Hi,

Let's consider the image attached. Also, an scenario where I have multiple power supplies. Should I connect all grounds to a common point?
I remember once that I had a circuit like the one attached, and didn't have the two grounds joined, so the circuit didn't work properly until I got the two grounds together (cathode ground and motor power supply ground), but why?

 

No. You do not connect the grounds. That will defeat the purpose of the opto-isolation.

 

No. You do not connect the grounds. That will defeat the purpose of the opto-isolation.

If I hadn't isolation, then I could connect the two grounds, right?

 

Hi,

Let's consider the image attached. Also, an scenario where I have multiple power supplies. Should I connect all grounds to a common point?
I remember once that I had a circuit like the one attached, and didn't have the two grounds joined, so the circuit didn't work properly until I got the two grounds together (cathode ground and motor power supply ground), but why?

You can use separate grounds on such a circuit, but then you'd need separate power supplies. Let's suppose that the circuit on the left is powered by a wall adapter, and the one on the right by a car's battery. The circuit would work perfectly, as long as each power source has a return path to its own ground.

 

If I hadn't isolation, then I could connect the two grounds, right?

Right. You could have a 5V power supply and another one delivering 12V, and you could connect both grounds. But then the use of an optoisolator would become almost pointless.

 

Right. You could have a 5V power supply and another one delivering 12V, and you could connect both grounds. But then the use of an optoisolator would become almost pointless.

Let's say that I'm not using opto isolation. What would happen if I do not connect both grounds?

 

Let's say that I'm not using opto isolation. What would happen if I do not connect both grounds?

Your circuit would not work, because there would not be a return path for the signal driving the transistor.

 

Your circuit would not work, because there would not be a return path for the signal driving the transistor.

That's the part which confuses me. Let's say I have 12V for the motor and 5V for the arduino and I'm not using isolation. The 12V has its own ground and arduino its own ground. If don't connect both grounds, you say that the circuit would not work, because there would not be a return path for the signal driving the transistor, but the return parth for the signal driving the transistor is not the 5V ground?

 

Your circuit should work if you were to reconfigure it this way:

​

You'd only have to calculate the best value for R2. I'd say that 270 ohms should do it.

 

the return parth for the signal driving the transistor is not the 5V ground?

Nope ... current needs to flow from the arduino's output, through the transistor's base and then to the collector to ground, and then back to the arduino for things to work.

EDIT: I meant to say "back to the arduino's power supply"

 

No. You do not connect the grounds. That will defeat the purpose of the opto-isolation.

That is not always the purpose of opto-isolation, many times it is to transition from one system voltage level to another.
A prime example is where a PC port is used as an output to turn on a AC activated device such as a AC motor, Many use an opto or SSR thinking they are getting galvanic isolation whereas the PC supply common and the AC motor neutral are both connected to earth ground.
IOW there are many situations where it can be used without galvanic isolation occurring.
IN the systems I have designed, I try to keep all power common points to a star point Ground (Earth).
Max.

 

Nope ... current needs to flow from the arduino's output, through the transistor's base and then to the collector to ground, and then back to the arduino for things to work.

EDIT: I meant to say "back to the arduino's power supply"

Got it. Thank you.

 

A final note.

Here's an easy way to visualize it: the word "circuit" itself, is derived from the word "circle", that's because current goes around and trough the components when they're active. If current doesn't have a way of returning to its source, then the circuit will never work.

 

That is not always the purpose of opto-isolation, many times it is to transition from one system voltage level to another.
A prime example is where a PC port is used as an output to turn on a AC activated device such as a AC motor, Many use an opto or SSR thinking they are getting galvanic isolation whereas the PC supply common and the AC motor neutral are both connected to earth ground.

In this case you are still getting galvanic isolation of the current path of the I/O at the signal translation point because earth ground is not normally a current/energy carrier in this type of circuit as the PC has AC power galvanic isolation in the DC power supply for the signal. OPTO separate grounding is also important to provide common-mode signal rejection in noisy environments.

 

IPC has AC power galvanic isolation in the DC power supply for the signal. OPTO separate grounding is also important to provide common-mode signal rejection in noisy environments.

The PC power supply DC common is connected to earth ground as well as the AC neutral.
So zero isolation.
Max.

 

The PC power supply DC common is connected to earth ground as well as the AC neutral.
So zero isolation.
Max.

"as well as the AC neutral" ? Are you sure that's how PC power supplies are wired?

 

The PC power supply DC common is connected to earth ground as well as the AC neutral.
So zero isolation.
Max.

Those two (earth ground and AC neutral) should never be bonded at the equipment (at least in the US they shouldn't be). The fact they might be bonded at the main power panel from the utility feed for safety doesn't defeat galvanic isolation of power at every transformer supplied appliance because of a safety frame-ground.

 

Those two (earth ground and AC neutral) should never be bonded at the equipment (at least in the US they shouldn't be). The fact they might be bonded at the main power panel from the utility feed for safety doesn't defeat galvanic isolation of power at every transformer supplied appliance because of a safety frame-ground.

That was my impression as well ... I have a psu laying around, I'm gonna check it for continuity at its input terminals ...

 

The use of the chassis symbol and Isolation in both cases without differentiating makes for a confusing case.

"Isolation" might be the wrong word. If we leave the Isolation part out, then the circuits can eliminate ground loops.

We might also have to assume that chassis ground is connected to Earth.

The schematic is definitely confusing.

 

That was my impression as well ... I have a psu laying around, I'm gonna check it for continuity at its input terminals ...

The bottom line? Where is the source of EMF driving the circuit at that any point. With a earth frame-grounded chassis and a neutral/earth grounded bond at the main panel only, when we look inside a PC with a transformer power supply the 'source' of EMF driving the PC circuits is not the utility. The source of EMF for circuits in the PC is the induced voltage in the secondary of the transformer. The galvanic isolation of the transformers magnetic field creates a separated and isolated current loop driven by the secondary EMF to transfer energy. If you ground the secondary AC or DC power to earth you still don't have a metallic current path to an EMF 'directly' from the utility power so galvanic isolation remains in effect.