Are dual winding transformers' outputs isolated ?

[xmen_xwk]

I have transformer that has 2 wires going in and 3 wires coming out, and transformer is marked as 12-0-12. So are those 12v ouputs are isolated ?

I actually need to build 2 isolated power supplies, because one power supply can have EMI problems as it will used to switch AC inductive motor. And other one will be used for powering the ICs.

 

[smijesh]

12-0-12 Marked transformers are not isolated, Two wire may be primary and three wire secondary (12-0-12). Better to use two transformer based on the power requirement (AC Inductive motor required more current than IC). Otherwise buy custom designed transformer for your requirement.

One dual output transformer
https://catalog.triadmagnetics.com/Asset/VPP24-2330.pdf

 

[KlausST]

Hi,

Two voltages (12V, 12V) referring to the same point (0V) never can be isolated to each other. It is impossible.

On the other hand I don't see the need to be isolated.
I assume you connect them anyway.

So in my eyes you can use the 12V-0V-12V transformer.
I'd even say you can do with a single supply and filters.
In any case you need good wiring and a good PCB layout.

Klaus

 

[xmen_xwk]

Hi,

Two voltages (12V, 12V) referring to the same point (0V) never can be isolated to each other. It is impossible.

On the other hand I don't see the need to be isolated.
I assume you connect them anyway.

So in my eyes you can use the 12V-0V-12V transformer.
I'd even say you can do with a single supply and filters.
In any case you need good wiring and a good PCB layout.

Klaus

Using Optocoupler, they are not connected. But what kind of filter should I use, I'm already using RC snubber across motor. A good Pcb layout? I have 9mm spacing between Line and neutral and no close traces, the signal traces are way far. An explanation of good Pcb layout would be nice.

 

[KlausST]

Hi,

An explanation of good Pcb layout would be nice.

It´s a wide field ... many parameters .. impossible to give enough "general rules".

Example:

I have 9mm spacing between Line and neutral

This may be good ... and not.

My personal taste:
There is no need for that wide spacing. Here you need to take care about "functional isolation". You don´t say which voltage. But even for 230V AC you may go down below 1mm.

In opposite is "safety isolation", here a 9mm distance for a 230V AC line is a good value.

This was isolation...now to EMI.
For EMI it is good practice to wire neutral and line as close as possible. Because every current that gose through line it comes back on neutral with opposite direction.
Both lines sending out electromagnetic fieds. And because the current is opposite, the magnetic field is opposite, too. This means they cancel each other.

If you see both lines as a transmitting antenna (for EMI frequencies) then the area between both lines gives a raw estimation of antenna qualitiy.
For sending out low EMI (this is what you want) you want a poor antenna quality = need small area = close distance.

****
Optocoupler, filter...
There is no general rule. It all depends on your circuit. Voltages, currents, signal waveforms, frequencies, wiring....

The more informations (pictures, schematics, wiring informations, motor datasheet, PCB layout....) you give the more detailed and helpful answers you will receive.

Best is to know your enemy, then you can focus on it. Least effort, best results.

Klaus

 

[xmen_xwk]

Hi,


It´s a wide field ... many parameters .. impossible to give enough "general rules".

Example:

This may be good ... and not.

My personal taste:
There is no need for that wide spacing. Here you need to take care about "functional isolation". You don´t say which voltage. But even for 230V AC you may go down below 1mm.

In opposite is "safety isolation", here a 9mm distance for a 230V AC line is a good value.

This was isolation...now to EMI.
For EMI it is good practice to wire neutral and line as close as possible. Because every current that gose through line it comes back on neutral with opposite direction.
Both lines sending out electromagnetic fieds. And because the current is opposite, the magnetic field is opposite, too. This means they cancel each other.

If you see both lines as a transmitting antenna (for EMI frequencies) then the area between both lines gives a raw estimation of antenna qualitiy.
For sending out low EMI (this is what you want) you want a poor antenna quality = need small area = close distance.

****
Optocoupler, filter...
There is no general rule. It all depends on your circuit. Voltages, currents, signal waveforms, frequencies, wiring....

The more informations (pictures, schematics, wiring informations, motor datasheet, PCB layout....) you give the more detailed and helpful answers you will receive.

Best is to know your enemy, then you can focus on it. Least effort, best results.

Klaus

That's the problem, I can't exactly know the load, it all depends on user. Think of it as digital switch, user presses a button and it turns on load. Since I don't know about the load, I use a 6amp 220v AC motor as worst case, if it can handle the motor, I believe it can pretty much handle many other loads having 5amp rating.