Using a zener with variable input and led - brain freeze - Page 1
Forum section: electronics / beginners

author: aries1470
author_rank: Regular Contributor
timestamp: July 26, 2019, 03:53:01 am
content: Hi everyone,I'm having a brain freeze, and just wanted to confirm for the following.Input voltage: 6 - 60V DC inputUsing a 5V1 zener, a 120Ohm resister and a Red led 2v1, just to show that power is coming in to the circuit.How will I lay it out?Am I missing something?Thanks in advance.

author: MrAl
author_rank: Super Contributor
timestamp: July 26, 2019, 05:55:24 am
content: You have to think about the power too. Over 2w in the resistor.

author: aries1470
author_rank: Regular Contributor
timestamp: July 26, 2019, 06:44:53 am
content: MrAIOuch, so the values are ok, but instead of a 1/8 or 1/4 W it will need to be a 2W resister?Thank you for the feedback.So it will need to be a 2W resister, so how many watts for the Zener then?, since I only need to drive the LED and nothing else from that, as I jsut need an indicater that there is power coming in.Will the 1N4733 1W be sufficient?Edit:This is what I am trying to do:The value for R2 would be 120Ohm, but what about R1?Also how many watts for each resister?Thanks.

author: MrAl
author_rank: Super Contributor
timestamp: July 26, 2019, 06:49:19 am
content: Hi,Actually if the power is 2w then you should go with a 4w, or better yet 5 watt resistor.You should be able to get away with a 1/2 watt zener if it is only 6 volts.4 watts is a lot of power though, the power resistor will probably get hot. Maybe go to a 10 watt unit even for lower temperature.

author: mikerj
author_rank: Super Contributor
timestamp: July 26, 2019, 07:08:55 am
content: If this is just a simple power on indicator then you would be far better off using a simple constant current source/sink design than a fixed resistor.It's also not clear to me how you intend to combine the zener and the LED with a single resistor. Presumably you want the zener to prevent the LED current changing over the supply range, in which case you need a second resistor to supply the zener, and then the led + resistor across the zener.

author: aries1470
author_rank: Regular Contributor
timestamp: July 26, 2019, 10:30:31 am
content: Yes, it is just a simple power indicator, but needs to work from 6V - 60V.It be going to a regulator, but the regulator has its own led power good indicator, but it also needs a simple indicator to show that there is power coming in.The least amount of components, the better.That's why I was looking at a Zener diode solution, as the zener will only power an led and nothing else.Open to more ideas

author: digsys
author_rank: Unknown
timestamp: July 26, 2019, 11:21:03 am
content: Just use a very high efficiency Led and run it at 1/10th - 1/20th current. I have never used Leds that draw more than 1mA for full brightness. They cost bugger all more.You will still need 2 resistors + zener (protect Led reverse voltage - unless you find one that can do 60V+)

author: mikerj
author_rank: Super Contributor
timestamp: July 26, 2019, 11:41:00 am
content: The simplest (one component) solution would be a constant current diode, though probably won't be the cheapest option.

author: kosine
author_rank: Regular Contributor
timestamp: July 26, 2019, 12:15:29 pm
content: Yes, that'll work. I was going to suggest the same transistor circuit, albeit with a zener on the base.However, you need a high voltage transistor such as a BC546, which is listed as max VCE 65V. The more common BC547 is only rated at 45V.If you're desperate to get something up and running, a lot of compact fluorescent light bulbs use high voltage transistors such as the 13007 series. If you've got a dead bulb anywhere they're a convenient source of high voltage components.

author: exe
author_rank: Unknown
timestamp: July 26, 2019, 06:13:55 pm
content: Quote from: digsys on July 26, 2019, 11:21:03 amInteresting idea. I recently bought green leds, they were already bright for me at 0.15mA. Just for fun I tried to see what's the minimum current that would still make it visible. It was 5-8uA at daylight.I wonder which color requires less current. It's not just about brightness, but also eye sensitivity is different to different colors.PS I also have red leds that are barely bright at 20mA.. These are new, I think it was this model:https://www.tme.eu/nl/en/details/l-424hdt/tht-leds-3mm/kingbright-electronic/I wonder how this is possible and what's the application for such low brighness? The efficiency is super-low, I only bought them because I didn't know how much brightness I needed.

author: aries1470
author_rank: Regular Contributor
timestamp: July 29, 2019, 01:05:44 am
content: Quote from: PA0PBZ on July 26, 2019, 11:52:41 amThank you for that schematic. So, what diodes are you using? would those be two "normal" diodes? since I didn't see them as zeners or schotcky.If so, would signal diodes also do, i.e. 1N4148? Since I am going to use very minimal mAFor the NPN transistor, I have available for easy purchase the following:(Yes, I will be doing two versions, one THT and one SMT),It will not dramatically increase the cost, and would help "debug" where a potential problem may be when the other people do not have a basic understanding of electronics - and don;t need to either. So this will help a lot :-)ON Semi KSC1009YTA- THT $0.074AUD - 100pcs - $7.40AUD B160V / E140V <- Will be going for thisON Semi BC546BTA- THT $0.116AUD - 100pcs - $11.60AUD B80V / E65VON Semi BC546CTA- THT $0.071AUD - 200pcs - $14.20AUD B80V /E65VON Semi KSP06TA- THT $0.139 AUD - 100pcs - $13.90AUD B80V / E80VON Semi MMBTA06LT1G- SMT $0.108AUD 50pcs - $5.40 B80V / E80VFMMTA42TA- SMT $0.067AUD 50pcs - $3.35 B330V / E300V <- Will be going for thisInfineon SMBTA06E6327HTSA1- SMT $0.052 100pcs - $5.20 B80V / E80VEdit:Added more SMT options.Edit 2:Link was correct for FMMTA42TA, but had MMBTA06LT1G in its name. Oopps, sorry.Thank you to everyone that has replied, as I was not aware of this option.

author: aries1470
author_rank: Regular Contributor
timestamp: July 30, 2019, 01:24:03 am
content: Hi guys,Just want to confirm if all will be okay using the following:2x 1N41481x KSC10091x 10K Resistor1x 100 Ohm Resistor1x LEDwill be assembling on a breadboard.Does anyone see any issues assembling as per image from @PA0PBZ?Thank you.

author: Zero999
author_rank: Super Contributor
timestamp: July 30, 2019, 11:26:40 am
content: It looks good to me. The current regulation in real life, probably won't be as good, as the simulation, but it'll certainly be much better than the zener circuit.