Confused about drivers!

[Deeksie]

New to here!! I'm a bit confused about drivers! Your help would be very appreciated!!

I understand that you should have some sort of driver to regulate the voltage/current to the LED's because the voltage of the batteries can fluctuate.

However, I don't get why people use a driver? I've read around a bit and seen there (as far as I can see) are far cheaper ways to regulate the power.....

Like a Zener Diode..............

Or a voltage regulator chip (I found several on REUK dot com, E.g.: called LM2940ct)

I fail to imagine that you've all missed this so I figure I'm missing something......!

Why do you use drivers that can be around $20, when (from what I can see) a cheap $2 voltage regulator would do the same job??

Thanks in advance!

Deeksie

 

[VegasF6]

The term "driver" could be pretty generic. Most of the quality drivers I have seen are "switch mode" drivers. The advantage being efficiency.

If you have an led voltage the same as your battery voltage, the most efficient driver would be a simple current limiting resistor. That is fine for the brief period of time that your voltages match, which won't be for very long.

What led or leds you want to drive and what battery configuration you are going to use obviously effect your choice. If you wanted to use something with a lower voltage, 1xAA, 2xAA, for instance, you would need a "boost driver" or voltage multiplier. If you had a higher voltage, maybe 8.4V or something you would need a way to drop voltage to achieve whatever current you wanted. Then there are boost/buck drivers which as you can probably guess, do both. So, you start with say 4.2V, driver operates in buck mode. Once battery voltage drops below what is required to drive led at needed current, driver operates in boost mode.

There are many other types of course. The LM2940 you mention is a linear regulator, sort of a variable resistor. It limits current to the led at a constant 1 amp while your battery voltage is higher than led voltage + drop out of the LM2940. In the case of the LM2940, drop out voltage can be from .5-1 volt so anything under that is no longer in regulation and will supply less power to the led, dimming as battery power runs down. When battery voltage is higher, the excess energy is converted to heat and disapated through the regulator. The higher the voltage, the more is turned into heat and wasted. That particular regulator wouldn't work for driving 1 led I don't believe as spec sheet shows 9V minimum output. There are other LDO regulators that do though.

A diode, be it xener or otherwise, a resistor, will also waste energy to heat.

Pulse width modulation, switch mode power supplies, hysteretic controllers, etc are more efficient, and when you have a limited amount of space, and energy, as in the case of a battery operated flashlight, efficiency is a good thing. For fixed lighting, or automotive use, these become less important.

If you look at some older builds here however, such as the space needle, etc you will see that these drivers are somewhat new. LED drivers have come a long way in a few years!

*edit* lm2940 is a voltage regulator not a current regulator as I wrote.

 

[neoseikan]

BTW, most cheap drivers cost their producers less than $2. And they sell it at $3~$5.

 

[Deeksie]

Thats great! Thanks guys.....!

I thought it might have something to do with efficiency! :thumbsup:

Deeksie

 

[Oznog]

Well, drivers need to put out constant current too. Putting a controlled, fixed voltage across an LED may get you 70% of the current you want, or 130%, and the current will increase as the LED warms up, potentially out of control.

A zener cannot act as a regulator for an LED.
A voltage reg can't either, not very well anyways. If you have a fixed voltage AND a ballast resistor, that gets rid of the variations in batt voltage, but only partially compensates for variations in LED forward voltage. And it'll be terribly inefficient for a battery application, lowering the runtime and makes a crapload of heat.

There are plenty of regs at KD/DX for like $3-$5. Some are linear type which can still provide current regulation as long as Vbatt>Vled, but are inefficient. The ones with inductors on them are buck, boost, or buck/boost and those are more efficient. They'll also typically work over a wider voltage range. A buck/boost for example can maintain the specified current to a white LED at 3.4v even as a lithium batt drops to 3v. A linear reg would dim because it can't boost the voltage to meet the LED's needs thus some of the cell's capacity will be unusable.