Resistors in LED circuit?

An incandescent bulb can be designed work at any voltage (within reason) but LEDs work at specific voltages. For white LEDs I think it's 3.6V, so you need to drop some voltage with a resistor. Obviously this does waste energy. The other options is a voltage regulating circuit, which also wastes energy. So other than finding batteries with the perfect voltage, you are stuck wasting some juice. Then add in the form factor you are trying to create, and there's another limitation. The X5 runs off of two CR123 lithiums which gives it a nice small size.

HTH

LEDs are current-operated devices. They require a minimum voltage to start conducting current. A series resistor limits the current to protect the LED. Without this current-limiting resistor, a LED would have a very limited operating voltage range. Anything voltage exceeding this range could quickly destroy the LED.

Originally posted by rlichter:
[QB]Several posts concerning the Inova X5 refer to resistors being used to "protect" the LEDs. Can anyone elucidate this concept?

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Several posts concerning the Inova X5 refer to resistors being used to "protect" the LEDs. Can anyone elucidate this concept? I mean, if the resistors are in series with the LEDs to drop the voltage, they are themselves consuming energy and dissipating it as heat. Hardly an efficient design. Also, in what sense would such an arrangement "protect" the LEDs? Why design a light to use a 6V source and run the LEDs at less than 6V?

You do need to protect the LED. An ideal diode (a theoretical model) has a forward voltage drop. If your voltage is below this, no current. If your voltage is above this, then the only thing to limit your current is the resistance in the other parts of the circuit. In other words, a small difference in the voltage of a diode can result in a very large current change. Even though a real LED is not an ideal diode, it comes close enough where it needs protection.

The resistor circuit is easy and cheap, but does waste power. Also, the brightness goes down as the batteries drop.

You can also use a linear regulator in current mode. This will allow a constant current throught the LED, but it needs more voltage (which requires an extra battery). These are not used too often.

The best option is to use a DC-DC converter (such as a boost circuit). These can give a fairly constant brightness, and can suck a battery completely dry. This is used in the ARC family of products. The disadvantage is that these circuits can be complex and can be daunting for a beginning to design. The parts also cost money, which raises the cost of the light.

Something that I have never heard anybody is it to use a JFET to control the current. A JFET can be wired to achieve a simple "constant current" mode. Has anybody tried this?

Hello there,

In addition to the other posts:

Most devices need current limiting
because their characteristic operating
voltage is lower then the intended source.
The exception to this are devices that are
designed to operate at a specific voltage.
An ordinary light bulb would need current
limiting if it was designed for a voltage
much less then 120vac.
The problem with the LED is that the technology
is so complex that right now there are no
ways to design LEDs for specific voltages,
so we have to take what we can get and go
from there. We end up using a voltage
supply that is higher then the LED voltage,
so we need current limiting. To add to
the problem, the LED voltage varies over
temperature, so if your ambient changes
much you may end up blowing out the device
if you use a fixed voltage source even though
it works well at room temperature.

The alternate to current limiting is
current regulation, which comes in many forms:
Switchers typically allow the LED to operate
at its' characteristic voltage while maintaining
a low loss in efficiency. Not all switchers
are that good though. Depending on the input
voltage range some linears will beat
most switchers. The decision to use resistor,
switcher, or linear regulator depends mostly
on the intended source supply type and the
type of LED.

Good luck with your LED circuits,
Al

Harrkev,
A jfet can easily be configured to supply constant current but then all you have is a linear regulator with all of its drawbacks.