open collector driving a opto relay
open collector driving a opto relay
(OP)
I had a question regarding open collector outputs. I want to use a digital output card to drive a opto relay. The led If is 7mA. I don't want to switch the common. I want to set the digital output to go high and provide 5V to the opto relay and source the current. Well I am not sure if the output is capable of sourcing that much current. The digital output can definitely sink the opto relay led current but like I said I don't want to switch the common or cathode of the LED. I could interface the digital output's open collector and drive it with a PNP transistor. OVerall I am trying to switch on one of the Crydom or Crouzet Solid state AC relays that require 3-32VDC with 8mA current.





RE: open collector driving a opto relay
A pnp transistor is a good idea. Make sure to put a resistor in series with the base otherwise you'll get some odd ringing when the transistor turns on due to the inherent lead inductance of the transistor.
If it isn't in a specification (and that would surprise me), you can check the current source capability with resistors. Start with a large resistor (say 50K for 50V/5K = 0.1mA) and measure the resistance and the voltage at the output pin. Move to a smaller resistor and repeat until the voltage at the output pin drops to 3V. Leave yourself margin (larger resistor) because there will be tolerance variations from part to part. I expect you'll be able to get 2-3mA, maybe more.
RE: open collector driving a opto relay
RE: open collector driving a opto relay
RE: open collector driving a opto relay
How about using a MOSFET as a switch in series with the led opto. Many mosfets have a threshold voltage around 2volts typical with a maximum of 4, so your 5v should be able to turn it on. They also require very little current and provide a very low on resistance (little power disipation).
RE: open collector driving a opto relay
RE: open collector driving a opto relay
And what reason could there possibly be that you cannot switch the common to an opto relay??
RE: open collector driving a opto relay
RE: open collector driving a opto relay
RE: open collector driving a opto relay
RE: open collector driving a opto relay
RE: open collector driving a opto relay