In Reply to: Re: Constant current sources to drive LEDs on long lines posted by Ron Harrison on 02/25/03 at 10:10 AM:
Thanks for the reply.
The rational behind the reasons I stated for using a constant current source obviously needs some expanding upon.
(As background - the remote indicator LEDs are on a small PCB without any other components, ie there are no dropping resistors.)
Driving them with constant current sources means that the brightness will be repeatable and constant regardless of line length.
The supply is unregulated, I don't want to draw the current from the meagre Vcc regulator.
The input voltage range is 10 to 32Vdc.
In the case of a fault, the maximim current will only be that which the current source delivers.
If multiple repeaters were needed, they could be strung in series, as the supply voltage allows (need both 'series' and 'termination' styles of remote LED indicator.)
Still need some sort of active device to drive the lines, eg inverter or opamp or discrete BJT etc,
so why not a current source???