Author Topic: Self Biased Zener Voltage Reference  (Read 2266 times)

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Offline soligenTopic starter

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Self Biased Zener Voltage Reference
« on: July 16, 2015, 11:16:51 pm »
HI EEVBloggers. First time poster here.

I'm not only new here, but also to electronics as a hobby.  I've been watching many videos and googling to learn as much as I can,  and I find EEVBlog a great resource.  I am very much a beginner.

I have been playing with voltage and current regulation, and decided to find a way to refine my voltage reference.  I've been researching ways to do this without simply buying a voltage reference (learning more this way) and I found the diagram below that I think fits what I am trying to do very well, as I am likely to have an extra op amp to use.  The only issue I have with this is the calculation for C.  Being an inverse of the time constant, I just cant seem to come up with values that seem reasonable.  If this instead were RC >= .001 then a 10k resistor gives a .1uF capacitor, which is reasonable. But, the formula in the diagram boggles me.

Can anyone help me understand this?  Is the formula on the diagram correct?  I don't really understand why the capacitor is needed.



original page: http://www.embedded.com/electronics-blogs/embedded-round-table/4425816/2/Voltage-references

Thanks
 

Online TimFox

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Re: Self Biased Zener Voltage Reference
« Reply #1 on: July 17, 2015, 01:35:06 am »
The equation for Rf x C is wrong (dimensionally inconsistent).
Probably should have been Rf x C > 1 msec.
E.g., 10 k ohm and C > 0.1 uF.
 

Offline T3sl4co1l

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Re: Self Biased Zener Voltage Reference
« Reply #2 on: July 17, 2015, 01:47:11 am »
FYI, LMCxx won't give best results here -- high noise voltage.  A bipolar amp with e_n/i_n ~= Z of the circuit will do best.

Unless it's a buried zener, this is probably orders of magnitude away from useful, though.  In which case, even a LM358 should be sufficient.

Tim
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Offline Jay_Diddy_B

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Re: Self Biased Zener Voltage Reference
« Reply #3 on: July 17, 2015, 02:21:45 am »
Hi group,
I believe that Rs, which is the start-up resistor is shown in the wrong position. It should be connected from cathode of the Zener to Vdd.

Rs is normally much greater in value than Rbias. This is done to reduce the variation in Zener current with changes in Vdd.

The whole idea of the circuit is to operate the Zener diode at constant current.

Regards,

Jay_Diddy_B
 

Offline soligenTopic starter

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Re: Self Biased Zener Voltage Reference
« Reply #4 on: July 19, 2015, 12:07:02 pm »
Thanks for the help.

No, this is not a buried zener, bu tthanks for mentioning it, as I didn't know what one was until I looked it up.

I'll be using a TLC272 op amp (at least to start) because it is what I have.

I'll be experimenting now.  I'll post back when I have something.
 


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