# Implementing LED into low voltage zener cutoff circuit

#### bigjoncoop

Joined Feb 1, 2019
184
Hey guys,

Well I'm just about finished up with my zener diode low voltage disconnect. Life in super crazy lately haven't had time to mess with it.

Instead of using a TL431, for now I just went with a single zener diode.

But the problem I am having is incorporating a led into the circuit. Between the battery positive and before the zener diode there is only just about a half a volt. Which is obviously not enough to light the LED.

So I remember seeing circuit to make a simple boost converter using a inductor a diode and the capacitor but I think that is way more than I need.

I think I should just add a simple 2N222 transistor in between the LED cathode and ground which will be controlled by the zener diode. And obviously a current limiting resistor for the LED...

Does the circuit above seen correct in order to add an LED to be lit as long as the zener is conducting and draining the battery and then LED should turn off when the voltage hits the set point which is right around 15 volts

#### crutschow

Joined Mar 14, 2008
32,934
To see the Zener voltage the cathode must be positive with respect to the anode.
Then the set point is the Zener voltage plus the transistor base-emitter voltage or about 15.7V.

You also need a resistor in series with the LED (since it is a diode) to limit its current.

Note that circuit has a much looser operating voltage tolerance and stability than a TL431 circuit.

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#### bigjoncoop

Joined Feb 1, 2019
184
To see the Zener voltage the cathode must be positive with respect to the anode.
Then the set point is the Zener voltage plus the transistor base-emitter voltage or about 15.7V.

You also need a resistor in series with the LED (since it is a diode) to limit its current.

Note that circuit has a much looser operating voltage tolerance and stability than a TL431 circuit.
yeah, i just threw those schematics together quickly. the zener is backwards and i forgot to add the series resistor for the led.

#### bigjoncoop

Joined Feb 1, 2019
184
@crutschow

u also mentioned -
Then the set point is the Zener voltage plus the transistor base-emitter voltage or about 15.7V...

are you saying that the led will turn off at 15.7v while the rest of the circuit will be operational until 15v? currently the circuit stops conducting (zener opens) at exactly 15v

#### Sensacell

Joined Jun 19, 2012
3,257
Zeners are not sharp switching devices- the LED will BEGIN to glow at some point and get brighter, not a definitive indication of anything.

That's why people have suggested the TL431.

#### crutschow

Joined Mar 14, 2008
32,934
are you saying that the led will turn off at 15.7v while the rest of the circuit will be operational until 15v?
No.
I'm saying, with the addition of the transistor the whole circuit will be operational until 15.7V.
The base-emitter voltage of the transistor adds to the Zener voltage.
currently the circuit stops conducting (zener opens) at exactly 15v
The Zener breakdown voltage has a tolerance, typically ±5%.

#### dl324

Joined Mar 30, 2015
15,849
yeah, i just threw those schematics together quickly. the zener is backwards and i forgot to add the series resistor for the led.
You also drew it sideways and upside down.

This is how it would normally be drawn:

#### LowQCab

Joined Nov 6, 2012
3,458

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#### DickCappels

Joined Aug 21, 2008
10,068
I would go back to the TL431 for now.

#### metermannd

Joined Oct 25, 2020
341
One other thing. Instead of three 82.6 ohm resistors to get 28 ohms, I would just go with a pair of 56 ohm resistors in parallel. That might be a bit cheaper.

#### Larrylab

Joined Feb 18, 2019
1
I'm very sorry, but you have a fatal error. In almost all cases, you must have a resistor connecting the base to emitter. Otherwise, any leakage current, such as from the internal collector-base diode or, in your case, current through the Zener as the voltage across it starts to approach the "knee" (15V), will flow into the base. This small current will be multiplied by beta and produce collector current. So, even if the voltage across the battery is well below the Zener voltage, collector current will flow thru the LED and drain the battery. This will be particularly serious on a hot day. Adding this resistor (maybe 1K), will slightly increase the turn- on voltage but will definitely stabilize the point at which the LED turns on - although it will still turn on gradually and a little unpredictably. Yes, a comparator (with a little hysteresis, of course, would help.