LED Driver VS. power Supply

[sfav8r]

OK, what is the difference between an LED Driver and a power supply? I'm currently working with a "constant voltage" LED driver that connects to a dimming module. The dimming module wants a constant 8v input which the LED driver provides. I don't see why I need a $200 LED driver that just sits there putting out a constant 8v with a 40w rating when I can get a really good "power supply" for $50.

I've asked the manufacturer who doesn't have a good technical explanation. They just say using anyone elses supply will void the warranty. Does anyone know why this should or shouldn't work?

 

[zbang]

LEDs are current devices with a forward voltage drop. You need to get the voltage supply over that drop in order to get current to flow, then control the current to control the intensity. Too much current and the smoke comes out, which is partly why each type of LED fixture usually has it's own type of supply (yes, I'm oversimplifying). The right "power supply" can run LEDs, but it won't be a $10 wall wart deal. Also, some drivers deliver high current pulses to get more apparent light and not heat the device too much. (I use a current-regulated bench supply set for about 50v max to test LED fixtures.)

See also http://en.wikipedia.org/wiki/LED_circuit, which actually looks pretty good.

 

[G._S._Ohm]

If you look at a datasheet for an LED, the brightness vs. current is predictable but the forward voltage drop for a given current varies widely and may not even be controlled by the manufacturer.
Therefore, it is a current operated device.
So are coils but for a different reason.

But if you have many identical LEDs in series, the voltages differences cancel out for statistical reasons and you might get away with having a predictable current through all of them.

For about $5 and a half dozen parts you can convert a voltage source into a current source, using an IC voltage regulator wired as a constant current circuit.

When does a voltage source become a current source?
The highest internal impedance for a device that was sold as voltage source that I could find was about 40 ohms. For indicating LEDs running at Vf of about 2 V, 150 ohms and 5 volts are used a lot to simulate a current source.

 

[sfav8r]

Thought I would follow up with my original post. I ordered a $35 power supply from Jameco that was rated at 7.8v. I replaced the 8v $175 "LED Driver" that was originally installed with this strip of lights. It works like a charm. I believe that since the dimming is handled by a separate dimming module, there is absolutely nothing special required of the power supply. If things change, I'll repost, but for now all is fine.