> On Mon, Aug 25, 2008 at 3:30 AM, Mike Harrison > wrote: > >> Resistors don't fail short - even the standards for Intrinsically safe >> equipment for use in flammable atmospheres state that resistors >> can be assumed to be infallible as regards failure to >> lower values. > > Not all resistors are equal. For IS (intrinsic safety) application, what > you > say can only apply to certain type of resistors (eg: metal film and > wire-wound). > > Xiaofan I believe carbon composition resistors fail to a short. Film and wire wound fail to an open. When I started in electronics, carbon composition resistors were the only ones around. They had a cylindrical body. I remember seeing my first carbon film resistor with the now familiar "dog bone" shape and thought they were quite strange. I haven't been following this thread closely, but have seen stuff about protecting the LEDs from reverse voltage. As I recall from the original schematic on the thread, short strings of LEDs were connected in parallel with polarity reversed. That handles the reverse polarity protection and also allows AC current to flow so the current limit capacitor does not just charge up to the peak voltage and have everything stop. Harold -- FCC Rules Updated Daily at http://www.hallikainen.com - Advertising opportunities available! -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist