dash lighting - LEDs don't work in tach

[morbias]

Just to clarify, the LED bulb units I used have resistors built in so the LEDs don't burn out.

[bjhines]

PWM controllers can pulse at virtually any frequency. 100hz up to Megahertz range are typical. The performance of the transistors and LEDs are the determining factor for frequency. Most PWMs are designed with frequencies above the audible range to eliminate noise problems(20,000 to 30,000hz). There are always issues with harmonics, and some transistors will get hot when cycled too rapidly with high current flow. The actual waveform can vary with good designs achieving slew rates measured in nanoseconds. Inductive loads require lower pulse frequencies to overcome low-pass filtering effects. This is why your cordless drill has an audible squeal when you push the trigger(1000hz-3000hz).

 

Diodes have a forward voltage threshold that must be reached to allow current to flow(~1v). Good diodes can pass a large current without overheating. LEDs are not good diodes, They have high internal resistance and will produce significant amounts of heat. All diodes will overheat if current is not limited. A resistor is the simplest current limiting device used. Most diodes work well with ~1000 OHMs to limit current in a 12 volt system. You cannot hook up more than just a few diodes in parallel(2 or 3 typical) with one resistor. Internal resistance varies too much(tolerances are crap for LEDs) to allow large parallel networks. Arrays must be designed with complex parallel/series arrangements of resistors and LEDs. In the case of dashboard lighting, ALL of the LEDs are in parallel so each must have it's own resistor. It's handy how they do that for you.

Edited January 21, 2011 by bjhines