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  #1  
Old 10-19-2008, 06:30 AM
Lebe Lebe is offline
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Default Can a LED replace a voltage regulator?

I need to drop from 5 to 3 volts in my circuit. I originally thought of just using 5/3.3 voltage regulator, but the thought just came into my head: couldn't I just use an LED with a voltage drop of 2V instead? It should work the same right or am I missing something?
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Old 10-19-2008, 06:37 AM
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Seems reasonable to me, though they aren't predictable, you'd have to have some sort of adjusting or compensating mechanism. I've never really seen a graph of voltage vs. current, but I'm under the impression it is pretty constant once their turned on.
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Old 10-19-2008, 08:36 AM
blocco a spirale blocco a spirale is offline
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As long as the current doesn't exceed the rating of the LED. 20-30mA.
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Old 10-19-2008, 12:44 PM
DickCappels DickCappels is offline
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Just be careful peak current. If your power supply has a fast rise time and there is a large decoupling capacitor on the other end of the LED, then the peak current through the LED could be pretty high.

And assure that there is a minimum load so your load can ever drop to "near zero", causing the voltage could rise (if, for example, your load is a microcontroller that goes to sleep).
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Old 10-19-2008, 01:10 PM
chrissyp chrissyp is offline
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The led will not act as a voltage regulator, it will only reduce the output voltage by a fixed amount. If the input voltage rises so will the output voltage , a true voltage regulator will maintain the output even with fluctuations in input voltage
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Old 10-19-2008, 01:56 PM
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That fixed amount is exactly what a zener does, only reverse biased. You can even use a regular voltage drop in much the same way from a regular diode. Modern LEDs drop as much as 3.5 volts, which puts them comfortably in the bottom of a zener diodes range, and from what I understand a zener that low isn't too stable, so an LED might be OK.
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Old 10-19-2008, 02:56 PM
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For more current, a rectifier diode will drop a lower voltage but handle much more current.
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Old 10-19-2008, 03:00 PM
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Quote:
Originally Posted by Lebe View Post
I need to drop from 5 to 3 volts in my circuit. I originally thought of just using 5/3.3 voltage regulator, but the thought just came into my head: couldn't I just use an LED with a voltage drop of 2V instead? It should work the same right or am I missing something?
While you can use the led as a regulator, I would caution you that the regulation will not be very tight.

hgmjr
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Old 10-19-2008, 03:11 PM
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Unless your load is very constant I suggest against it, because the Vf of the LED will change quite a bit depending upon the current flowing through it.

It will be fairly good if you have a load that will always be between 10mA to 20mA, but you'll have several mV variance even in that range. If your load is reactive (ie: capacitive or inductive) the transient loads would likely cause the LED to fail.

Standard silicon rectifiers such as a 1N4001-1N4007 would be much more robust, but even those will have a significant variation in Vf with load current.

Months back, I graphed a few diodes for Vf over a range of currents. I've attached a graph for a 1N4002 diode. What this graph doesn't show is how the Vf rapidly increases as the diode reaches it's maximum rating - because the test only went to 100mA. A similar test of a 1N4148 diode showed a marked increase in Vf above 50mA.

You could perform your own test using an LED, some resistors, a power supply and a couple of meters. If you are careful and record your results accurately, you may learn quite a bit on how LEDs perform in a circuit.

If you exceed the maximum current rating of the LED, it will rapidly burn out. But, LEDs are cheap nowadays
Attached Images
File Type: png 1N4002-bDiodeVfvsCurrent.PNG (24.2 KB, 13 views)
File Type: png 1N4148DiodeVfvsCurrent.PNG (27.1 KB, 12 views)
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Old 10-19-2008, 06:57 PM
Audioguru Audioguru is offline
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Simply look at the poor voltage regulation on the datasheet of an LED.
My MV8191 LEDs have a max continuous current rating of 40mA.
Their voltage changes a lot when their current changes.

The actual voltage could be anything from maybe 1.3V to 2.4V at 20ma.
Attached Images
File Type: gif MV8191.gif (10.1 KB, 5 views)
File Type: png MV8191 red LED.PNG (3.5 KB, 4 views)
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