are zeners THAT noisy? - Page 1
Forum section: electronics / beginners

author: exe
author_rank: Unknown
timestamp: August 04, 2025, 06:37:10 pm
content: Hello forum,I wanted to build a yet another cheap voltage reference. I took a zener of unknown origin of 5.4V nominal, and tried to find a current with zero tempco using my fingers. It turned out to be around 0.4mA (quite low, huh). Trouble is, at such currents I see a lot of noise when measuring voltage, sometimes exceeding 10mV! The noise slowly disappears as I increase current, and at some 10mA it seems to be quiet.So, is it me doing something wrong, or zeners are really that noisy? I read in the literature that they are noisy, but never seen any specific numbers.How I connected zener: it's connected to a power supply via a 510 ohm resistor. To change current I change voltage. I verified that noise is present across zener, or across resistor, but not if I connect my o'scope to power supply directly.Still, the noise shape looks strange to me. As if it picks up some environmental noise. Could it be that zener impedance drops as I increase current? So, it is not zener's noise, but artefact of my measurement?

author: bson
author_rank: Unknown
timestamp: August 04, 2025, 06:59:47 pm
content: The Zener reverse conduction is based on junction breakdown, so yeah, it's noisy. Like radioactive decay it's made of random discrete events. The probability of an event increases with voltage. I doubt (but don't know) that the periodicity is related to this, in which case it's a noise spur - but maybe it is, maybe it's just external mains interference adding a little voltage to the junction and so modulating itself onto the noise.

author: golden_labels
author_rank: Super Contributor
timestamp: August 04, 2025, 07:54:39 pm
content: The base sine is 50 Hz. The spikes appear regularly, 45° out of phase with the sine. Non-deterministic noise is not that regular. Unless it’s external noise being picked up, I’d say power supply generates that.Are you using long leads? Perhaps they form an LC circuit with the diode, amplifying something in the power supply. What happens, if you add some smoothing capacitor (4.7–47 µF) just before the resistor and diode?

author: Vovk_Z
author_rank: Super Contributor
timestamp: August 04, 2025, 08:01:34 pm
content: Those screenshots look like some Zeners intuitively generate 50 or 60 Hz noise, it depends on country of origine.If seriously, then I have to say that there is no sense to talk about noise mesurements until.there is such clearly large mains noise.

author: TimFox
author_rank: Super Contributor
timestamp: August 04, 2025, 09:07:25 pm
content: Current through the potential gap in a reverse-conducting Zener diode generates shot noise.However, a 50 Hz sine or distorted sine wave is not "noise": it must be picked up from somewhere.Note that glass-packaged diodes can easily have photocurrent that responds to the periodic current through a bulb or LED in your room lighting (usually at 100/120 Hz).Pure shot noise depends on the current: the mean square noisecurrentis given by<in2> = 2q Ix (bandwidth in Hz)whereqis the charge on the electron andIis the mean (DC) value of the current through the gap.To calculate the noisevoltageacross the device, multiply by the small-signal resistance (dV/dI) at the operating point.Thermal noise has the same spectrum, but is independent of DC current value.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 04, 2025, 10:09:46 pm
content: The shown signal is more like mains hum coupled to the zener.Yes the zener impedance depends on the current and gets higher with less current. 400 µA are actually not that low and may well be feasible.Zener diodes can be quite noisy. Not all are the same however. There are really noisy one and relatively low noise ones. Not sure if is the type of batch that makes the difference. There is also a dependence on voltage.A few weeks ago I just did a test with some cheap zeners and found quite some difference. Attached are examples for a rather noisy and surprisingly low noise diode. The current is chosen for relatively low TC, but not fine adjusted (just the test current of my DMM).

author: langwadt
author_rank: Super Contributor
timestamp: August 05, 2025, 01:37:24 am
content: try replacing the zener with a resistor that gives roughly the same voltage

author: coppercone2
author_rank: Super Contributor
timestamp: August 05, 2025, 06:37:15 am
content: out of curiosity does anyone have a noise figure for voltage ref vacuum tubes compared to zeners in rms noise to 20MHz?

author: iMo
author_rank: Super Contributor
timestamp: August 05, 2025, 08:41:13 am
content: Quote from: exe on August 04, 2025, 06:37:10 pmI would use batteries and an AC preamplifier, an oscilloscope is not sensitive enough. Also limit the bandwidth. The impedance of a zener could change from X ohm to XX ohm based on the current.Also you may use a current source (battery powered). Use shielding as well (ie. a cookie box).Here is a zener measurement with my cheapo meter:https://www.eevblog.com/forum/metrology/low-cost-lm399-noise-indicator/msg5056336/#msg5056336

author: 5U4GB
author_rank: Super Contributor
timestamp: August 05, 2025, 08:48:47 am
content: Quote from: exe on August 04, 2025, 06:37:10 pmYes. They've been used as shot noise sources in random noise generators precisely because of this property. For example here'san old AD article on itwhich gives the characteristics of several zener diodes under different conditions.

author: Roehrenonkel
author_rank: Frequent Contributor
timestamp: August 05, 2025, 09:19:03 am
content: Hi coppercone2,Quote from: coppercone2 on August 05, 2025, 06:37:15 amThey can be very low noise:https://amplifier.cd/Technische_Berichte/Rauscharme_Gleichspannungsquelle/Gleichspannungsquelle.html

author: mtwieg
author_rank: Frequent Contributor
timestamp: August 05, 2025, 11:42:51 am
content: Quote from: exe on August 04, 2025, 06:37:10 pmYes, the zener impedance will drop as current is increased. This is likely why the interference disappears as you increase current.Yes, zener diodes are often used for noise sources, but it's incorrect to assume that zeners are inherently more "noisy" than other components in general. Typically when zeners are used as noise sources, they have voltages of 6V or higher, meaning their reverse breakdown is dominated by avalanche breakdown instead of zener breakdown. The majority of their noise comes from avalanche noise, not shot noise (though they appear similar).https://www.analog.com/en/resources/design-notes/building-a-lowcost-whitenoise-generator.html

author: golden_labels
author_rank: Super Contributor
timestamp: August 05, 2025, 12:39:41 pm
content: Hey, folks! I suppose everybody may enjoy the zener noisness topic as a tangent. But not as the main topic, replying directly to OP, whose case is clearly not noise.

author: coppercone2
author_rank: Super Contributor
timestamp: August 05, 2025, 01:55:47 pm
content: I need to go through translation but I think last image tells me what I wanted to know. impressive

author: 5U4GB
author_rank: Super Contributor
timestamp: August 06, 2025, 01:46:24 am
content: Quote from: coppercone2 on August 05, 2025, 01:55:47 pmI'm just enjoying the fact that a post about valve technology is coming from someone with the name Roehrenonkel :-). It's a bit difficult to translate and retain the effect but it's something like "Uncle Valve" (not toob, I'm not in the US).

author: RoGeorge
author_rank: Super Contributor
timestamp: August 08, 2025, 08:27:54 am
content: Quote from: exe on August 04, 2025, 06:37:10 pmThat bigperiodicripple of about 1-2 divisions on the oscilloscope is mains hum (together with its periodic spikes). If it were to power the same circuit from a battery, and keep it inside a shielded box, you wouldn't see that 50Hz hum+spikes. That is captured from the surrounding, not coming from the Zener diode.It is easier to accidentally capture mains hum when the circuit is high impedance, and that is why at low currents, the hum ripple is bigger. When the current is bigger, the slope in the I/V plot is steeper, and the dinamic resistance goes smaller.Image fromhttps://wiki.analog.com/university/courses/alm1k/alm-lab-zener-diodeThe DC current through the Zener will enforce the bias (working) point. If the current is too small, the diode is near zero, or near the knee of the curve at best (to the left side of the vertical axis, somewhere in the blue zone). When you increase the current, the Zener is biased to work somewhere in the green or yellow zone.The slope in that plot is the dynamic internal resistance of the diode junction. The stepper the slope, the smallest the resistance. A horizontal line means infinite resistance (dynamic resistance, the dv/di, not V/I). A vertical line means zero resistance.On top of that, there is a smallrandomnoise (0.1 or less oscilloscope divisions amplitude) produced by the Zener diode. That noise is of a relatively wide spectrum and reasonably flat in amplitude, such that sometimes the noise from a reversed polarized diode (like a Zener, or the base-emitter junction of a BJT) is used to probe the bandwidth of RF circuits, filters, generate random numbers, etc.