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Followup to Zener/LED Question

 
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Dennis Johnson



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

PostPosted: Sun Mar 05, 2006 6:34 pm    Post subject: Followup to Zener/LED Question Reply with quote

I've already received a few excellent responses to my question about using zener diodes to regulate power to an LED light. Thanks so much for your very thoughtful answers. It's obvious that my understanding of LEDs is flawed. I have a couple of basic questions to get me back on track and that will allow me to fully understand your replies:

Assume an LED with a Vf of 2.1 volts and 20 mA rating:

1. If I hook the LED to a power supply fixed at 2.1 volts but with an unlimited capacity to supply current, what would happen to the LED?

2. If I hook up the LED to a power supply set to 48 volts or so but with the current limited to 20 mA, what would happen to the LED?

I have a handful of miscellaneous LEDs and I'd try the experiment myself if I had a power supply.

It's a great day when I can learn something new, particularly if it's useful!

Thanks,
Dennis Johnson
Lancair Legacy --- currently revising my instrument panel LED lighting plan


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PostPosted: Sun Mar 05, 2006 6:56 pm    Post subject: Followup to Zener/LED Question Reply with quote

Basically, you want a current limiter in the circuit. That is what the
resistor does. So your 48V/20ma example probably work (depending upon
the current spec of the LED). But if you hook to 2.1V supply, you may
get no output if the Vf is > 2.1V (high tolerance) or high current,
light, and maybe the magic smoke, if the Vf is < 2.1V
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PostPosted: Sun Mar 05, 2006 7:29 pm    Post subject: Followup to Zener/LED Question Reply with quote

Dennis Johnson wrote:

Quote:


I've already received a few excellent responses to my question about using zener diodes to regulate power to an LED light. Thanks so much for your very thoughtful answers. It's obvious that my understanding of LEDs is flawed. I have a couple of basic questions to get me back on track and that will allow me to fully understand your replies:

Assume an LED with a Vf of 2.1 volts and 20 mA rating:

1. If I hook the LED to a power supply fixed at 2.1 volts but with an unlimited capacity to supply current, what would happen to the LED?


Well, it depends on whether the power supply is exactly the 2.1 volts of

the zener Vf. If it is exactly the 2.1 volts then the zener will light
up with the 20 mA brightness. If it is a little higher (2.2V) it will
light up much brighter since it will draw much more current. A little
higher still and - Toast!

If you plot voltage on the X axis and current on the Y axis, a resistor
would be a straight line pointing up and to the right. A zener would be
"L" shaped with a little bit of rounding at the apex of the "L"- very
little current until it reaches the zener voltage and then the current
would rise very fast for just a little more voltage. An LED is similar
except the apex is a little more rounded than the zener. Zeners and
LEDs MUST have something to limit the current or they will overheat and
die when the voltage rises a little above the Vf or zener voltage.

Quote:
2. If I hook up the LED to a power supply set to 48 volts or so but with the current limited to 20 mA, what would happen to the LED?


It would light to whatever brightness that 20 mA creates. If you use a

resistor, the brightness would change if you varied the 48 volts. If
you used an active current limiter such as I described earlier, the
brightness would stay constant. If you plotted the active limiter as
above, you would get a straight horizontal line from about 3V to 38V
(the maximum voltage limit of the LM317). Below 3V or so you would get
essentially no current.

The only drawback to the active current limiter is that it doesn't allow
the LED to change brightness if you wanted to dim it with a normal
dimmer. You have to use one of the digital dimmers (pulse width
controlled) to dim it.

Quote:
I have a handful of miscellaneous LEDs and I'd try the experiment myself if I had a power supply.

It's a great day when I can learn something new, particularly if it's useful!

Thanks,
Dennis Johnson
Lancair Legacy --- currently revising my instrument panel LED lighting plan

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PostPosted: Sun Mar 05, 2006 7:38 pm    Post subject: Followup to Zener/LED Question Reply with quote

Ooops! Actually, in my previous post it would be a backwards "L" shape.

Dennis Johnson wrote:

Quote:


I've already received a few excellent responses to my question about using zener diodes to regulate power to an LED light. Thanks so much for your very thoughtful answers. It's obvious that my understanding of LEDs is flawed. I have a couple of basic questions to get me back on track and that will allow me to fully understand your replies:

Assume an LED with a Vf of 2.1 volts and 20 mA rating:

1. If I hook the LED to a power supply fixed at 2.1 volts but with an unlimited capacity to supply current, what would happen to the LED?

2. If I hook up the LED to a power supply set to 48 volts or so but with the current limited to 20 mA, what would happen to the LED?

I have a handful of miscellaneous LEDs and I'd try the experiment myself if I had a power supply.

It's a great day when I can learn something new, particularly if it's useful!

Thanks,
Dennis Johnson
Lancair Legacy --- currently revising my instrument panel LED lighting plan


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that a significant number of electrons may have been temporarily inconvenienced.
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