super Contributor
timestamp: March 11, 2019, 04:07:56 pm
content: Frankly, I do not know. I updated above, I've tried to reverse and it does not oscillate and it simulates as above..I see some dependency on the collector resistor. Try with lower value, ie 20k as in my above simul.

author: iMo
author_rank: Super Contributor
timestamp: March 11, 2019, 04:39:11 pm
content: I've taken the model of a LTZ1000 Vref from herehttp://www.ko4bb.com/getsimple/index.php?id=a-spice-model-for-the-ltz1000-heaterand a stripped down version simulates without oscillation.

author: iMo
author_rank: Super Contributor
timestamp: March 11, 2019, 04:49:02 pm
content: It looks like the LM358 does not like that wiringWith LT1013 I get no oscillation in your schematics.PS: the TEMP simulation depends on the zener model's tempco, usually not defined.

author: nnills
author_rank: Contributor
timestamp: March 11, 2019, 05:15:21 pm
content: I put the breadboard in a esd bag with some foam in the opening(to stop airflow). It is now stable to 100 uV!(over 1 hour)

author: iMo
author_rank: Super Contributor
timestamp: March 11, 2019, 06:50:44 pm
content: Have you coupled the zener and the transistor thermally?

author: Andreas
author_rank: Super Contributor
timestamp: March 12, 2019, 06:08:55 am
content: Quote from: imo on March 11, 2019, 04:49:02 pmNo Op-Amp likes 100nF (C3) across the output without furhter measures.(see my LTZ1000 cirquit with a additional series resistor for the negative input and a additional capacitor from Output to negative input of the OP)Quote from: imo on March 11, 2019, 06:50:44 pmEven then there will be some T.C.For a good compensation between zener and VBE you will have to increase the zener voltage to 5.6-6.2 (around 5.9) Vwith best regardsAndreas

author: IconicPCB
author_rank: Super Contributor
timestamp: March 12, 2019, 07:04:03 am
content: What Andreas said ...Go with a slightly higher zener voltage and then insert a low value resistor ( in the order of tens of ohms between the zener and the transistor base.Adjust the resistance for prefered zero tempco temperature ( say room temperature?)