Idea for cheap LED "regulator" ?

[Klaus]

Was thinking about how to limit the current the easiest way for multiple Nichia or Luxeon LEDs.

Right now we either just use like 3+ cells and drop the voltage with a resistor - or 3+ cells and regulate down with a buck? or 2 cells and regulate up with a booster - all of these options ahve pros and cons.

One idea came up lately where I´m not shure about and would like to throw into the discussion - why not just take 3+ cells and limit the voltage (effectively also limiting the current) using a Zener Diode ?

The value of the Zener could be fine-tuned to the best driving voltage of the LED at hand - as long as the cells (primary or rechargeable, whatever) deliver more than that the LED would only see that voltage.

Choosing a low-drop Zener the losses couldl be minimized - I think this could be some sort of cheap regulation as the voltage wouldn´t be higher with fresh cells (as it is with a resistor) and won´t cost much when the cells gets depleted.

I think we do have some much more experienced people "on board" who might correct me if this makes no sense but I do see some advantages against just a dropping resistor - certainly a real regulator is "better" but also much more complex.

Comments ?

Klaus

 

[Silviron]

Yes, I hope that someone with a little (LOT) more electronics knowledge would put in their two cents on this too.

I had the same idea last summer, played around with it a bit, and had no success. I could get what appeared to be a regulated current out of a Zener and a capacitor and resistor circuit (I forget the details, something I found in a power supply / regulation theory book) that worked fine until I introduced the LED into the circuit.

I did quite a bit of research on the web too, and just about everything I could find on the subject gave me the feeling that it wouldn't work well, althought nothing really said WHY it wouldn't work.

 

[MrAl]

Hi again Klaus and others,

Im working on a circuit like this with a
very small size to fit into flashlights
that will work from a little over 3 volts
to 6 volts dc input. The output will be
350ma (for the LS) but of course any current
less then that will be possible too.
This means it will work with one led to
about 10 or more leds, regulating the current
perfectly.

Myself and someone else will be selling
the completed boards very soon now.
Price should be very low too, because
we are just trying to raise a little
money for future projects like this,
not make a huge profit :-)

This will be a 'pop in' board, that doesnt
require adjustment at all.

I'll post a note when we are done with the
board and have tested it completely.
Preliminary tests have already been done.

Take care,
Al

 

[Klaus]

Thx Al,

keep us posted

Klaus

 

[Jonathan]

Zener diodes only regulate the voltage _across_ them when there is current flowing _through_ them. In other words they are 'shunt' regulators.

The way one uses a zener diode as a voltage regulator is to place the diode in series with a current limiting resistor and connect this to your power supply. The current limiting resistor is necessary in order to prevent too high a current from flowing through the diode.

You then connect your load across the zener diode. As the load draws more current, the zener draws less current, always maintaining the same voltage across itself.

You could use it as a voltage regulator for an LED array, but you would still have the inefficiency of the current limiting resistor, in addition to the inefficiency of the current flowing through the zener.

Another problem with this approach is that LEDs are really not well suited to being driven with a regulated _voltage_. Because LEDs draw very little current until their threshold voltage, and then the current starts increasing rapidly as voltage goes up, unless you get the voltage selected 'just right', you will not get the desired LED current.

The only reason that the zener diode would help at all is that it has an even steeper voltage versus current curve than an LED. The addition of a Zener to a resistor dropping LED circuit would help regulation when the battery is fresh...but it would lower the efficiency of the entire circuit.

-Jon

 

[Klaus]

Thx Jon,

nice explanation - and while I knew about that "just right" selection the other parts of your write-up were unknown to me - uuhh - I never told you guys I´m an electronic guru.

Klaus

 

[springnr]

Actually folks been known to use leds in place of zeners for low voltage applications. A red led at 2v has better V-I curve than a zener and is cheaper.