12V Arduino digital input - Page 1
Forum section: electronics / beginners

author: Lucky-Luka
author_rank: Unknown
timestamp: April 23, 2022, 08:39:34 am
content: Hi allI want to detect a 12V signal using an Arduino Nano v3.I've decided to use a NPN transistor to do so. Is it a valid solution? Is it better than using a resistive voltage divider?I've tried the circuit and it looks like it works fine.I've tried to use the 100k res at the base but I don't really know if it's the best value. How to chose it?Thanks

author: madires
author_rank: Super Contributor
timestamp: April 23, 2022, 08:55:32 am
content: A BJT is fine! For calculationg the base resistor please seehttps://wiki.jmehan.com/display/KNOW/Transistors+-+Calculating+Base+Resistor++for+Switchingfor example.

author: Ian.M
author_rank: Super Contributor
timestamp: April 23, 2022, 09:01:28 am
content: The max. hFEof a 2N2219 is 300, (400 at elevated temperatures). As little as one uA leakage current across the switch could therefore pull down the D2 pin enough to register as logic low.The transistor adds complexity and increases the sensitivity of the circuit to noise and leakage, making it significantly worse than a simple potential divider. It does however prevent input over-voltage reaching the Arduino.

author: Lucky-Luka
author_rank: Unknown
timestamp: April 23, 2022, 09:08:12 am
content: Interesting. I wanted to prevent over-voltage even if I think it shouldn't be present. It is connected to an alarm system for home.So should I use a zener diode? how?Thanks

author: DavidAlfa
author_rank: Super Contributor
timestamp: April 23, 2022, 09:16:24 am
content: Just use a resistor divider.The input pin has 1uA current leakage typical, this circuit provides 15, should be more than enough.The ESD diode will clamp higher voltages to VCC, the resistor value is high so very little current will actually go into it.Even direct short with 240Vac will input just 0.72mA peak, will hardly cause any issue.Code:(12V)->-*->(4.95V)||

author: Lucky-Luka
author_rank: Unknown
timestamp: April 23, 2022, 09:27:53 am
content: Where is the ESD diode?

author: Psi
author_rank: Super Contributor
timestamp: April 23, 2022, 09:35:12 am
content: With that 470K in series with the source you aren't going to have any ESD issues.The IO protection is going to handle a surge of 50mA. To get 50mA through 470K you need 0.05 * 470000 = 23,500VIf it's an arduino based on an AVR then it has reasonable spec ESD diodes inside the chip on each pin. And it is ok to use them so long as you limit the current. (Atmel/Microchip even have an APP Note where you can feed mains voltage into an IO pin through 1meg resistor to look at the AC waveform or do zero crossing).

author: Lucky-Luka
author_rank: Unknown
timestamp: April 23, 2022, 10:11:33 am
content: Will 15k and 10k resistors do the trick?Aren't those original resistors values too big?

author: Terry Bites
author_rank: Super Contributor
timestamp: April 23, 2022, 10:42:26 am
content: As Ian says,1 uA of base current could give rise to a collector current of 400uA. That’s a drop of 4V across a 10k resistor. So use ~ 1kIf the input is from a long line, RFI can cause false triggering. If that's the case then you need some filtering and input protection. egDon't ground the far end. Run a ground from your MCU system instead.The base current or Iin= (12-0.7)/(Rb) . So the collector current I(Rc) =hfe*Iin.You have 11.3/10k =1.13mA. Looking at the 2N2219 data sheet, the hfe is at least 100 at low currents.Ic =100*1.13mA =111.3mA. The collector resistor will limit this to 5/1k or 5mA. More than enough to pull the collector below the GPIOs low threshold. 5mA may be a lot of current in your design.A small MOSFET can be used instead- with or with out protection components. [ Specified attachment is not available ]BTW. A 2N2219 is over kill, it will happily switch half an amp. It can work at 100MHz at 200mA or more.

author: tunk
author_rank: Super Contributor
timestamp: April 23, 2022, 10:57:54 am
content: If the alarm system has a battery, then the voltage most likely will be higher than 12.0V.If you use a voltage divider, adjust the resistors values to reflect this.

author: mikerj
author_rank: Super Contributor
timestamp: April 23, 2022, 11:09:47 am
content: Quote from: Lucky-Luka on April 23, 2022, 09:27:53 amWithin the microcontroller. There is a diode between the pin and ground, and the pin and Vdd which are both reverse biased under normal operation. The majority of CMOS ICs will have this.

author: Peabody
author_rank: Super Contributor
timestamp: April 23, 2022, 02:43:16 pm
content: Will there ever be a time when the Arduino will be powered down but the 12v signal will still be active? If so, using a voltage divider could possibly back-power the Arduino through the protection diode on the GPIO pin, depending on the resistor values.

author: rstofer
author_rank: Super Contributor
timestamp: April 23, 2022, 02:52:18 pm
content: It adds complexity but there's a reason they invented the opto-isolator. You may not need the Schmitt Trigger because a) you don't care about the edges heading toward the uC or the uC has Schmitt Trigger inputs.https://learnabout-electronics.org/Semiconductors/opto_52.php

author: Ian.M
author_rank: Super Contributor
timestamp: April 23, 2022, 03:02:50 pm
content: This is not rocket science and there is no need to over-complicate it.A potential divider with upper resistor 5K1 and lower 3K6 will divide 12V down to 5V. That choice of resistor values is for the closest ratio to the desired 7:5 within the E24 series. The decade chosen is to maintain a relatively low input impedance, drawing a bit over 1mA from the 12V source so it isn't excessively sensitive to 'phantom' voltages due to leakage currents.To protect the MCU against over-voltage you need to clamp the voltage at the potential divider tap. While you could clamp to the supply rails, e.g. using a BAT54S dual series Schottky diode, that risks the 5V rail rising catastrophically if more current gets dumped into it than the load on the rail.As its a digital input, you can use a Zener to clamp it*, cathode to the divider tap, dumping the clamping current to ground. A 5.1V 5% Zener rated for 100mW or more is likely to be suitable. That will provide protection to over +/-35V continuous if you use 1/4W resistors, or over +/-70V with a 1W upper resistor in the divider, the limit being that resistor's dissipation when negative voltage is applied. If there is a significant risk of overvoltage use a fusible resistor for the upper (5K1) one in the divider. As the Zener voltage can rise a little above the MCU's 5V supply (or the supply may be lost), you also need a resistor between the divider tap and the input pin to limit any current flow through the internal protection diodes, and 10K should do nicely.A capacitor in parallel with the Zener may be added to help suppress RFI and glitches, and 10nF with the divider values above will give a -3dB cutoff frequency of 7.5KHz, and have negligible effect on signals under 1KHz. Note that whenever you use a RC filter feeding a digital input, to avoid glitches and possible excessive current consumption from the digital device's supply while the input transitions, the input should have a Schmitt trigger characteristic.* Simple Zener clamping is unsuitable for ADC inputs unless Vref<<Vcc as low voltage Zeners have a relatively soft knee so will tend to leak current reducing the measured voltage as the ADC approaches its full scale count at Vdd.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: April 23, 2022, 03:44:37 pm
content: It's easy to overcomplicate a thing like this, but it's also easy to make it too simple.A safe way is to add a voltage divider in front of the transistor. If you put a resistor between the base and emitter, then it will de sensitize the transistor and your transistor won't do anything a s long there is approx 500mV across this resistor. It's a good beginners project to build this on a breadboard.Without such a resistor, the transistor can easily pick up mains hum, especially when you have long wires. You can (and should!) also build this on a breadboard. It's a good learning experience to see what happens and where the limits are.For the actual circuit I would probably just use a voltage divider with a fairly high input impedance. For example 47k and 68k, and combine that with an extra filter capacitor directly on the uC pin to filter out the mains hum and spurious switching.for a real robust situation you can add scottky diodes or a TVS for protection but it's not really needed as long as the input impedance of your circuit is high enough. The AVR pins have internal protection diodes and these can handle a few mA continuously. There is even an official application note in which a mains voltage zero cross detector is made with only a series resistor.Long wires can pick up lots of weird stuff and this may very well require extra filtering. Not only on the input, but also on the power supply.You can also make it more robust in software. For example, sample the input at a fixed frequency, and only accept a change if quite a number of the same input state has been sampled.Also:Buying (and using) an EUR 10 Logic analyzer is always a good thing. These things should be bundled with every "arduino starter kit". They work with the open source Sigrok / Pulseview project, and are great for quickly visualizing simple things as on/off digital switches to I2C, UART, SPI and some 100 other serial based protocols.As an intermediate step for the hardware, I recommend to build a very simple circuit on your breadboard. Just build your input section, and then write a simple program that only reads the input pin, and then outputs it to another pin in a tight loop. On that second pin you can either attach the logic analyzer, or a simple LED (with series resistor). This gives you quick feedback to what your input circuit does for example when you touch the wire, or when the wire is close to mains voltage carrying wire.

author: Lucky-Luka
author_rank: Unknown
timestamp: April 23, 2022, 06:32:28 pm
content: Quote from: Doctorandus_P on April 23, 2022, 03:44:37 pmcan you show me what kind of instrument you have in mind?Is the updated version of the circuit ok for you?

author: Doctorandus_P
author_rank: Super Contributor
timestamp: April 24, 2022, 01:35:30 pm
content: It's one of these boxes:https://www.sparkfun.com/products/18627The original "design" is from Saleae, and they put the thing in a very expensive custom milled fancy aluminimum box. However, it is not much more then a Cypress CY7C68013A, a voltage regulator and some input protection. Lot's of chinese shops sell these things (Including shipping) for less then the shipping alone from sparkfun.The CY7C68013A microcontroller is a bit of a weird thing. It does not have any flash memory. When you connect it to your PC, it is up to the driver for the device to load firmware into it via the standardized DFU protocol.That last schematic looks OK.The 100nF capacitor only keeps out relatively high frequencies. If you also want to suppress the mains hum, then you need a much bigger capacitor. Something like an 1uF or 10uF electrolytic capacitor in parallel to your ceramic capacitor. For experiments on your breadboard, it's also nice to try out a 1000uf capacitor. such a capacitor is big enough to introduce a clearly visible delay.Sigrok / Pulseview also works with generic development boards for this uC (It recognizes these because they have a different VID/PID pair in the Eeprom), and then it can work with 16 channels, but these generic boards do not have any protection on the input pins.

author: Ian.M
author_rank: Super Contributor
timestamp: April 24, 2022, 02:13:43 pm
content: Yes, the 'Salae' Logic 8 clones are widely available on Ebay, Amazon and the usual far east e-bazaars, and as long as you stick to the open source Sigrok software and fx2lafw firmware, there are no ethical implications in owning/using one.Caution: some have a 'CLK' pin in place of the second GND pin. This is *NOT* a 'sample now' strobe clock input, but is the CY7C68013A clock output (up to 48MHz), so is a multiple of the sample rate, and is pretty much useless unless you are using it for data acquisition with an parallel ADC or similar. Do *NOT* connect when using it as a logic analyser.Also, if bundled with generic test clips, the clips are almost invariably not fitted with pins to accept the supplied duPont leads, and most likely are not easily solderable. See:https://www.eevblog.com/forum/beginners/soldering-to-strange-metal-contacts/specifically my replies #20 and #23 which show how they *can* be soldered.

author: DavidAlfa
author_rank: Super Contributor
timestamp: April 24, 2022, 03:06:17 pm
content: Quote from: Doctorandus_P on April 24, 2022, 01:35:30 pmRomless mcus aren't that strange!In this case it's beneficial, you can run any firmware you want, not requiring flashing the system, risk of bricking...But sure,you can add a default rom. The bootloader reads the eeprom, the first byte defines wether there's a firmare on the eeprom to be loaded(0xC2) ot not(0xC0). And can also boot from external parallel flash.Just happened that I bought one 16-ch version, and received 2.USBee AX-Pro / Saelae eeprom values:https://saturn.ffzg.hr/rot13/index.cgi?action=display_html;page_name=saleae_usbee_cloneBut I programmed these VID/PID so Sigrok recognizes them as 16-ch by default.https://www.sigrok.org/blog/sigrok-firmware-fx2lafw-013-released16ch is limited to 12MHz, as it has double data to send!

author: Lucky-Luka
author_rank: Unknown
timestamp: April 26, 2022, 02:03:12 pm
content: I've completed the circuit and it looks like it works in real life.I am doing an add on for a home alarm system.I bring power and signal with the same cable from the central unit to the Arduino Circuit. Basically I have an AI camera connected to the central unit that act as a sensor and I want to add an Arduino circuit controlled speaker that say something before the alarm goes off.Do you spot any major flaw in the design?When the speaker is working the current draw doesn't exceed 0.2A and the speaker is supposed to work for several minutes just when the alarm goes off.Thanksp.s. I've used a trimmer in the voltage divider because I deal with alarm systems that give both 12V and 15V signal and power to my board.

author: ledtester
author_rank: Super Contributor
timestamp: April 27, 2022, 03:09:34 am
content: Quote from: Lucky-Luka on April 26, 2022, 02:03:12 pmIn that case I would use a transistor like you had in your first post. You won't have to worry so much about the input voltage vs. what you're running your MCU at. For instance, what if you wanted to interface with a 3.3V MCU?To elaborate... you can design the input circuitry of the transistor to fully saturate it over a wide range of input voltages -- like 10 to 30V. On the output side you pull up the collector to whatever Vcc is for your digital logic -- whether it be 3.3, 5V or even higher (e.g. for CMOS logic chips). Now you have a design you don't need to tweak for different situations. The technique goes by the name of "open collector output" (or "open drain output" if a MOSFET is used).

author: Lucky-Luka
author_rank: Unknown
timestamp: April 27, 2022, 07:23:38 am
content: Maybe an optoisolator then?

author: Ian.M
author_rank: Super Contributor
timestamp: April 27, 2022, 08:12:40 am
content: Increase R2 to 6.2K in the Zener clamped divider I posted back in reply #13, and you can use it from 9V to 24V, However it is still beneficial to tweak it for the expected input voltage as it is desirable to have a switching threshold roughly in the middle of the range between max. input low and min. input high. If the threshold is too low, (or too high) it will have poor noise immunity. A simple potential divider effectively multiplies the threshold of the gate its feeding by the inverse divider ratio.Ledtester's proposal suffers from the same deficiency, with the added disadvantage that the threshold for a simple common emmitter BJT is rather temperature dependent.Unfortunately, low voltage Zeners have a rather 'soft' knee on their Ir vs Vr characteristic curve, so simple Zener clamping is far less suitable for driving 3.3V (or lower) logic. This can be worked around by using a three resistor divider and clamping a higher divider tap with a higher voltage Zener.

author: Lucky-Luka
author_rank: Unknown
timestamp: April 27, 2022, 08:33:14 am
content: Then the opto solution should avoid these problems. No?I want to use this board with 12v-15v input.

author: Ian.M
author_rank: Super Contributor
timestamp: April 27, 2022, 08:49:15 am
content: No. Optocouplers are for isolation or for *EXTREME* level shifting (e.g. when you need to get a logic signal from near-ground to riding on top of a 400V power rail). If you use them with a common ground directly across the input and output sides of the opto, its pointless as its doing exactly the same job as a transistor (BJT) but with much crappier hFE.

author: Lucky-Luka
author_rank: Unknown
timestamp: April 27, 2022, 08:59:28 am
content: They have common groud in my board.It should work. Don't you think?

author: Ian.M
author_rank: Super Contributor
timestamp: April 27, 2022, 09:26:46 am
content: It wont work. You have either made a brain-dead mistake with the opto connections or simply don't understand how to use optocouplers.

author: Lucky-Luka
author_rank: Unknown
timestamp: April 27, 2022, 10:09:23 am
content: I have made an error connecting the output of the optocoupler.Having said that, do you think it can work (if properly connected) for my purpose?

author: Zero999
author_rank: Super Contributor
timestamp: April 27, 2022, 10:13:39 am
content: An opto-coupler will work, but why bother? It's easier and cheaper to just use a BJT.

author: Lucky-Luka
author_rank: Unknown
timestamp: April 27, 2022, 10:30:04 am
content: Quote from: Zero999 on April 27, 2022, 10:13:39 amI've noticed that there are at least 3 school of thought...I have started the thread with a BJT solution.Then I was told "The transistor adds complexity and increases the sensitivity of the circuit to noise and leakage, making it significantly worse than a simple potential divider. It does however prevent input over-voltage reaching the Arduino."Other than the voltage divider I thought the opto-coupler would behave better than the BJT solution that I have proposed at the beginning..

author: Zero999
author_rank: Super Contributor
timestamp: April 27, 2022, 11:54:30 am
content: I think the voltage divider is the best way to do this. It's simple, cheap and effective.R1 R2VIN────────╱╲╱╲╱╲╱───────┬───────╱╲╱╲╱╲╱───────── 0V18k MCU 10kIOHere's the calculator I used to get the above values, except I inputted 12V for VIN, because I assumes it's automotive and is more typical.https://www.random-science-tools.com/electronics/divider.htm

author: Lucky-Luka
author_rank: Unknown
timestamp: April 27, 2022, 12:15:43 pm
content: It's not automotive. I connect to an alarm system of a house. I can have both 12V and 15V as HIGH signal.

author: Ian.M
author_rank: Super Contributor
timestamp: April 27, 2022, 12:40:56 pm
content: An optocoupler *can* work (now you've sorted out the output connections so the base is open, not the collector), but if you strap the LED collector and the phototransistor emitter to the *SAME* ground, you've just turned half a buck of optocoupler into an extremely crappy transistor (less than unity current gain and rather high 'Vbe'), not even worth five cents. OTOH it lets you keep the input and output side grounds separate (bring the input ground from the signal source down the same cable), which eliminates ground loops and vastly reduces EMI susceptibility.However they don't tolerate much reverse voltage across the Opto-LED, so adding an anti-parallel diode across it to protect it is a good idea, and if you want to increase its turn-on threshold, either a Zener in series for a reasonably well defined threshold, or a resistor in parallel with the LED + antiparallel diode, (giving a much less well defined threshold based on the LED Vf) can also be helpful. Also the CTRR can drop significantly with LED ageing so if you don't want premature failures in the field, *ALWAYS* design with a factor of two excess CTRR for your application circuit.LTspice sim attached.

author: Lucky-Luka
author_rank: Unknown
timestamp: April 27, 2022, 01:09:57 pm
content: So at the end of the day both solutions (voltage divider and opto) can be used without a clear "winner"...Shouldn't I use a 4.7 zener in the voltage divider to be more precautionary?I could avoid the 10K res to the input in that way. No?

author: Ian.M
author_rank: Super Contributor
timestamp: April 27, 2022, 01:40:02 pm
content: Quote from: Lucky-Luka on April 27, 2022, 01:09:57 pmA divider is *CHEAP* and effective, and if high enough resistance, far more tolerant of overvoltage."... more precautionary? ..."A picture is worth a thousand words - show us what you are thinking with a schematic, or fire up a simulator and answer your own question!

author: Lucky-Luka
author_rank: Unknown
timestamp: April 27, 2022, 01:57:32 pm
content: like this

author: madires
author_rank: Super Contributor
timestamp: April 27, 2022, 02:11:53 pm
content: All roads lead to Rome.Any solution comes with some pros and cons. Choose the one which fits the best and fix the remaining drawbacks in your specific application.

author: Ian.M
author_rank: Super Contributor
timestamp: April 27, 2022, 02:25:59 pm
content: What the <beeping> <beep> is a 100uF capacitor directly connected to an I/O pin doing in there?If the Arduino looses power with any other loads being driven by it or its 5V Vcc rail, that can destroy it by transient current through the offending pin's internal protection diodes.For %DEITY%'s sake add a 10K resistor (or at least >=1K) between the big capacitor and the Arduino pin to limit the current!Better - leave out the big capacitor and de-glitch the input signal in software.

author: pigrew
author_rank: Frequent Contributor
timestamp: April 27, 2022, 02:37:36 pm
content: Quote from: madires on April 27, 2022, 02:11:53 pmSo... what's the application?Is protection from automotive load dumps necessary? Just ESD?What's the bandwidth needed? Is it just a relatively slow pushbutton, or actually a sensor that needs to be detected by the MCU within 5us?The input capacitors would aid in "debouncing" a switch. Is that what you want? (yes, they are too big... more like 10nF would be reasonable) (If so, verify your MCU's inputs have Schmitt triggers.. IIRC, they do.).As an aside, I've started to prefer low-side buttons, so something likeVcc<->resistor<->button<->GND. That way, if you accidently short the button's terminals to GND, you won't have any high current paths. On the other hand, if you're using it with a voltage divider, this will cause power consumption when the switch is open. Though, the idle power consumption could be avoided if you then use a high-side PNP transistor to translate the levels (similar to what the first post had, except upside-down).

author: Lucky-Luka
author_rank: Unknown
timestamp: April 27, 2022, 02:51:02 pm
content: How many times do I have to repeat what the application is about?Again: I connect this Arduino board to the central unit of an alarm system for homes.The signal stay high for as long as the central system is programmed keep it high when the alarm goes off.When the signal goes high the speaker say something.In some application the signal can be 12v while in others it can be 15v.I have chosen that big cap because another "super contributor" user told me"Long wires can pick up lots of weird stuff and this may very well require extra filtering. Not only on the input, but also on the power supply." ... "The 100nF capacitor only keeps out relatively high frequencies. If you also want to suppress the mains hum, then you need a much bigger capacitor. Something like an 1uF or 10uF electrolytic capacitor in parallel to your ceramic capacitor. For experiments on your breadboard, it's also nice to try out a 1000uf capacitor. such a capacitor is big enough to introduce a clearly visible delay."I haven't understood how I can achieve the following in my circuit:"As an aside, I've started to prefer low-side buttons, so something like Vcc<->resistor<->button<->GND. That way, if you accidently short the button's terminals to GND, you won't have any high current paths."

author: ledtester
author_rank: Super Contributor
timestamp: April 27, 2022, 03:41:33 pm
content: Quote from: Lucky-Luka on April 27, 2022, 02:51:02 pmHow close will the Arduino be to the central unit? Just a few inches or will there be a long cable run between the two?

author: pigrew
author_rank: Frequent Contributor
timestamp: April 27, 2022, 03:42:24 pm
content: Quote from: Lucky-Luka on April 27, 2022, 02:51:02 pmApologies for not reading the whole thread. I only looked at the first schematic (with a button) and the last schematic (with only input protection). Since it's not a button (like was shown in the first post), ignore this advice... it doesn't apply.Quote from: Lucky-Luka on April 27, 2022, 02:51:02 pmYou'll get conflicting advice, it's your job to pick the wheat from the chaff and decide what's correct. There are also many correct answers.Since your Vdd bus has a somewhat large load (speaker driver), the capacitor could parasitically power the speaker through the MCU through its high-side ESD protection diode.... this likely would damage the MCU.... The MCU's internal IO pin ESD protection diodes can't handle more than 1 mA (with Vdd=+5V). With such a large capacitor, a series resistor would be needed between the capacitor and the MCU to limit the current to below that level. You could calculate the R*C time constant of the RC filter to be some reasonable time, like a few milliseconds, to choose a smaller capacitor value.I also wouldn't worry about picking up mains hum if your GND, VDD, and signal wires are all coming from the same source, and routed with the same path.The ATMega has a wide input voltage range. It accepts between 0.6*Vdd and Vdd+0.5V as a HIGH. You could design your divider to a slightly lower voltage (to be safer)... so 15V could be divided down to 5V meaning 12V down divide down to 4 V, and everything would still work properly. (With Vdd=5V, the input high range is 3 to 5.5V).A divider by itself, with a small capacitor should be good enough. I don't think the Zener is necessary.If the resistor divider limits the current to <1 mA and you have enough capacitance, it's OK to use the MCU's internal ESD protection diodes.Worst case assume your input is 12V and input power is 0V (user hooked up the input live without hooking up the power first). Assuming 0.5 mA and Vin=18V (extra safe worst case), you'd want >36k of resistance between the input and the MCU. Only if you need faster (perhaps >10 kHz) signals, would I worry about more advanced protection schemes.

author: Lucky-Luka
author_rank: Unknown
timestamp: April 27, 2022, 03:50:47 pm
content: Quote from: ledtester on April 27, 2022, 03:41:33 pmSeveral meters: more or less as many meters as between the central unit and the outdoor alarm siren.

author: Lucky-Luka
author_rank: Unknown
timestamp: April 27, 2022, 04:25:24 pm
content: I was simulating the voltage divider circuit.I cannot understand why a 4.7V zener lower the voltage at the input pin. I thought that it doesn't act if it doesn't see voltages higher than 4.7V.

author: pigrew
author_rank: Frequent Contributor
timestamp: April 27, 2022, 04:31:04 pm
content: Quote from: Lucky-Luka on April 27, 2022, 04:25:24 pmCheck the Zener's datasheet. This one is designed to be 4.7V at 20 mA. It will start to conduct about 380 mV earlier (~4.32 V), so limit the voltage to closer to 4.35 or 4.4V. The 380 mV is Z_VT * I_ZT, and is approximate... You see 3.3V? Perhaps the zener breakdown is much less sharp than I expected based on the Z_VT. It's designed to be a voltage regulator, not a TVS diode.What you're thinking about are TVS diodes. They are rated with a standoff voltage where the reverse leakage current is minimal. The also have a breakdown voltage where they start to conduct (same as the Zener voltage), which will be a few volts above the standoff voltage.

author: Ian.M
author_rank: Super Contributor
timestamp: April 27, 2022, 04:45:52 pm
content: Low voltage zeners have a rather soft knee. Higher voltage ones where avalanche breakdown dominates have a much sharper knee. Also 1N750's nom. 4.7V zener voltage is specified at 10mA test current and you are only putting about 60uA through it . . .