# How to use Opto couplers

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#### joe_1

##### Member
Hello,
I am looking at 4N35 datasheet, and having difficulty understanding it : https://www.vishay.com/docs/81181/4n35.pdf
The concept of the opto coupler is that you apply little voltage to the Anode\Cathode, then bigger voltage will be allowed to pass on the other side.
I have 3.3V from micro controller, and on the other side I want to turn ON/OFF 24V DC voltage only (no AC).
Is it doable with this ?
How do I connect the 3.3V, 24V to the 4N35 ?
Thanks.

##### Well-Known Member
The photo diode just turns on a 'transistor' because it is isolated from the diode you can use/switch whatever level of voltage you need for the output side, as long as it is with limits for the opto spec.
Just as a relay with a low voltage coil can switch high voltage, AC or DC by way of the contacts, IOW they are isolated.
Max.

#### crutschow

##### Well-Known Member
Why do you think you need optical isolation in your circuit?
How much current in the 24V load?
If you don't need isolation, just use the 3.3V signal to turn on a BJT or MOSFET (logic-level type).

#### Tony Stewart

##### Well-Known Member
There are many ways to connect a power FET switch to optoisolator. "0" or "1" in and "0" or 1" out and both input and output may be low side switch or high side switch depending how you define specs for Input hi,lo and power down on primary side and secondary ground switch or power switch depends on current rating . The Opto is a current transfer ratio (CTR) of 50% typ over temperature range (40% min) to 100% min (25'C) on 4N35 . THis means output current is not less than input current = 100% but very low current. 10mA typ.

. So Rs in series for input current, Ii= V(Rs)/Rs = (Vdd-Vf(LED-Vo(CMOS))/Rs = 10mA typically used CMOS driver can be hi or low then LED is tied cathode low or Anode high respectively with series Rs to limit current.

Rs tends to be selected for 3.3V supplies around Vs(Rs)=2V/10mA = 200 Ohms +/-10% so 180~ 220 Ohms is OK if you know worst case temp and tolerances.

Outputs are floating collector-emitter , which for a power switch is best to use a MOSFET and for high side switch, one uses an Pch MOSFET with sufficiently low RdsON. If using 4V threshold switch then must be >=12V and < 30V so 24V is OK. then Gate must have a pull down to ground R to turn ON with no input power , or if you prefer pullup R to 24V to gate to keep off when no source current is applied, then the Collector drives the Gate to ground for 24V Vgs and xx mOhm across switch to load.

If Pull R is used, then Collector can only sink 40% to 100% (25'C) of input 10mA so using 4mA max @ 24V = 6K ohm pullup. to say 10k Ohm. Then both C-E wires go to remote Gate FET and RTN or 0V of remote 24V supply with 6~10k between Pch Gate and Drain to 24V+ with Source going to load with some protection as required by EMI transients or load cap if hard wired.

Using twisted pairs gives better Noise immunity and grounds are not shared.

There are many ways to change wiring depending on logical function , positive or negative logic. (inverted)

THis is just one example.

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