Author Topic: zener reference instability  (Read 689 times)

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Offline nnills

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zener reference instability
« on: March 10, 2019, 08:23:46 pm »
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 zener
  • The 12V supply
  • The LM358
  • The breadboard
  • long-ish leads

Conditions: Temp: 22C   Vin: 12.225V   VinAC: 130uV
Schematic: PDF attachment
« Last Edit: March 10, 2019, 08:29:01 pm by nnills »
 

Offline Kleinstein

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Re: zener reference instability
« Reply #1 on: March 10, 2019, 08:58:48 pm »
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
 

Offline alex-sh

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Re: zener reference instability
« Reply #2 on: March 11, 2019, 02:07:28 pm »
Power LM358 directly from 12V. How noisy is your 12V PSU? You may need some additionally filtering
LM358 - According to the datasheet - Single Supply: 3V to 32V
 

Offline nnills

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Re: zener reference instability
« Reply #3 on: March 11, 2019, 02:25:10 pm »
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).
« Last Edit: March 11, 2019, 02:32:28 pm by nnills »
 

Offline nnills

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Re: zener reference instability
« Reply #4 on: March 11, 2019, 02:49:46 pm »
Quote
Power LM358 directly from 12V.
It is. I have a plug pack directly connected to the breadboard
 

Offline imo

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Re: zener reference instability
« Reply #5 on: March 11, 2019, 03:06:39 pm »
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.
« Last Edit: March 11, 2019, 04:20:14 pm by imo »
 

Offline nnills

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Re: zener reference instability
« Reply #6 on: March 11, 2019, 03:57:12 pm »
Quote
PS: it works better when the opamp's inputs are reversed..  :)
Was this a joke ???? On my breadboard it doesn't work, gives 2.170V instead of 5.578V.
 

Offline imo

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Re: zener reference instability
« Reply #7 on: March 11, 2019, 04:07:56 pm »
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.
 

Offline imo

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Re: zener reference instability
« Reply #8 on: March 11, 2019, 04:39:11 pm »
I've taken the model of a LTZ1000 Vref from here

http://www.ko4bb.com/getsimple/index.php?id=a-spice-model-for-the-ltz1000-heater

and a stripped down version simulates without oscillation.
 

Offline imo

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Re: zener reference instability
« Reply #9 on: March 11, 2019, 04:49:02 pm »
It looks like the LM358 does not like that wiring :)
With LT1013 I get no oscillation in your schematics.
PS: the TEMP simulation depends on the zener model's tempco, usually not defined.
« Last Edit: March 11, 2019, 04:52:01 pm by imo »
 

Offline nnills

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Re: zener reference instability
« Reply #10 on: March 11, 2019, 05:15:21 pm »
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) :)
 

Offline imo

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Re: zener reference instability
« Reply #11 on: March 11, 2019, 06:50:44 pm »
Have you coupled the zener and the transistor thermally?
 

Offline Andreas

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Re: zener reference instability
« Reply #12 on: March 12, 2019, 06:08:55 am »
It looks like the LM358 does not like that wiring :)

No 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)

Have you coupled the zener and the transistor thermally?
Even 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) V

with best regards

Andreas
 

Online IconicPCB

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Re: zener reference instability
« Reply #13 on: March 12, 2019, 07:04:03 am »
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?)
 


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