Can a LED replace a voltage regulator?

Discussion in 'General Electronics Chat' started by Lebe, Oct 19, 2008.

  1. Lebe

    Lebe Thread Starter New Member

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    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?
  2. Bill_Marsden

    Bill_Marsden Moderator Staff Member

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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.
  3. blocco a spirale

    blocco a spirale Active Member

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    As long as the current doesn't exceed the rating of the LED. 20-30mA.
  4. DickCappels

    DickCappels Well-Known Member

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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).
  5. chrissyp

    chrissyp Active Member

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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
  6. Bill_Marsden

    Bill_Marsden Moderator Staff Member

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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.
  7. beenthere

    beenthere Retired Moderator

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    For more current, a rectifier diode will drop a lower voltage but handle much more current.
  8. hgmjr

    hgmjr Moderator Staff Member

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    While you can use the led as a regulator, I would caution you that the regulation will not be very tight.

    hgmjr
  9. SgtWookie

    SgtWookie Expert

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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 Files:

  10. Audioguru

    Audioguru New Member

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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 Files:

  11. steveb

    steveb Senior Member

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    A solution like this is a kluge. You can probably make it work in a pinch, but it will have drawbacks such as 5V output when current is very small, and poor regulation with temperature and current changes. But, maybe you enjoy watching the LED light getting brighter and dimmer based on electrical loading - hey that could be fun.

    For the most demanding applications, you can buy a cheap tiny (sot23 package) 5V to 3.3V low-dropout regulator that will outperform anything else you can make in every possible way. Size, stability, frequency response, noise, output current, reliability etc. will all be superior. I recommend Texas Instruments as a starting point to see what is available. These modern devices are really good and not very expensive.

    If cost is critical, and performance not too demanding, just use a resistor and 3V zener in the classical regulator configuration.

    Steve
  12. Lebe

    Lebe Thread Starter New Member

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    thanks everyone for your inputs, using an LED sounds too risky for my project....I'm just gonna play it safe and go with the voltage regulator:p
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