# What happens if two linear optocouplers are paralleled, if their output voltages are different?

Joined Oct 26, 2012
434
If the outputs are connected, what voltage would be detected at the combined output? The higher voltage? Or, would the lower voltage device malfunction in some way?

Since they are optically isolated, i think it's not possible for one output to feed into the other output.

There would not be a load on the combined outputs.

Example part: FOD2712A - Optically Isolated Error Amplifier
https://www.mouser.com/datasheet/2/308/1/FOD2712A_D-2313415.pdf

Something is wrong with this sim, but i don't know what. Getting "convergence failed".
http://tinyurl.com/yotdpz9r

Last edited:

#### MrChips

Joined Oct 2, 2009
30,790
You need to put a load resistor, for example 5kΩ to a positive voltage source, for example +5V.

Also, you need current limiting resistors on the emitters.

Joined Oct 26, 2012
434
You need to put a load resistor, for example 5kΩ to a positive voltage source, for example +5V.

Also, you need current limiting resistors on the emitters.
Not sure if this is what you mean. Not getting an error anymore.
http://tinyurl.com/yo5te6r8

But i think the key is to put diodes on the outputs to prevent backflow.

#### Ian0

Joined Aug 7, 2020
9,792
Just put them in parallel with a single pull-up resistor.
The output of an optoisolator is a current that it proportional to the current through its LED.
So the voltage across the pullup resistor will be V=(Iin1*CTR1+Iin2*CTR2)*Rpullup.

Joined Oct 26, 2012
434
Just put them in parallel with a single pull-up resistor.
The output of an optoisolator is a current that it proportional to the current through its LED.
So the voltage across the pullup resistor will be V=(Iin1*CTR1+Iin2*CTR2)*Rpullup.
You mean the R as shown in my reply above?

#### Ian0

Joined Aug 7, 2020
9,792
You mean the R as shown in my reply above?
No, the pull-up resistor as mentioned by @MrChips .
Also note that when @MrChips says “emitters” he means the LEDs (because they emit light) not the emitters of the transistors.

#### Alec_t

Joined Sep 17, 2013
14,312
You mean the R as shown in my reply above?
The R is ok but you need to disconnect the collectors from ground.

Joined Oct 26, 2012
434
The R is ok but you need to disconnect the collectors from ground.
Collectors are connected to nothing?

#### BobTPH

Joined Jun 5, 2013
8,934
Collectors both connect to one side of pullup. Other side of pullup connects to +5V.

Emitters of the transistors both connect to ground.

#### Ramussons

Joined May 3, 2013
1,409
If the outputs are connected, what voltage would be detected at the combined output? The higher voltage? Or, would the lower voltage device malfunction in some way?

Since they are optically isolated, i think it's not possible for one output to feed into the other output.

There would not be a load on the combined outputs.

Example part: FOD2712A - Optically Isolated Error Amplifier
https://www.mouser.com/datasheet/2/308/1/FOD2712A_D-2313415.pdf

Something is wrong with this sim, but i don't know what. Getting "convergence failed".
http://tinyurl.com/yotdpz9r
An Opto Coupler is not a Voltage Source, it is a Switch. Connecting 2 in Parallel will make it behave as a OR gate triggered by light.

#### BobTPH

Joined Jun 5, 2013
8,934
An Opto Coupler is not a Voltage Source, it is a Switch. Connecting 2 in Parallel will make it behave as a OR gate triggered by light.
Actually it is more of a current sink. Within some range, two linear optos in parallel will add their effects.

#### Ramussons

Joined May 3, 2013
1,409
Actually it is more of a current sink. Within some range, two linear optos in parallel will add their effects.
Whether Sink or Source depends on the setup. Collector Resistor or Emitter Resistor.