Help me understand diode datasheets. - Page 1
Forum section: electronics / beginners

author: MarkS
author_rank: Unknown
timestamp: February 22, 2023, 02:23:42 pm
content: In a previous post--here--, I was informed that a zener diode in my circuit would destroy accuracy. And, did it ever! I decided to simply remove the diode from the project, as I really am overcomplicating things and I'm protecting the circuit against a long shot possibility. This circuit isn't critical and I don't need to be so thorough.Still, I was thinking about it and realized that with a 10:1 voltage divider, dropping to a max of 3.3v, the maximum input voltage would be 36.3v. Since the zener affected accuracy at the higher voltages read, I though that maybe I could use a 36v zener and place it prior to the divider. That would, I'm guessing here, clamp the max voltage to roughly 36 volts and since I'm only interested in reading up to 14 volts, the accuracy hopefully would not be impacted as severely.Regardless if that would actually work or not, it did expose a glaring lack of knowledge on my part. I was looking at the1n5221and realized that I have no idea how to read the various charts and other data. What am I look at? What am I looking for? Moreover, it seems that I do not understand forward/reverse voltage as well as I had thought.I really need a good diode primer.

author: TimFox
author_rank: Super Contributor
timestamp: February 22, 2023, 02:46:32 pm
content: If you place a 36 V Zener ahead of the divider, it should not affect the accuracy at 14 V.However, should the input try to get to, say, 50 V, there must be a resistor in series with the top end of the Zener to limit the current through it, or it will blow up with a high current from a "stiff" voltage source.ThetypicalI-V curves will give you some idea of how much current will flow through that resistor at 14 V.The relevant parameters for a higher-voltage Zener are the voltage at the specified test current (varies with voltage and type) and the maximum current it can sustain (which is the rated power divided by the Zener voltage).Another important parameter is the "knee", a specified voltage at a relatively low current (where the diodes "starts" to conduct).In general, 3.3 V Zeners have a very "weak" curve of voltage vs. current, while 36 V "Zeners" are actually "avalanche diodes" and have a much sharper curve of voltage vs. current.The boundary between "true Zeners" and "avalanche diodes" is somewhere around 5 or 6 V.If you don't mind wasting some current, you could install an intermediate voltage Zener, > 3.3 V, with 1 mA current through it from another resistor, and connect a low-leakage regular diode to an intermediate point on your higher-resistance voltage divider. The high resistors in the divider would limit the current through the Zener to a save level, while the low-leakage diode is reverse-biased at normal operating voltages: the error can be calculated from the leakage current of that diode and the resistance in the divider.

author: pcprogrammer
author_rank: Super Contributor
timestamp: February 22, 2023, 02:56:05 pm
content: I glanced the thread you mentioned and think a TVS diode is a better solution to suppress spikes.To not wreck havoc on the max 3.3V input to the ADC, (Not familiar with the RP2040 ADC converter, but that value seems high) you can use a TVS with a higher voltage and use three resistors to make your divider. First resistor connects to the battery, second resistor and TVS. Second resistor connects to RP2040 and third resistor, which is then connected to ground.A TVS can also handle bigger currents then a zener diode. Something likethisfor exampleBy the way it has been a long time since school, butOhmslaw is I = V / R. What you mentioned in the other thread has to do with power (P = V * I) and there James Watt springs to mind, but am not certain of that. But others might already have mentioned that there.

author: MarkS
author_rank: Unknown
timestamp: February 22, 2023, 09:04:06 pm
content: Quote from: pcprogrammer on February 22, 2023, 02:56:05 pmI had a brain fart. I understand the math, but I was having trouble applying it. I don't want to blame everything on my cancer, but I have a lot on my mind and trying to focus is difficult at times.

author: MarkS
author_rank: Unknown
timestamp: February 23, 2023, 05:21:28 pm
content: We kind of got off on a side issue, and I'm grateful for it, but my main question involved understanding diodes and diode datasheets.

author: pcprogrammer
author_rank: Super Contributor
timestamp: February 23, 2023, 06:35:18 pm
content: Quote from: MarkS on February 22, 2023, 09:04:06 pmSure I can understand that.Quote from: MarkS on February 23, 2023, 05:21:28 pmNot my specialty either, but for your quest I suppose the diode curves are of relevance. A zener is a bit of a different beast of course and it's forward voltage is like a normal diode. With the anode connected to a positive voltage it will have a drop of max 1.1V depending on the current. (From the 1N5221 datasheet)In its normal usage where the anode is connected to ground and the cathode on a positive voltage it will regulate based on the current within a certain span. This is what the curves tell you. There is also temperature to take into account.Maybe someone with a better understanding and ability to explain can chime in.