Resistorless Zener regulator circuit?

[tylernt]

Consider the simple Zener regulator circuit from Wikipedia:

Zener diode - Wikipedia, the free encyclopedia

I understand the resistor is required to keep excess current from destroying the Zener. However, assume this circuit is powered by a 9v (PP3) battery, and the output load is two 20mA/3.6v LEDs in series which will never be disconnected (so no open circuit protection is required). We're using a Zener of approximately 7v to avoid damage to the LEDs.

Can the resistor be omitted in this scenario, to improve the length of time the LEDs will light as the battery is depleted? I don't think there will be any efficiency gain (as the Zener will still be dissipating as heat whatever current the resistor would have dissipated), but: without the resistor to drop the voltage to the LEDs, I'm thinking they will light longer after the battery drops below the Zener voltage and the Zener falls out of the circuit.

Thoughts?

 

[crutschow]

Using a zener is not the correct way to control power the LEDs. You just need a resistor in series with the LEDs to limit the current to the desired value. You never connect an LED directly to a battery or other voltage source.

 

[tylernt]

Using a zener is not the correct way to control power the LEDs. You just need a resistor in series with the LEDs to limit the current to the desired value. You never connect an LED directly to a battery or other voltage source.

I believe you can directly connect LEDs to a power source without a resistor as long as the cumulative Vf of the LEDs is comfortably more than the supply voltage. For example,

GRYNX » Versatile compact 9v LED lamp

?

 

[JimB]

I believe you can directly connect LEDs to a power source without a resistor as long as the cumulative Vf of the LEDs is comfortably more than the supply voltage.

If the Vf of the LEDs is greater than the supply voltage, it will not work.

For example,
GRYNX » Versatile compact 9v LED lamp

That is just plain bad design, the LED current is limited by the internal resistance of the battery.
A really bad idea.

JimB

 

[tylernt]

If the Vf of the LEDs is greater than the supply voltage, it will not work.

I guess the guy with 3 LEDs on a 9v battery must be using LEDs each with a Vf of less than 3.2v and not the "almost 4v" he claims? Otherwise they would not light at all.

That is just plain bad design, the LED current is limited by the internal resistance of the battery.
A really bad idea.

Hm. The Photon Freedom keychain lights use the internal resistance of 2xCR2016 cells with no other regulation, and I've never heard of anyone burning out an emitter. (I mention Photon as they're a reputable company with a warranty on their lights.) Seems to work for them, though of course we're not comparing apples to oranges.

So back to the original question... using, say, an incandescent bulb instead of LEDs on the output of a resistorless Zener regulator... what will happen?

 

[crutschow]

So back to the original question... using, say, an incandescent bulb instead of LEDs on the output of a resistor less Zener regulator... what will happen?

Incandescent bulbs will work that way since they are designed to work with a voltage supply. LEDs are designed for use with a current supply (or current limited supply). Comparing incandescent bulbs with LEDs is indeed comparing apples with oranges.

 

[BrownOut]

To answer your question, a series resister must always be used with a zener. After the "breakover" voltage, zener current increases without (practical) bound. Never never connect a zener without a resistor to limit current.

 

[3v0]

Thanks brownout that made sense to me. Once the Zener starts conducting it needs current limitation because it is not a resistive device.

 

[crutschow]

Thanks brownout that made sense to me. Once the Zener starts conducting it needs current limitation because it is not a resistive device.

And so are LEDs.

 

[tylernt]

If the Vf of the LEDs is greater than the supply voltage, it will not work.

BTW, I have a "dead" lithium coin cell here that reads 2.74v open circuit. I also have a Cree C503C-WAN with a Vf spec of 2.8v. The battery will light the LED, so either my LED is out of spec, or LEDs will light below their Vf.

 

[nickelflippr]

At least you have a spec grade led and a corresponding data sheet, which is more than most. Cree has a chart for Vf vs. If, it varies with current and within certain recommended Vf bounds.

I have some Cree P4's The green and blues have operated at a at 3.0Vf (about 9-10ma) no problem, plenty bright. Hit them accidentally with 5.5V and got to 3.9Vf (about 50ma), oops! So take the Vf as an average if you will, at the stated forward current, which for the P4's is 3.4Vf at 20ma for the green and blue ones.

 

[3v0]

Maybe suggested rather then average.

 

[nickelflippr]

Cree uses the word "typical", maybe that's the word? Recommended If of 10-30ma for long term operation. Edited previous post, as typical Vf of 3.4V was at 20ma not 35ma (max recommended If rating).

 

[audioguru]

A certain LED part number does not have a single voltage. The LEDs are not exactly the same so the voltage is a range of voltages. The datasheet has a graph that shows the "typical" voltage being fairly high at high current and lower at low current.

You read the datasheet wrongly. The typical forward voltage is 3.2V and the range is from 2.8V to 4.0V at 20mA.

 

[Martel]

What's the idea of using a Zener without a current limiting device ?

The only place i see zeners w/o resistors is in an OVP that shall not crowbar the supply. And say welcome to power zeners and heat sinks !

 

[tylernt]

What's the idea of using a Zener without a current limiting device ?!

I was hoping for a small parts-count regulated LED driver (don't like a single series resistor because the LED dims as the battery is depleted). Instead I've chosen an LM317 with one resistor in current-limit mode (LM317 Current Calculator - Electric Circuit).

Speaking of the LM317, what happens when Vin drops below the LED Vf plus the LM317 voltage drop (in the diagram linked to above)? Does it allow unlimited current to flow, or zero?