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Optoisolators are often encouraged for isolating signals controlling higher voltage loads. The typical example is a low-voltage microcontroller controlling a motor or solenoid operating at a higher voltage. In this hypothetical scenario, the power supply for the microcontroller and the power supply for the high voltage load are entirely separate. In a lot of real cases, though, the microcontroller is actually being powered by the same power supply, just via a voltage regulation circuit. So this is my question:

Let's say there's a single 10 V power supply. One subcircuit regulates the 10V down to 5V that powers a microcontroller. The microcontroller sends a signal that controls a solenoid operating directly from the 10V supply. Does putting an optoisolator between the microcontroller and the solenoid control circuit serve any purpose or is it pointless because, ultimately, the two power supplies are connected anyway?

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    \$\begingroup\$ For 10V, maybe not; but for higher voltages, an opto-isolator is often used as a level-translator for gate drive of a high-side driver. \$\endgroup\$ – Tut Oct 15 '13 at 20:56
  • \$\begingroup\$ That's a fair point. I was just using 10V as an example. It could be 100V. Although that would make for some very inefficient voltage regulation... \$\endgroup\$ – Computerish Oct 15 '13 at 21:03
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Your concerns are not ungrounded**. Lack of ohmic isolation of power supply and ground rails defeat the purpose of the opto-isolator. Ground loops remain unbroken. EMI can propagate through power and ground rails.

Often, there is an isolated DC-DC converter, which provides ohmic isolation of the power supply and ground. Here's just one example of such power supply, from the dozens of models that are available. You can also roll your own.

** pun intended

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If the motor/solenoid supply and the micro are not galvanically isolated it's difficult to see how a case can be made to use an opto isolator in the drive circuits from micro to motor/solenoid.

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Shortly - no, using opto-isolators in these conditions is pointless at all.

It is, btw, common mistake of the beginners.

Once I saw even more "advanced" design - opto-isolator, then power MOSFET that drives a relay that switches the load and all this powered by one single power source.

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Depends on what you're using to "regulate" the 10V down to 5V. If you're using a voltage regulator, isolation won't buy you much. If you're using a 10V to 5V isolated DC to DC converter (not all converters are isolated), that device will give you isolated 5V with respect to an isolated ground. If you're careful not to connect your grounds or connect your power supplies through any devices, optoisolation will work

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Opto-isolators are useful in cases where there is a substantial potential difference between the processor's control voltage and the thing it's controlling. For example, if one is using a processor to switch the positive of a thousand-volt electronic flash supply, even if the negative rail shares the processor's ground, having an opto-isolator may be preferable to trying to couple the signal via other means.

Opto-isolators may also be useful in cases where the controlling circuitry and the circuitry being controlled nominally share a ground rail, but a potential difference may sometimes exist between points on that rail. For example, a system may have a nice quiet ground rail when it's idle, but an extremely noisy one during operation. If one may need to take some analog measurements while things are idle, but will need reliable control even when things are noisy, one might use a shared ground for the analog measurements but use optos for the control signals. Note that one must be careful to ensure that the noise on the controlled side of things does not disrupt processor operation, but there are a variety of means through which that can be accomplished.

One other point is that even if one part of the overall system will require a common ground, having things designed so that one can break that ground connection while troubleshooting parts which do not require it can sometimes be helpful. That can often only be done, however, if things are connected via optos.

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  • \$\begingroup\$ Great points about debugging by breaking grounds and quiet sections of the ground rail. Thanks! \$\endgroup\$ – Computerish Oct 15 '13 at 21:28
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Does putting an optoisolator between the microcontroller and the solenoid control circuit serve any purpose or is it pointless because, ultimately, the two power supplies are connected anyway?

It most often doesn't serve a needful purpose, but occasionally might. If the two are separated by a large distance, even on the same power supply, there might be issues with the signal. If the solenoid driver is not well designed then surge damage can be prevented with an optoisolator. If you're dealing with a dodgy power supply that puts noise on the power lines an optoisolator may help.

However if you're designing the whole device, and don't have to connect to other people's lousy solenoids, drivers, or powersupply, can specify and test every component, and know that they will all be physically close with a single ground point, then an optoisolator is overkill.

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