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Electrical engineering: dropping voltage on a circuit

Posted on 2013-06-21
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Last Modified: 2013-10-10
I have a 12v power source. Some components on the circuit require 12v, but one of the components I'm powering is an LED which takes a maximum voltage of 4v. What is the best way to drop the circuit's voltage to accomodate that LED's maximum? I could use a resistor I guess, but it seems like a waste of electricity since I think it just gets converted to heat.
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Question by:zumpoof
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by:dbrunton
ID: 39267359
Try the calculator at http://ledcalc.com/ for resistance required.

I suspect you won't have that big a resistor.
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by:aburr
ID: 39267367
I bet you can get one as big as you need at Radio Shack.
A resistor is the simplest way to go. There is not better (ie energy efficient) way to go.
You have to drop the current for the LED through 8 volts no matter how you do it. You will generate the same amount of heat no matter how you do it. (Unless you can find a use for that energy such as running a fan ...)
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by:Dave Baldwin
ID: 39267513
Also, the resistor value should be for the 'normal' LED forward voltage, not the max.  I think you would find that if you get it up the max voltage, it would be a lot brighter than you wanted.  For a very short time.
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by:zumpoof
ID: 39267519
Thanks for the answers guys. So, to be clear, a resistor is still a better choice for this over a voltage regulator or a transformer?
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by:Dave Baldwin
ID: 39267572
Yes, by far.  LED forward voltage varies some with each device and it depends on the current thru it.  A fixed voltage regulator would make some brighter, some dimmer, some blown out.  A transformer is an AC device and LEDs require DC current.  To use a transformer, you would have to add a diode and a resistor to make it work.  The only other acceptable method is a constant current source.  That takes 3 resistors and a transistor for a simple one.
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by:zumpoof
ID: 39268999
Thanks DaveBaldwin. Why would a 'fixed voltage regulator' cause such brightness inconsistencies? I figured those things just magically dropped voltage somehow and then you could use the new voltage normally.
As for the transformer, yeah I'd need to add a rectifier to switch it to DC. I just figured since transformers can drop voltage that it might be a better choice than a resistor.
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by:ozo
ID: 39269010
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by:Dave Baldwin
Dave Baldwin earned 250 total points
ID: 39269561
A 'fixed voltage regulator' would cause such brightness inconsistencies because LED forward voltage is not that precise and consistent.  For that reason, LEDs are never driven by voltage, they are driven by current.  And current is usually set by allowing the voltage to drop across a resistor.  

If you have a data sheet for the LED you want to use, you will see a range of values for all the parameters.  All electronic components have tolerances in their parameters.  Things like LEDs can vary quite a bit unless you have the manufacturer preselect them for you.
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viki2000 earned 250 total points
ID: 39291935
An LED works by providing constant current to it in the limit of the its rated voltage.
The resistor is the cheapest and simplest way to go, but with low efficiency. As was mentioned, the 12V-4V=8V multiplied by LED current will be power loss as heat on the resistor.

A better way is to use a constant current source with higher efficiency, usual a switching power supply, as a step down dc-dc converter.
But for 1 LED is not worth it.

If you have a set of LEDs, as an LED lamp or LEDs strips for lighting, then you do it with switchable power supplies, DC-DC converters with higher efficiency.

The voltage regulator, most of them linear, will have also bad efficiency, the excess power will be lost on the transistor inside the regulator. A 3-terminal voltage regulator may be used as constant current source in the proper connection using 1-2 resistors.

If your 12V does not vary, quite stable, and if has enough current to power up your LED, then go with a resistor. If you have variations in voltage, then you need the constant current source.

If the LED is used for lighting and is high-power LED, then you have to know that the light of the LED is direct proportional with the current in LED. So is better to maintain the current as much as possible in the prescribed limits of the LED datasheet.

Nowadays are common HIGH-POWER LEDs for lighting. In such case you better use a dedicated power supply and forget your 12V source. But I guess is not your case.

First point to start: parameters of your LED as power, voltage, current. Only after that you decide if a simple resistor solution is a waste of energy or not.
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