Re: How much current for encoder LED?

On 2 Apr, CAD_CAM_EDM_DRO@yahoogroups.com wrote:

> How much current does the average LED need for operation in a
> quadrature encoder? With an unknown encoder I should start with 5V
> and what size resistor? Very important not to fry this encoder.

A safe value for most all LEDs is 20ma, with failure coming typically
over 50ma or so. However, because of the very close distance between
emitter and detector, maybe only 5 to 10 ma will be required.

Note that LEDs have a typical forward drop of about 1.7 volts, so your
resistor will need to have about 3.3 volts across it. At 10ma, that's
a 330 ohm resistor.

Here's the thing, though.

The only "encoders" I've actually held in my hand are from HP. These
things are not just LEDs and phototransistors. They have electronics
inside, over and above the optical stuff.

As such, they require a genuine source of 5 volts, with no current
limiting resistors. Inside the device they have their own current
limiting electronics.

You may want to investigate further.

Alan




--

Alan Rothenbush | The Spartans do not ask the number of the
Academic Computing Services | enemy, only where they are.
Simon Fraser University |
Burnaby, B.C., Canada | Agix of Sparta