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Zener Diodes and LEDs

 
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Dennis Johnson



Joined: 10 Jan 2006
Posts: 89
Location: N. Calif.

PostPosted: Sun Mar 05, 2006 3:52 pm    Post subject: Zener Diodes and LEDs Reply with quote

I'm making LED instrument floodlights and LED lights for an annunciator panel. I expect the system voltage to vary between about 13.8 volts when the engine and alternator are running, down to about 12.4 volts running just off a lightly loaded battery.

When connecting the LEDs in a series string of three, the resistance required to get the voltage to the recommended level for the LEDs at the 13.8 system voltage is different than when the system voltage is 12.4. Although it doesn't make a huge difference in the amount of light that comes out of the LEDs, it is noticeable.

I'm wondering (which often gets me into trouble) if I couldn't use zener diodes instead of resistors to solve the problem of the light output varying with system voltage. For example, if I have an array of three LEDs, each with a Vf of 1.7 volts, could I use a 5.1 volt zener diode instead of resistors?

I don't have any experience with zener diodes, but the catalogs make it sound like they produce a stable voltage output when operated within their amperage limitations, sort of like a cheap power supply. Digikey sells them for only a little more than resistors, so cost isn't an issue. Admittedly, the varying light output isn't a huge problem, but if it's just as easy and cheap to use zeners, why use resistors?

Thanks,
Dennis Johnson
Lancair Legacy #257, wiring in (slow) progress


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brian



Joined: 02 Jan 2006
Posts: 643
Location: Sacramento, California, USA

PostPosted: Sun Mar 05, 2006 4:30 pm    Post subject: Zener Diodes and LEDs Reply with quote

Dennis Johnson wrote:

Quote:
I'm wondering (which often gets me into trouble) if I couldn't use zener diodes instead of resistors to solve the problem of the light output varying with system voltage. For example, if I have an array of three LEDs, each with a Vf of 1.7 volts, could I use a 5.1 volt zener diode instead of resistors?

Actually, that would make the problem worse. You would get a huge change
in light output with only a tiny change in voltage. Light output from an
LED is a function of the current through it and the current through an
LED is not linear with applied voltage. The resistor acts as a ballast
to make the current change more linear with voltage so that a 0.3V
change doesn't cause the LED to go from no output to full output.

What you really want to make is a constant current source. This would
make the light output independent of bus voltage.

Another way to stabilize light output is to regulate your bus voltage
down to something like 9V, using that to power your LEDs.

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Dan.Beadle(at)hq.inclines
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PostPosted: Sun Mar 05, 2006 5:19 pm    Post subject: Zener Diodes and LEDs Reply with quote

You can use a resistor and Zener diode to make a constant voltage source
for the LEDs as you say. This is not terribly efficient, but easy to
assemble.

You don't say what the current is that you need. Let's assume that it
is 100mA (.1A). If the system voltage is at 13.8V and you want to use
just fixed resistors, the voltage drop through the resistor would be
13.8-5.1 (target) or 8.7V. The R required would be Vd/I = R or 8.7/.1 R = 87 ohms...

So far, so good. But if the voltage in drops a bit, the same current
still gets dropped 8.7V by the R... causing the LEDs to dim a little.

You can get a constant 5.1V at the LEDs with a 5.1V Zener. Pick the
current you want and the lowest voltage you want. Let's stay with the
.1A and set the lower drop out at 10.1V for a Vd of 5V. To get a
current of .1A, you would need R to be 50 ohms. But at 13.8V, the drop
would not be enough. Zener to the rescue:

| -> LED1 -> LED2 -LED3 |
Hook up as Vin (14v) -> R ->| | ->
gnd
| -> Zener(Band-Cathode)---Zener Anode |

When the voltage goes above 10.1V, the Zener will conduct enough to
cause additional voltage drop through the R, holding the voltage at 5.1V
to the LEDs...

Finally, you have to look at the wattage for both the R and the Zener.

Continuing the example, the max drop across the R would be something
like 14.5-5.1 or 9.4V. Watts = V*V/R = 9.4v * 9.4 / 50 , so 1 Watt
would work in this example

For the Zener, the voltage drop is 5.1V, the current is the current
through the R (V/R or 9.4/50 = 188mA), less any current diverted to the
LEDs. Assuming they draw .1A, the current through the Zener would be
188-100, or 88mA. So the wattage would be V*I or 9.4*0.088 = .88W. So
use a 1W or larger Zener.

Hope this helps.

Dan

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retasker(at)optonline.net
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PostPosted: Sun Mar 05, 2006 5:27 pm    Post subject: Zener Diodes and LEDs Reply with quote

Short answer.. No. Longer answer below...

Resistors limit the current through the LEDs to whatever you want (20 mA
maybe). More source voltage just means a little more current and a
little brighter LED. The extra voltage is dropped across the resistor,
not the LEDs. LEDs actually have a voltage current characteristic
similar to zeners which limit the voltage across themselves.

If you string the LEDs and a zener together, they will not emit any
light until the voltage reaches whatever minimum is required across the
LEDs and the zener - 10.2 volts in your case - and then burn out as the
voltage tries to go higher..

If what you are suggesting is to place a 5.1V zener in parallel with the
LEDs and a resistor in series with the zener and LEDs, that will sorta
work as long as you don't mind wasting some extra power. Since LEDs
have slightly different voltage drops and since this changes somewhat
with temperature and since the same happens with a zener, using a simple
circuit like you are suggesting won't really work. You would have to
use a zener nd two resistors to make something that would work and you
would be throwing away a significant amount of power, since what you
would be doing is constructing a simple shunt regulator. Basically you
would use the zener and a resistor to create a shunt regulated voltage
source and then string the LEDs and an additional resistor across the zener.

A better way to accomplish what you want to do would be to use a three
terminal regulator such as the LM317 and hook it up as a current sink in
series with the LEDs. This requires nothing more than a single resistor
and the LM317 and what happens is that the regulator operates to keep
the same current through the LEDs regardless what the external power
does - keeping the LEDs the same brightness as the voltage varies from
12.4 (or less) to 13.8. You can hook the power source to the regulator
and then the regulator to the LEDs and then to ground or you can hook
the power source to the LEDs and then to the regulator and then to
ground - make no difference as long as they are connected in series.

Download the datasheet from: http://www.national.com/pf/LM/LM317L.html
(about 1/4 down the page) and then look at the bottom of page 17 of the
datasheet for a current regulator. You do not need the pot, just a
fixed resistor works fine calculated per the accompanying formula (Vref
is 1.25V). So to get 20 mA you would use I=1.25/R, or R = 1.25/.02 or
62.5 ohms (or the closest standard resistor value).

The part is available from Digikey or Mouser or any number of other
electronics supply houses.

Or you can just use a resistor and live with the slight change in
brightness.

Any questions, ask Smile .

Dick Tasker

Dennis Johnson wrote:

Quote:


I'm making LED instrument floodlights and LED lights for an annunciator panel. I expect the system voltage to vary between about 13.8 volts when the engine and alternator are running, down to about 12.4 volts running just off a lightly loaded battery.

When connecting the LEDs in a series string of three, the resistance required to get the voltage to the recommended level for the LEDs at the 13.8 system voltage is different than when the system voltage is 12.4. Although it doesn't make a huge difference in the amount of light that comes out of the LEDs, it is noticeable.

I'm wondering (which often gets me into trouble) if I couldn't use zener diodes instead of resistors to solve the problem of the light output varying with system voltage. For example, if I have an array of three LEDs, each with a Vf of 1.7 volts, could I use a 5.1 volt zener diode instead of resistors?

I don't have any experience with zener diodes, but the catalogs make it sound like they produce a stable voltage output when operated within their amperage limitations, sort of like a cheap power supply. Digikey sells them for only a little more than resistors, so cost isn't an issue. Admittedly, the varying light output isn't a huge problem, but if it's just as easy and cheap to use zeners, why use resistors?

Thanks,
Dennis Johnson
Lancair Legacy #257, wiring in (slow) progress








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PostPosted: Sun Mar 05, 2006 5:53 pm    Post subject: Zener Diodes and LEDs Reply with quote

Better solution than mine. As you point out, the LEDs need constant
current, not constant voltage. Selecting a the zener higher voltage,
with current limiting behind it to the LEDs would work fine. But, with
less efficiency than a current regulator configuration.

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mthomson(at)showmeproduct
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PostPosted: Sun Mar 05, 2006 8:42 pm    Post subject: Zener Diodes and LEDs Reply with quote

Looks like this is what you need. See
http://www.periheliondesign.com/Vregflyer.htm

Malcolm.

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tshank(at)core.com
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PostPosted: Mon Mar 06, 2006 7:06 pm    Post subject: Zener Diodes and LEDs Reply with quote

Yes you can use zeners but you need resistors also. What you want to do
is to provide a stable voltage to drive the LED's so that the intensity
does not change with the battery voltage. Using the example of three
LED's each with a 1.7 volt drop connected in series with a desired
current of 20 mA. With the three LED's connected in series and the
cathode of the end one connected to ground connect the anode of the
other end of the string to a 45 ohm resistor. Connect that resistor to a
320 ohm resistor, connect that resistor to battery voltage. Connect a 6
volt zener diode cathode to the junction of the two resistors. Connect
the other end of the zener diode to ground. By the way I didn't check to
see if these values are exactly available this is just an example. When
operated at the minimum voltage of 12.4 volts at 20 mA the voltage drop
across the 320 ohm resistor will be 6.4 volts leaving 6 volts across the
zener diode which will not conduct. Now when operated the total voltage
drop across the three LED's will be 5.1 volt leaving 0.9.volts across
the 45 ohm resistor thus the current will be 20 mA. When the battery
voltage rises to 13.8 volts the zener will conduct current causing an
increased voltage drop across the 320 ohm resistor to maintain the
voltage across it at 6 volts and the LED's will have a stable current
source.

Tim Shankland
Malcolm Thomson wrote:

[quote]

Looks like this is what you need. See
http://www.periheliondesign.com/Vregflyer.htm

Malcolm.

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