How low voltage indicator work

[srilankaaa]

Hello
How low voltage indicator work zener diode and resistance

 

[stenzer]

Hi,

see the picture below. Here the voltage on the cathode will drop faster than on the anode side. The voltage drop at D1 is constant (until undercutting the zener voltage of 6.2 V), thus the voltage across R1 (Cathode) will decreases when V(PLUS) decreases. The LED will start to light up when the required forward voltage across the LED will drop. This means that the voltage at the cathode and consequently the PLUS voltage has to drop by about the forward voltage of the LED to light up, and to indicate a voltage drop. By choosing a specific LED color, you can select a rough indication threshold voltage [1].

[1] https://www.circuitbread.com/ee-faq/the-forward-voltages-of-different-leds

BR

 

[D.A.(Tony)Stewart]

I do not appreciate any useful information in this design on the schematic.

The ramp voltage is negative which is irrelevant and thus demonstrating LED voltage is obfuscating any useful purpose. The 12V applied initially passes thru the 6.2V Zeners then ramps down in 10 ms. When the Zeners stop conducting the resistors start and the LED turns on, as it ramps down.

So the LED starts being reverse biased OFF to forward biased ON with 3 mA until the applied ramp drops below the LED Vf and finishes its slow 10 ms ramp down to 0V.

Maybe someone thought it was useful for something and it is possibly the worst LED test circuit for brightness of Vf forward voltage, I've seen.

Now, What was your question?

oops I read #2 before Reading #1 (my bad)

Now I see the Zeners act as an asymmetric bridge or window comparator for the LED to conduct with low current and loe Vf of 1.5V instead of 2.1V and the zeners drop in vboltage with low current too.,

thus the battery threshold is 11V for a dim LED..

Just pulling your leg, but really not a good LED tester.

Adjust Battery voltage slider and observe all node V and branch currents.

https://tinyurl.com/2xy7nn73

The LED continues to drain battery down to 6V.

 

[D.A.(Tony)Stewart]

Why use 6.2V Zeners when LEDs are cheap and readily avail. Zener-like diodes.

https://tinyurl.com/ysvp8zsn

The Red middle LED goes out when it is full charged. The Zeners were also wasting current. This just adds more glitter.

 

[srilankaaa]

Hello
Led will on with two resistance 1k

 

[srilankaaa]

Hello

Hi,

see the picture below. Here the voltage on the cathode will drop faster than on the anode side. The voltage drop at D1 is constant (until undercutting the zener voltage of 6.2 V), thus the voltage across R1 (Cathode) will decreases when V(PLUS) decreases. The LED will start to light up when the required forward voltage across the LED will drop. This means that the voltage at the cathode and consequently the PLUS voltage has to drop by about the forward voltage of the LED to light up, and to indicate a voltage drop. By choosing a specific LED color, you can select a rough indication threshold voltage [1].

The voltage across cathode will drop faster than anode side can you explain

 

[stenzer]

For the left leg the LED is blocking the current toward the anode. Thus, D1 and R1 are forming a simple voltage divider, splitting the battery voltage. Here, ~6.2V will drop across D1 (defined by the selected zener diode) and the remaining voltage (V1 - 6.2V) across R1 (neglecting small forward current across the LED here).

At the begining of the simulation, the anode voltage is also about 6.2V, which is defined by the zener diode D2. This voltage will start to decrease when a (significant) current path is provided across the LED, which is th case when the cathode voltage is lower than the anode voltage by the LED's forward voltage (voltage drop across R1). Note, here R1 and D3 are in series, seen from the anode, providing the current path to lower the anode voltage.

 

[D.A.(Tony)Stewart]

When current * V matches Vz of Zener, LED has 0V across it. So if LED starts dim at 1.6V and full brightness at 2V you need to choose a Zener = Vz @ rated threshold current such that 2*Vz - 1.6V = your battery at 50% capacity (dim LED) and full brightness will then occur when Vf of RED LED rises ~ 0.4V @ 5mA With ultrabright > 10,000 mcd LED's 30 deg, this is very bright @ 5 mA.

So define your own threshold. Pick a part And learn more about Zener threshold and "knee resistance"..l All diodes have a knee resistance which drops at rated max. current. So current spec is important here. The voltage, Vz changes 10% or more. then you have knee threshold tolerances.

BTW LEDs are diodes too so the same rules apply, except the Vth threshold very tightly controlled for a given wavelength, so you mainly just have to be considerate of the Vf vs If current and knee or bulk resistance where diode square law R drops below the bulk linear resistance then it comes Vth+IR=Vf
for LEDs or Vz +I*Rz=V Note: This is how Nexperia does it (and I prefer this method to ON Semi's) with low Vz at 0.250 mA or less 50 uA. These are stable while Rz has wide process variables , just like LEDs so at max current there may be a wide tolerance for Vf. but not at low currents.

Except for LED's they never specify Vth, I figured this out on my own. They only spec Vf @ I rated and give typical curves.

here try out my simulation and see if you read the scope thresholds with mouse pointer on bottom voltage when LED current starts and stops. then report back what voltage you read. Rev A link http://tinyurl.com/ypvzlc6x

Then read about Dr. Clarence Melvin Zener at Bell Labs, whose name will live forever.

Zener Diode - Definition, Working, Circuit Symbol, V-I Characteristics, Applications, and FAQs

Zener Diodes are heavily doped diodes that are designed to operate in reverse direction. When a reverse voltage is applied to a Zener diode, on reaching the Zener voltage, the pn junction breaks down.

byjus.com

I have a Rule of Thumb for all diodes (Stewart's Law) There is a version for arrays of LED's and Zener's but it is basically Take the rated power dissipation for the package and invert it. That is pretty close to the incremental or bulk resistance after the "knee" Rs= 0.5/Pmax +/-50% tolerance covers most diode designs from 0.1W to 100W. (hockey pucks)

Now re-examine my formula for this design and improve it.
The keeners will slow down the simulation and stop it when the Zener current is 250 uA and verify Vz.

If you do this I predict you are or will become an expert in Analog design.

--- Updated Dec 26, 2023 ---

 

[srilankaaa]

hello
how led is on can you explain how resistance is truning on led on battery is low

 

[D.A.(Tony)Stewart]

hello
how led is on can you explain how resistance is truning on led on battery is low

Can you run my linked simulation and answer this yourself with Ohm's Law?

Look at traces with mouse pointer.
If you don't understand Ohm's Law , how can I explain.

 

[srilankaaa]

Hello
i cannot understand how led is on all time if resistance is there

 

[stenzer]

The resistors are not preventing the illumination (turn on) of the LED. And the LED is only turned on if the battery voltage is low enough. The main purpose of the resistors is to limit the current through the LED to avoid damaging it. Here, the current is about (12V - 1.6V * 2)/2000Ohm = 4.4mA, have a look at the attached simulation result in reply #2, which shows 4.2mA when the LED is turned fully on.

 

[D.A.(Tony)Stewart]

Can you run my linked simulation and answer this yourself with Ohm's Law?

Look at traces with mouse pointer.
If you don't understand Ohm's Law , how can I explain.

Zeners draw all the resistor current until the LED turns on.

This looks better than LTspice http://tinyurl.com/ywb67auh which side goes up more with VBat so that LED turns off and why