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# How can I reduce the output voltage of a zener diode? Answered

Hello,

My main goal here is to get an LED to light up when it "detects" over 30V, regardless of the specific output of the voltage source.

I attached here an image of the circuit that I currently have. The output of the zener diode is about 30V. What I want to do now is reduce this output to 2V in order to light up an LED that has a 1.8-2.2 VDC forward drop and a max current of 20mA.

I was thinking of attaching a voltage divider to this circuit in order to lower the 30V to 2V, but I'm not sure how, especially with the zener diode involved.

Can anyone help me?

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## 12 Replies

iceng (author)2013-01-25

Here is a ckt that II found some time ago.
It looks like you can adjust it.
Make some experiments and report back.

A

frossiove (author)2013-01-25

I'm using multisim to test my circuits, but multisim doesn't seem to have a model for a TL431. Moreover, in this circuit, it looks like R3 is dependent on the battery. I want to make a circuit that indicates when it detects a voltage greater than 30V, regardless of what battery is being used.

steveastrouk (author)2013-01-25

No, read the datasheet on the TL431. Its a rare beasty, a shunt voltage regulator. A.'s circuit is very clever. R3 ISN'T dependent on the battery voltage. R2 and R3 program the Voltage on pin 3 V = (1+r2/r1)*2.5 V Which is INDEPENDENT of the supply.

frossiove (author)2013-01-25

Just so you know, I really appreciate the time you're putting in to help me. But I still have a few questions.

1) Can you explain how you got that formula for the voltage at pin 3? I'm curious as to why R3 isn't part of your equation Vo = (1+R2/R1)*2.5 .

2) If this equation is correct, Vo = (1+4.7/2.2)*2.5 = 7.841V. Isn't that too much voltage for just two red LEDs?

3) Apparently, R3 determines the cutoff voltage, but how is this cutoff voltage calculated? For my purpose, I want the cutoff voltage to be 30V, right?

steveastrouk (author)2013-01-26

1.) Its should be 1+R3/R1.
2.) The output voltage interacts with R2, since all the current flows through it to the TL431 and the diodes. Essentially your 7.8 volts is the open circuit volts of the circuit.

R3 AND R1 determine the operating voltage.

steveastrouk (author)2013-01-26

Sorry, its should have been 1+R3/R2.

R1 will limit the current

steveastrouk (author)2013-01-25

The way to do this is to use a transistor. As it is now, the voltage on node 5 will increase FROM 0 to 30V, and an LED will start dim and get brighter.

I muse about trying to put the LED in SERIES with the zener, then it will turn on at about 31V, but if you want brightness from 31V, and current of, say, 10mA in the diode, by 130V, you would have nearly 4 x more current in the LED.

frossiove (author)2013-01-25

I believe 30V would damage the LED. I want it so that when the voltage source is over 30V, only 30V will be result from the zener diode because then I should be able to decrease this voltage to 2V. If the source is below 30V, I don't want the LED to light up.

For example, if the source is 15V, 15V will result from the zener diode and my thought process is that if it goes through the same process that can decrease 30V to 2V, 15V should be decreased to lower than 2V, therefore not lighting up the LED.

I did some experiments, but it seems that using any voltage divider to decrease 30V to 2V will make the overall output depend on the specific output of the source.

For example, I attached here a picture of what worked for specifically a 130V source.

steveastrouk (author)2013-01-25

No, put the LED in series, with the zener in your model and see what happens. Provided R1 is correct, the LED will only turn on when there is more than 30V available, and the maximum current will be limited by the seris resistor. You CAN'T HAVE more than 1.2V across a red LED.

frossiove (author)2013-01-25

Okay. I did what you said and the LED turns on only when the voltage source is at least 37V, not 30V. I attached an updated picture of the circuit. What do I need to change to fix this?

Here are the specs of the zener diode that I'm using:
5 W Zener Regulator 30 V, SUR40

And the LED that I'm using has an On Current of 5 mA.

By the way, should I even be using a zener diode for this task? Is there another device that I should be using instead?

steveastrouk (author)2013-01-25

Its an interesting piece of circuit theory: see how simple it CAN be.

The answer would be to use a 23V Zener.....

What kind of theoretical LED is this ? The colour affects the forward voltage.

See what happens to the forward current as you increase the voltage.

The simplest way to be better than this has been kicked to death in a recent question in the forums. The original circuit referred to is here.
https://www.instructables.com/id/9v-battery-status-indicator-circuit/

Orngrimm and I have kicked it to death. There are various alternatives, like using a schmitt trigger that can do the same job.

frossiove (author)2013-01-25

Thanks! But how did you calculate the zener voltage to be 23V instead of 25V? Since the current through the LED has to be at least 5mA for the LED to turn on, solving the equation (30-Vz)/1000=0.005 got me 25V for the zener voltage. However, 23V seems to be the correct solution.

Moreover, do you still recommend that I try to implement a transistor like you mentioned in your first post?