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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

You are correct that digital outputs typically sink more current when they go low than the source when going high.  

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

Hi, open collector outputs can supply NO output current, they only sink it. All you need is a resistor from the output to the power rail (pull up) to supply the current. For the values you gave 220 ohm will do.

RE: open collector driving a opto relay

(OP)
Oh yes 220 will do. On this digital output board the pullup resistor is internal and is pulled up to 5VDC only giving me uAmps. I was thinking of using an inverter module from phoenix contact. Based on the circuitry I can drive the open collector output from the digital IO card to the base of this transistor (Din rail module). Module is one of phoenic contact p/n 2964319. I think this should work.

RE: open collector driving a opto relay

If I understand what you are saying correctly, you want to use the IO card, which is capable of supplying 5v at uAmps of current to turn on the LED of an opto.  You also want to use the IO card as an active high signal as opposed to using it as a current sink.

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

Good catch cbarn24050.  That sourcing test wouldn't have been very interesting...  

RE: open collector driving a opto relay

I am amazed that anything calling itself a Digital-Output-Card cannot supply/sink at least many tens of mA!  Even the weakest microprocessor can sink 8mA!

And what reason could there possibly be that you cannot switch the common to an opto relay??

RE: open collector driving a opto relay

(OP)
itsmoked my system is already wired, software complete. looking for a quick modular solution/

RE: open collector driving a opto relay

If you want the simplest implementation, just using a pullup resistor will do it.  When the interface drives high, the contribution to the LED "If" will be coming from the resistor. When the interface drives low, it sinks the current from the resistor to keep the voltage output to a low voltage enough that the LED will not light.  A 300 ohm resistor will give some 10mA.  If your interface board can sink 10mA, that should do it.

RE: open collector driving a opto relay

(OP)
Felixc, I know but there's internal 10k ohm resistor pulled up already. This only gives me 500 microamperes.

RE: open collector driving a opto relay

I basically repeated what cbarn said.  Right, I think I now what this interface card is like.  The 300 ohm resistor will parallel with the 10K.  No problem there.  Adding a 300 ohms in parallel with that internal 10K will give the current that the opto needs.  Give it a try.  Just a resistor, nothing else!

RE: open collector driving a opto relay

(OP)
Ahhh.  Ok Felixc. Thanks!

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