Hello

I have always been told that if we are using a circuit with two different voltages (say 5V and 12V) we should as much as possible separate the grounds.

What happens if instead of two power sources we use only one (say 12V) and some circuit involving a regulator to generate the other (5V) (something like a DC-DC)

say:

12V--------------------------[78L05]-----------------------5V
            |      |             |          |             |
           ==      ==            |          ==          == (capacitors)
            |      |             |          |             |
GND---------------------------------------------------------------

Is it ok now to use just ONE GROUND connection for both 5V and 12V circuits???

Thanks in Advance

Kansai

 

I don’t know if it’s ok for what you’re doing but it’s done all the time duel voltage systems like this one.
7Arrows

 

I have always been told that if we are using a circuit with two different voltages (say 5V and 12V) we should as much as possible separate the grounds.

Grounds of separate power systems are often kept separated for a whole lot of other reasons but most are connected together in a lot of designs.

One cannot simply use a generalization to cover all situations.

 

Hello

What happens if instead of two power sources we use only one (say 12V) and some circuit involving a regulator to generate the other (5V) (something like a DC-DC)

It is OK to use one ground with the voltage regulator circuit as you have shown. In fact, I don't think it will work if the 12V source and regulator are referenced to separate grounds.

John

 

Also remember that a schematic often is a simplified drawing. Then it comes to building the hardware the connections in the schematic do not have to replicated 100% as drawn in the schematic. If you build your voltage regulator on breadboard or PCB you will probably select a star ground topology

 

If you are using a mixing high power devices and logic, it is a good idea to run the respective grounds separate from each other to the respective devices, i.e. one ground to the high power circuits and another to the logic devices. Both would be tied together at the actual power supply. Reasoning for this it to keep the high power devices from imparting unwanted voltages into the ground line of the logic devices.