Trying to use zener diodes to restrict AC waveform to +2V/-2V.  Not working. - Page 1
Forum section: electronics / beginners

author: JoeN
author_rank: Frequent Contributor
timestamp: December 21, 2015, 05:34:18 am
content: Trying to use this arrangement:http://www.electronics-tutorials.ws/diode/diode_7.htmlThis is my circuit:This diode:http://www.onsemi.com/pub_link/Collateral/MMSZ4678T1-D.PDFI 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?

author: c4757p
author_rank: Super Contributor
timestamp: December 21, 2015, 05:41:01 am
content: Any rating measured at a forward current of anything is a forward voltage specification. They're Zeners, you need the reverse voltage. Datasheet saysthatwas tested at 50 microamps (which would be the lowest Zener test current I've ever seen)And no, the Zener voltage does not go down as current increases - Zeners have a positive differential resistance just like most devices.

author: IconicPCB
author_rank: Super Contributor
timestamp: December 21, 2015, 05:43:48 am
content: I see Vz=1.8@ Izt=50microAmpsEDIT.. too late ...she cried,... as she waved her wooden leg aboutEDIT use three small signals diodes in series in inverse parallel with another three small signal diodes in series

author: Paul Moir
author_rank: Frequent Contributor
timestamp: December 21, 2015, 05:49:30 am
content: 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.

author: Brumby
author_rank: Unknown
timestamp: December 21, 2015, 07:00:21 am
content: Quote from: Paul Moir on December 21, 2015, 05:49:30 amYes. 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.Quote from: IconicPCB on December 21, 2015, 05:43:48 amThis is an approach more likely to get closer to your desired voltage.

author: Ian.M
author_rank: Super Contributor
timestamp: December 21, 2015, 07:56:13 am
content: 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.

author: JoeN
author_rank: Frequent Contributor
timestamp: December 21, 2015, 08:52:59 am
content: Quote from: Ian.M on December 21, 2015, 07:56:13 amI 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.Quote from: Brumby on December 21, 2015, 07:00:21 amThanks. I will try that out. Yeah, I ordered the 1.8V zeners because that is the lowest zener voltage I could find also.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: December 21, 2015, 09:02:35 am
content: Another option (more expensive but much more accurate) is to use a limiting opamp, for example OPA698.CheersAlex

author: JoeN
author_rank: Frequent Contributor
timestamp: December 21, 2015, 09:16:47 am
content: Quote from: Alex Nikitin on December 21, 2015, 09:02:35 amGiven 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.

author: Tim F
author_rank: Regular Contributor
timestamp: December 21, 2015, 09:51:54 am
content: 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.

author: IconicPCB
author_rank: Super Contributor
timestamp: December 21, 2015, 01:22:27 pm
content: 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

author: TimFox
author_rank: Super Contributor
timestamp: December 21, 2015, 04:44:21 pm
content: 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.