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Light Control
| Re: Building SSR boards( resistor ) |
Author: Dan Baldwin
Date: 02-27-03 14:11
Calculating resistor values for control side of an opto:
First from the spec for the opto determine the current flow necessary to guarantee it will trigger. This is rated in ma (1/000s of an amp). optos generally call for 5ma, 10ma or 15ma. I then add a few ma to it to make sure...
I use 5ma optos so I design for 8ma to be supplied to the opto.
R = V/I (Resistance = Voltage / Current ) (Ohms =Volts/Amps)
Lets compute it for 8ma with a 5 volt supply and here is where some people make a mistake.
R = 5/0.008 = 625ohms .... (THIS IS WRONG because it really isn’t 5v)
Why is it wrong? Well the opto uses a LED and LEDs have an inherent property that causes a forward voltage drop this is listed in the specs as Vf. A Vf of 1.3 is pretty typical for an opto. Different color/types of LEDs have different Vf values.
So the actual calculation is:
R = (Control Voltage-Vf) / I
For control 5v - 5ma opto I do:
R = (5-1.3)/0.008 = 462 ( I use a 470 ohm resistor )
For control 5v - 10ma opto I do:
R = (5-1.3)/0.013 = 284 ( I use a 330 ohm resistor )
For control 5v - 15ma opto I do:
R = (5-1.3)/0.018 = 202 ( I use a 220 ohm resistor )
....
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| Topics |
Author |
Date |
|
Art Ritchie |
02-26-03 04:05 |
|
Ted Dodson |
02-26-03 22:11 |
    |
Re: Building SSR boards( resistor ) |
|
Dan Baldwin |
02-27-03 14:11 |
|
ron matlik |
02-28-03 09:02 |
|
Dan Baldwin |
03-01-03 18:00 |
|
ron matlik |
03-02-03 00:31 |
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Robert M |
03-04-03 15:03 |
|
Dan Baldwin |
03-06-03 12:00 |
|
Tim Fischer |
03-14-03 16:53 |
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Dan Baldwin |
03-14-03 22:37 |
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