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I am building a Ham transceiver to computer interface, which has the RS232 port controlling the PTT on the radio. The circuit currently calls out for a 1.2K resistor in series with the RTS line and the 4N25 optocoupler's input. If I use a 4N35 optocoupler, which has higher minimum CTR and lower Vce(sat) test conditions, what sort of change should I make to the resistor's value?

Chipresistor122
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  • You probably don't need to make any change, as this is digital, so the four regimes are working, borderline, not working, and damage. To bound things, find the maximum legal RS232 voltage, divide by the resistance as if the LED has no forward voltage, and see if the current is safe. Might also be worth considering the maximum reverse voltage, which technically a resistor alone cannot protect against... – Chris Stratton Nov 19 '20 at 16:47
  • Both opto's are low on diode reverse voltage levels so, if you want this checked out, you need to post a schematic to ensure that there isn't some circuit deficit to trip up a blanket recommendation. The 4N35 is also a lot slower so you also need to specify what max baud rate expectations you have if using for anything else other than a PTT. Also embed data sheets for both devices. – Andy aka Nov 19 '20 at 16:51
  • Thank you both for your replies to my question. I have wired up the circuit with the 4N35 and the 1.2K Ohm resistor, and it seems to be doing the job. No problem in keying up the PTT so far. – Chipresistor122 Nov 22 '20 at 02:03

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You can use a 4N35 as a drop-in replacement for the 4N25 in this particular circuit. The value of 1.2K is not critical here in general for either component since you'd be operating well into saturation, so no need to change its value. Remember you're dealing with low-power signals here so the amount of forward current you need is very minimal, there is nothing that will be sinking current. So the CTR, which essentially is about how much current draw you will get, wont make much difference, nor will the resistor.

Jeffrey Phillips Freeman
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