All About Circuits Forum Can a LED replace a voltage regulator?
 Register Blogs FAQ Members List Today's Posts Search Today's Posts Mark Forums Read

 General Electronics Chat Discussion forum for general chat about anything electronics related, including asking questions about material in the All About Circuits E-book, Worksheets, and Videos.

#1
10-19-2008, 06:30 AM
 Lebe New Member Join Date: Sep 2008 Posts: 4
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?
#2
10-19-2008, 06:37 AM
 Bill_Marsden Super Moderator Join Date: Mar 2008 Location: Dallas, TX (GMT-5 w/ DST) Posts: 19,045 Blog Entries: 5

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.
__________________
..
"Good enough is enemy of the best." An old engineering saying, Author unknown.

General info:
If you have a question, please start a thread/topic. I do not provide gratis assistance via PM nor E-mail, as that would violate the intent of this Board, which is sharing knowledge ... and deprives you of other knowledgeable input. Thanks for the verbage Wookie.
#3
10-19-2008, 08:36 AM
 blocco a spirale Senior Member Join Date: Jun 2008 Posts: 229

As long as the current doesn't exceed the rating of the LED. 20-30mA.
#4
10-19-2008, 12:44 PM
 DickCappels Senior Member Join Date: Aug 2008 Location: Thailand Posts: 863

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
10-19-2008, 01:10 PM
 chrissyp Member Join Date: Aug 2008 Location: guildford,surrey, uk Posts: 82

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
10-19-2008, 01:56 PM
 Bill_Marsden Super Moderator Join Date: Mar 2008 Location: Dallas, TX (GMT-5 w/ DST) Posts: 19,045 Blog Entries: 5

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.
__________________
..
"Good enough is enemy of the best." An old engineering saying, Author unknown.

General info:
If you have a question, please start a thread/topic. I do not provide gratis assistance via PM nor E-mail, as that would violate the intent of this Board, which is sharing knowledge ... and deprives you of other knowledgeable input. Thanks for the verbage Wookie.
#7
10-19-2008, 02:56 PM
 beenthere Senior Member Join Date: Apr 2004 Location: Missouri, USA (GMT -6) Posts: 15,815 Blog Entries: 10

For more current, a rectifier diode will drop a lower voltage but handle much more current.
__________________
First comes the hardware, then the software.
#8
10-19-2008, 03:00 PM
 hgmjr Super Moderator Join Date: Jan 2005 Location: Tennessee, USA (GMT-6) Posts: 9,030 Blog Entries: 11

Quote:
 Originally Posted by Lebe 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
#9
10-19-2008, 03:11 PM
 SgtWookie Expert Member Join Date: Jul 2007 Location: In the vast midwest of the USA; CST Posts: 22,039

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
 1N4002-bDiodeVfvsCurrent.PNG (24.2 KB, 13 views) 1N4148DiodeVfvsCurrent.PNG (27.1 KB, 12 views)
__________________
General info:
If you have a question, please start a thread/topic. I do not provide gratis assistance via PM nor E-mail, as that would violate the intent of this Board, which is sharing knowledge ... and deprives you of other knowledgeable input.
#10
10-19-2008, 06:57 PM
 Audioguru Banned Join Date: Dec 2007 Location: Ontario, Canada Posts: 9,411

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
 MV8191.gif (10.1 KB, 5 views) MV8191 red LED.PNG (3.5 KB, 4 views)

 Tags led, regulator, replace, voltage

 Related Site Pages Section Title Worksheet Negative feedback opamp circuits Worksheet Performance-based assessments for semiconductor circuit competencies Worksheet Regulated power sources Worksheet Bipolar junction transistors in active mode Worksheet Zener diodes Textbook Transistor ratings and packages : Bipolar Junction Transistors Textbook The common-collector amplifier : Bipolar Junction Transistors Textbook Voltage regulator : Discrete Semiconductor Circuits Textbook Special-purpose diodes : Diodes And Rectifiers Textbook Zener diodes : Diodes And Rectifiers

 Similar Threads Thread Thread Starter Forum Replies Last Post mejdi_electronique General Electronics Chat 4 01-12-2012 12:54 AM ridge84 Embedded Systems and Microcontrollers 4 11-29-2009 09:35 PM iONic The Projects Forum 2 08-10-2008 12:34 AM Pootworm General Electronics Chat 5 10-28-2007 12:58 PM Tyco The Projects Forum 4 03-21-2007 03:31 PM

 Thread Tools Display Modes Linear Mode

 Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is Off Forum Rules
 Forum Jump User Control Panel Private Messages Subscriptions Who's Online Search Forums Forums Home Electronics Forums     General Electronics Chat     The Projects Forum     Homework Help     Electronics Resources Software, Microcomputing, and Communications Forums     Programmer's Corner     Embedded Systems and Microcontrollers     Computing and Networks     Radio and Communications Circuits and Projects     The Completed Projects Collection Abstract Forums     Math     Physics     General Science All About Circuits Commmunity Forums     Off-Topic     The Flea Market     Feedback and Suggestions

All times are GMT. The time now is 01:34 AM.