Author Topic: Trying to use zener diodes to restrict AC waveform to +2V/-2V. Not working.  (Read 3864 times)

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Offline JoeNTopic starter

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Trying to use this arrangement:



http://www.electronics-tutorials.ws/diode/diode_7.html

This is my circuit:



This diode:

http://www.onsemi.com/pub_link/Collateral/MMSZ4678T1-D.PDF

I think the problem is the diode itself.  While it is rated at a zener voltage of 1.8V, my semiconductor tester is saying Vr=3.273V @ 5.0mA.  The waveform that results from a -5V/+5V input is at least that large, not -2V/+2V.  So I guess to get it down to 1.8V you need to actually pass more current?  The current is going through a 10K resistor to a 50 ohm input (a comparator).  I guess I should just lower the resistor value way down, maybe to 220 ohm, let 23mA flow at 5V?  Would that do it?  The data sheet says that the ratings are for a forward current of 10mA.  Or am I doing this all wrong?
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Offline c4757p

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Any rating measured at a forward current of anything is a forward voltage specification. They're Zeners, you need the reverse voltage. Datasheet says that was tested at 50 microamps (which would be the lowest Zener test current I've ever seen :o)

And no, the Zener voltage does not go down as current increases - Zeners have a positive differential resistance just like most devices.
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Offline IconicPCB

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I see Vz=1.8@ Izt=50microAmps

EDIT.. too late ...she cried,... as she waved her wooden leg about

EDIT use three small signals diodes in series in inverse parallel with another three small signal diodes in series
« Last Edit: December 21, 2015, 05:48:48 am by IconicPCB »
 

Offline Paul Moir

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Fig 9 has the useful graph.  Still it shouldn't be 3.2v. 
Also don't forget you'll get the forward drop from the other diode in addition to the reverse breakdown of the breaking down zener.
« Last Edit: December 21, 2015, 05:52:26 am by Paul Moir »
 

Offline Brumby

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Also don't forget you'll get the forward drop from the other diode in addition to the reverse breakdown of the breaking down zener.

Yes.  This is important.  You are going to get around 0.7v drop across the forward biased diode, together with the reverse biased diode operating in the zener range at 1.8v will give you clipping around the region of 2.5v.  You would need 1.3v zeners to do what you are trying to do.  In looking around at available zeners, the lowest value I've come across is 1.8v, so that's a problem.

EDIT use three small signals diodes in series in inverse parallel with another three small signal diodes in series

This is an approach more likely to get closer to your desired voltage.

 

Online Ian.M

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I notice pin 2 of U8 is labelled 'RF', so you must also consider the junction capacitance and the maximum signal frequency.  The junction capacitance can be quite high. e.g those Zeners has a reverse biassed capacitance of about 200pF, which with a 10K resistor feeding it would give a cutoff frequency of about 80KHz.
 

Offline JoeNTopic starter

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I notice pin 2 of U8 is labelled 'RF', so you must also consider the junction capacitance and the maximum signal frequency.  The junction capacitance can be quite high. e.g those Zeners has a reverse biassed capacitance of about 200pF, which with a 10K resistor feeding it would give a cutoff frequency of about 80KHz.

I call it RF because I am using an SMA connector there more for convenience than anything.  My problem happens with very low frequencies too.  I appreciate your explanation, that does help me think about these things and knowing this is no help at high frequencies is helpful to know.

This is an approach more likely to get closer to your desired voltage.

Thanks.  I will try that out.  Yeah, I ordered the 1.8V zeners because that is the lowest zener voltage I could find also.
« Last Edit: December 21, 2015, 08:54:40 am by JoeN »
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Online Alex Nikitin

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Another option (more expensive but much more accurate) is to use a limiting opamp, for example OPA698.

Cheers

Alex
 

Offline JoeNTopic starter

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Another option (more expensive but much more accurate) is to use a limiting opamp, for example OPA698.

Cheers

Alex

Given that this is just a personal project and I am not putting it in a bunch of stuff to sell, I will have to look at that.  Thanks.  It has quite a bit of bandwidth too, which eliminates the bandwidth issue that was raised.
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Offline Tim F

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If you use multiple small signal diodes as in Brumbys post the capacitance won't be much - a handful of pF. Good enough to get you a few MHz of bandwidth with a 10k resistor.
 

Offline IconicPCB

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Re: Trying to use zener diodes to restrict AC waveform to +2V/-2V. Not working.
« Reply #10 on: December 21, 2015, 01:22:27 pm »
The other option is to use paralleled  NPN/PNP transistors with bases  biased at +/- 1.7V and emitters joined together as a clipping circuit.

Much higher performance than diode option
 

Online TimFox

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Re: Trying to use zener diodes to restrict AC waveform to +2V/-2V. Not working.
« Reply #11 on: December 21, 2015, 04:44:21 pm »
Note that low-voltage devices are true "Zener diodes".  Somewhere around 5 V, the normal devices are "avalanche diodes" that have much sharper increase of current with voltage above the breakdown point than do true Zeners.
 


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