question about the led lightbars?

[justDIY]

referencing this article:
http://led.linear1.org/led-light-bars-from-super-bright-leds/

what are the switching diode's role on the light bars?? are they just being used to "eat" the obligatory 0.7v so the resistor doesn't have to?

[Rob]

I think they are to protect you from reverse polarity hookup of the power supply. Rectifier diodes cost pennies, so it's cheap insurance.

[justDIY]

since its basicly a board full of diodes from an electrical standpoint (?), how does the electricity know to blow the switching diode before the light emitting diodes?


example, lightbar is reverse biased with an unregulated high-current source (ie straight to a car battery)

[Rob]

Aw geeze, are you gonna make me look up datasheets on recitifier diodes?

they have a high PIV compared to regular (light-emittting in this case) diodes. A few hundred volts is what I recall. So there's no blowing, just not any conducting of the current in the wrong direction.

[Rob]

http://www.fairchildsemi.com/ds/1N/1N4001.pdf

First spec on the datasheet, they call it V_RRM and it goes from 50V for the 1N4001 on up to a kilovolt for the 1N4007. Peak Inverse Voltage was the way I learned it.

Check out that reverse characteristics graph on page 2.

[justDIY]

Ahha ... well this is good to know... learn something new every day!

But, in order for the diode to protect from reversed polarity, then it must be in series with the load?

So, the voltage drop and energy dissipated applies; like a 1 amp load * drop of 0.7v = 700 mW of energy to dissipate?

[Rob]

Not energy but power. Power is the time rate of change in energy.  But P=V*I holds true for all circuit elements.