05-05-2021, 04:25 PM
If you have looked at the ground loops discussion and the simple digital output description you have all the background you need to appreciate why an optocoupler on a digital output can help keep the gremlins at bay. Start from the basic buffered digital output circuit which uses a single transistor to switch amps of current under the control of a logic-level digital output.
That digital output signal is generated from the logic power supply in the data acquisition system. That means it is referred to the power ground at the data system. The emitter of the power transistor in the figure is referred to that same ground. It has to be, in order for the digital output signal attached to the base to correctly turn the transistor on and off. That means the high current passed by the buffer transistor needs to flow in a connection to the same ground as the data system ground. As described in the section on grounding and ground loops, that can be done best by providing a separate conductor to carry the large current.
Still, there is a possible problem lurking here. Lets say you have a 24 volt power suppy powering the load at the digital output buffer. That 24 volt supply may also run other elements in your system, perhaps a motor, a fan, or a heater. Those higher current loads must also be referred back to the 24 volt power supply ground, which is now the same as the logic supply ground. The plain mechanics of connecting many high current, (that is to say larger diameter,) wires to a single-point ground quickly become awkward. Also, the electric fields generated by the wires carrying the high load currents then would necessarily be physically closer to the sensitive data system. Remembering that such fields interact in proportion to the square of the distance, a little extra separation goes a long way. We would rather be able to separate the high current loads both electrically and physically from the analog inputs and controls. Fortunately, there is an easy way to isolate a digital output signal by using light to carry the information across an isolation barrier. A class of components called optocouplers perform that function. A light-emitting diode (LED) shines on a phototransistor. Without a direct electrical connection, the diode can turn the phototransistor on or off. There is a lot to know about optocouplers, but for this simple application the smallest, cheapest, most common sort of optocoupler is fine for the job. We'll use a 817 type optocoupler, which is a 4-pin device.
The digital output itself now needs only to turn on or off the LED inside the optocoupler. The phototransistor then provides the base current needed to turn on the buffer transistor. The optocoupler won't reliably provide a lot of current, so we generally use a Darlington for the power stage. That way, we won't be limited by the optocoupler current transfer ratio or by the buffer transistor current gain. The resistor R1 sets the LED current, and the resistor R2 limits the transistor base current. If you know the base current will stay in the safe zone for your buffer transistor, as limited by the optocoupler, you can omit R2.
Now, the higher power circuitry associated with the digital load is completely separate from the powering of the data system. That frees up the geometry of your system so that you can arrange the various elements for convenience, instead for minimizing loop area and radiated energy. With individually isolated digital outputs, different loads could even run from different power supplies. You would be unconstrained. Note that you may want to reconnect the grounds that we have just disconnected when we added the optocoupler. No, that would not make the whole exercise futile. With isolation, you get to make a low-current ground connection, and in the most favorable location. That is a big difference. To visualize, think of two star ground points, one for grounding noisy, high current loads and one for quiet low current power and signal grounds. Adding the optocoupler allows you to physically and electrically separate those two star grounds, and, if you choose, to connect them together with a low-current, gremlin-free, conductor.
We still didn't address using FETs for buffering digital outputs. Please check back later.
Tom Lawson
May 2021
That digital output signal is generated from the logic power supply in the data acquisition system. That means it is referred to the power ground at the data system. The emitter of the power transistor in the figure is referred to that same ground. It has to be, in order for the digital output signal attached to the base to correctly turn the transistor on and off. That means the high current passed by the buffer transistor needs to flow in a connection to the same ground as the data system ground. As described in the section on grounding and ground loops, that can be done best by providing a separate conductor to carry the large current.
Still, there is a possible problem lurking here. Lets say you have a 24 volt power suppy powering the load at the digital output buffer. That 24 volt supply may also run other elements in your system, perhaps a motor, a fan, or a heater. Those higher current loads must also be referred back to the 24 volt power supply ground, which is now the same as the logic supply ground. The plain mechanics of connecting many high current, (that is to say larger diameter,) wires to a single-point ground quickly become awkward. Also, the electric fields generated by the wires carrying the high load currents then would necessarily be physically closer to the sensitive data system. Remembering that such fields interact in proportion to the square of the distance, a little extra separation goes a long way. We would rather be able to separate the high current loads both electrically and physically from the analog inputs and controls. Fortunately, there is an easy way to isolate a digital output signal by using light to carry the information across an isolation barrier. A class of components called optocouplers perform that function. A light-emitting diode (LED) shines on a phototransistor. Without a direct electrical connection, the diode can turn the phototransistor on or off. There is a lot to know about optocouplers, but for this simple application the smallest, cheapest, most common sort of optocoupler is fine for the job. We'll use a 817 type optocoupler, which is a 4-pin device.
The digital output itself now needs only to turn on or off the LED inside the optocoupler. The phototransistor then provides the base current needed to turn on the buffer transistor. The optocoupler won't reliably provide a lot of current, so we generally use a Darlington for the power stage. That way, we won't be limited by the optocoupler current transfer ratio or by the buffer transistor current gain. The resistor R1 sets the LED current, and the resistor R2 limits the transistor base current. If you know the base current will stay in the safe zone for your buffer transistor, as limited by the optocoupler, you can omit R2.
Now, the higher power circuitry associated with the digital load is completely separate from the powering of the data system. That frees up the geometry of your system so that you can arrange the various elements for convenience, instead for minimizing loop area and radiated energy. With individually isolated digital outputs, different loads could even run from different power supplies. You would be unconstrained. Note that you may want to reconnect the grounds that we have just disconnected when we added the optocoupler. No, that would not make the whole exercise futile. With isolation, you get to make a low-current ground connection, and in the most favorable location. That is a big difference. To visualize, think of two star ground points, one for grounding noisy, high current loads and one for quiet low current power and signal grounds. Adding the optocoupler allows you to physically and electrically separate those two star grounds, and, if you choose, to connect them together with a low-current, gremlin-free, conductor.
We still didn't address using FETs for buffering digital outputs. Please check back later.
Tom Lawson
May 2021