Electronics > Beginners
Using a zener with variable input and led - brain freeze
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aries1470:
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.
MrAl:
You have to think about the power too.  Over 2w in the resistor.
aries1470:
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.
MrAl:
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.
mikerj:
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.
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