1N4733 Zener Diode Datasheet - Page 1
Forum section: electronics / beginners

author: sy
author_rank: Regular Contributor
timestamp: December 18, 2023, 06:37:58 am
content: Hello, I am trying to use this diode to regulate at 5.1V as stated in the datasheet.I'm using a 15-20V DC supply with a 462Ohm in series with the 1N4733.The upper and lower current limits should be:I_upper=(20-5.1)/462=32mAI_lower=(15-5.1)/462=21mAWhich is less than the current specified in the datasheet of 49mA.Based on this I would have assumed, if anything, the voltage at the output to drop below 5.1V, but when I measure with a DMM the output is 8.59V? And leaving it there for a while it increases like +0.01V every second or two...I've checked if the diode could potentially be damaged, but it seems fine using the diode test on the multimeter.Can someone help explain this to me please?

author: srb1954
author_rank: Super Contributor
timestamp: December 18, 2023, 07:34:17 am
content: Quote from: sy on December 18, 2023, 06:37:58 amAre you sure that your resistor value is correct? You should not get up to 8.59V if the Zener is OK and the resistor is within a suitable range. If your resistor is too low the current through the Zener will be too high, it will overheat and its voltage will exceed its nominal voltage and may increase further as the diode warms up.What is the minimum and maximum current drain of the circuit connected to the Zener? You need to know these values before you can calculate a suitable dropping resistor.

author: PSR B1257
author_rank: Frequent Contributor
timestamp: December 18, 2023, 07:40:16 am
content: Have you verified the marking on the diode?Have you checked the DMM against another DMM, or a more less known voltage source (e.g. 5V USB)How do you came up with a 462Ohm resistor (this value is not even part of the E192-series)?What kind of load do you want to supply?Why was a Zener Diode chosen? They are inherently inaccurate and also temperature- and current-depended.

author: Zenith
author_rank: Frequent Contributor
timestamp: December 18, 2023, 08:23:05 am
content: Quote from: PSR B1257 on December 18, 2023, 07:40:16 amI wonder if it's a 1N4738, nominally an 8.2 Volt Zener, or a 1N4739. 8 and 9 can look like 3 with poor printing, or if the component has been handled a lot and the ink is worn.I'd be inclined to try another Zener.

author: sy
author_rank: Regular Contributor
timestamp: December 18, 2023, 11:42:30 am
content: Quote from: Zenith on December 18, 2023, 08:23:05 amOMG thank you, I just double checked with others I had lying around and it is indeed a 1N4738.Quote from: PSR B1257 on December 18, 2023, 07:40:16 amI just measured one of the resistors I had lying around with my multimeter and it came up as 462OhmI'm just experimenting and learning to use them as regulators after reading through some book.What is the effect of temperature on the Zener diode? I noticed after touching the resistor it was quite hot (stupid idea). Would it have some effect on voltage creeping by like +0.01 I saw earlier...?Quote from: srb1954 on December 18, 2023, 07:34:17 amBecause, when I try the circuit the current draw is less than expected.Here is the new values:Dropping resistor: R=(20-8.2)/31mA=380Ohm (the one I used was around 375Ohm)However, the voltage drop across the terminals is approx 8.68V. I switched the diode out for a different one and got a different reading at 8.46V. Both times they drew only 27mA of current for the 20V input.I'm not too concerned about the later tested diode, however, with the first one it creeps outside the 8.61V range. Are there any potential concerns around this?Edit: actually it's kinda concerning xD Why is the voltage differing so much? +0.26V to +0.48V

author: Zenith
author_rank: Frequent Contributor
timestamp: December 18, 2023, 01:11:03 pm
content: As I recall, the rule of thumb for Zener current is that it shouldn't exceed 10% of the current required to have the Zener at its maximum dissipation, in this case 1000mW.I make that to be 0.1W/8.5V=0.0117A - about 12mA20-8.5=11.5 - the voltage drop across the resistor.11.5V/0.0117A=982R. The closest convenient value is a 1K resistor. I'd be inclined to make it higher, say a 2.7K.If you have a high Zener current, the Zener will self-heat and its behaviour won't be what you expected. If you have too low a Zener current, the characteristics are non-linear and subject to slight changes in the Zener current. Instinctively, there seems something wrong in a Zener being hot to the touch.Zeners have a lot of problems, such as being noisy and temperature dependent, but they are useful enough voltage references. They can be used as a constant voltage power supply, but it's a very crude approach.If you want a 5.1V supply, a 7805 regulator is cheap and a more obvious solution. If you want exactly 5.1V use a variable regulator where you set the voltage with resistors. If you only want a few mA and you want great stability, use a voltage reference such as a TL431.

author: PSR B1257
author_rank: Frequent Contributor
timestamp: December 18, 2023, 02:04:58 pm
content: QuoteGot it, so not nominal 462Ohm but actual.QuoteNot really. As the diode gets hotter, the voltage decreases.QuoteThat's common for Zener Diodes. There are manufacturing tolerances to start with. And on top of that, there characteristic curves are not perfectly "straight". If you plot U=f(I) you get a certain gradient around the nominal Zener-voltage. As the current increases/decreases the voltage also increases/decreases.Hence the question, what circuit you want to supply?

author: srb1954
author_rank: Super Contributor
timestamp: December 18, 2023, 07:51:48 pm
content: Quote from: PSR B1257 on December 18, 2023, 02:04:58 pmZener diodes with nominal voltages above 6V have a positive temperature coefficient so the voltage willincreaseas the diode gets hotter.

author: TimFox
author_rank: Super Contributor
timestamp: December 18, 2023, 08:01:46 pm
content: Easy demonstration: put a Zener diode across a good DVM input, feeding it either from a good current supply or a voltage supply with appropriate series resistor.When you pinch it with your fingers, you can easily see the voltage change.

author: Jeff1946
author_rank: Regular Contributor
timestamp: December 18, 2023, 09:35:20 pm
content: For currents less than 100mA you might want to consider using a TL431. The voltage can be set with two resistors and is much more stable than a zener diode. Checked with Mouser cost about $0.41 for one or $0.33 apiece for 10. Definitely worth having some.

author: Messtechniker
author_rank: Frequent Contributor
timestamp: December 18, 2023, 09:44:39 pm
content: Quote from: srb1954 on December 18, 2023, 07:51:48 pmZeners have their sweet spot regarding tempco at around 6 V.Seehttps://en.wikipedia.org/wiki/Zener_diode

author: PSR B1257
author_rank: Frequent Contributor
timestamp: December 19, 2023, 07:48:23 am
content: QuoteOff course, my bad

author: rdl
author_rank: Super Contributor
timestamp: December 19, 2023, 03:15:54 pm
content: I tried using a zener as a regulator years ago. I concluded a zener was not the best way to go.

author: Zenith
author_rank: Frequent Contributor
timestamp: December 19, 2023, 05:42:59 pm
content: Quote from: rdl on December 19, 2023, 03:15:54 pmI've seen it done with a few commercial products where they cheaped out completely. One was a mains adapter for a Sinclair calculator. As you say, not the best way to go, and there are better solutions that don't cost that much more.