Author Topic: Using a zener with variable input and led - brain freeze  (Read 1388 times)

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Offline aries1470Topic starter

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Using a zener with variable input and led - brain freeze
« on: July 26, 2019, 03:53:01 am »
Hi everyone,

I'm having a brain freeze, and just wanted to confirm for the following.
Input voltage: 6 - 60V DC input

Using 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.
 

Offline MrAl

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Re: Using a zener with variable input and led - brain freeze
« Reply #1 on: July 26, 2019, 05:55:24 am »
You have to think about the power too.  Over 2w in the resistor.
 
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Offline aries1470Topic starter

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Re: Using a zener with variable input and led - brain freeze
« Reply #2 on: July 26, 2019, 06:44:53 am »
MrAI

Ouch, so the values are ok, but instead of a 1/8 or 1/4 W it will need to be a 2W resister?  |O :palm:
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.
« Last Edit: July 26, 2019, 07:04:56 am by aries1470 »
 

Offline MrAl

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Re: Using a zener with variable input and led - brain freeze
« Reply #3 on: July 26, 2019, 06:49:19 am »
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.
 

Offline mikerj

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Re: Using a zener with variable input and led - brain freeze
« Reply #4 on: July 26, 2019, 07:08:55 am »
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.
 

Offline aries1470Topic starter

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Re: Using a zener with variable input and led - brain freeze
« Reply #5 on: July 26, 2019, 10:30:31 am »
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 :)
 

Offline digsys

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Re: Using a zener with variable input and led - brain freeze
« Reply #6 on: July 26, 2019, 11:21:03 am »
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+)
Hello <tap> <tap> .. is this thing on?
 

Offline mikerj

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Re: Using a zener with variable input and led - brain freeze
« Reply #7 on: July 26, 2019, 11:41:00 am »
The simplest (one component) solution would be a constant current diode, though probably won't be the cheapest option.
 

Online PA0PBZ

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Re: Using a zener with variable input and led - brain freeze
« Reply #8 on: July 26, 2019, 11:52:41 am »
795081-0
Keyboard error: Press F1 to continue.
 
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Offline kosine

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Re: Using a zener with variable input and led - brain freeze
« Reply #9 on: July 26, 2019, 12:15:29 pm »
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.
 
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Offline exe

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Re: Using a zener with variable input and led - brain freeze
« Reply #10 on: July 26, 2019, 06:13:55 pm »
I have never used Leds that draw more than 1mA for full brightness. They cost bugger all more.

Interesting 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.
 
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Offline aries1470Topic starter

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Re: Using a zener with variable input and led - brain freeze
« Reply #11 on: July 29, 2019, 01:05:44 am »
(Attachment Link)
Thank 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 mA  ;D
For 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 this ;)
ON Semi BC546BTA - THT $0.116AUD - 100pcs - $11.60AUD B80V / E65V
ON Semi BC546CTA - THT $0.071AUD - 200pcs - $14.20AUD B80V /E65V
ON Semi KSP06TA - THT $0.139 AUD - 100pcs - $13.90AUD B80V / E80V

ON Semi MMBTA06LT1G - SMT $0.108AUD 50pcs - $5.40 B80V / E80V
FMMTA42TA - SMT $0.067AUD 50pcs - $3.35 B330V / E300V  <- Will be going for this  ;)
Infineon SMBTA06E6327HTSA1 - SMT $0.052 100pcs - $5.20 B80V / E80V

Edit:
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.
« Last Edit: July 31, 2019, 12:36:01 am by aries1470 »
 

Offline aries1470Topic starter

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Re: Using a zener with variable input and led - brain freeze
« Reply #12 on: July 30, 2019, 01:24:03 am »
Hi guys,

Just want to confirm if all will be okay using the following:
2x 1N4148
1x  KSC1009
1x 10K Resistor
1x 100 Ohm Resistor
1x LED

will be assembling on a breadboard.
Does anyone see any issues assembling as per image from @PA0PBZ?

Thank you.
 

Online Zero999

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Re: Using a zener with variable input and led - brain freeze
« Reply #13 on: July 30, 2019, 11:26:40 am »
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.
 
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