[amsat-bb] KR400 controller question

[Zach Metzinger]

On 1/7/19 3:28 AM, Paul wrote:
> Thanks George, I tried it with and without the resistor and couldn't see 
> any difference with the LEDs so wondered if it was just put in to 
> increase the life of the bulb. I'll leave them in and see how we go.
> 
> The controllers now look like this adding a splash of colour to the 
> shack. http://www.ukdx.org.uk/tv/rotator/400s.jpg

Paul,

Given this circuit:

Vsupply --- 1N4001>| --- /\/\/\/\ --- LED>| --- "Gnd"

You can calculate the proper resistor value with the formula:

Vsupply - Vf - Vdiode
----------------------
      Iled

Vf is the LED forward voltage, Vdiode is the forward voltage of the 
1N4001. Putting some numbers in there for a typical application with the 
CREE C503B-RAN-CZ0C0AA2):

(28 - 0.7 - 2.1) / .020 = 1260 ohm

The absolute maximum rating of this LED is 50mA, so we can round down to 
a 1.2kohm resistor safely. No need for additional resistors beyond this one.

To add additional LEDs, you can either put them in series and add up all 
the forward voltages in that equation, or you can go parallel and add 
pairs of resistor+LED.

Since you've got a half-wave rectifier, you might want to add a 
capacitor after the 1N4001 to smooth out the ripple (and the obnoxious 
flickering). A good value would be about 10uF.

A more efficient way to drive LEDs is to use an IC, which can also 
provide PWM dimming via digital or analog (knob) input.

https://www.maximintegrated.com/en/products/power/led-drivers/MAX1570.html

73,

--- Zach
N0ZGO