Connecting the grounds of two power supplies

HI

Can the grounds of two different power supplies each supplying different voltages be connected?

If not, why?

And if they can...

what about the theory of isolation? For instance in the case where a microprocessor operating at 5V is connected to a device using 240V, the have to be isolated from each other?

Yes the two grounds can be connected to each other, but both should be DC voltages

I am not aware of any standard that will prevent you from connecting grounds of different power supplies ..
Of course, you have to implement safety precautions not to expose users to dangerous high voltages ..
As an example have a look at light dimmers, touch-button light switches (here you touch mains through some MΩ resistor) etc. etc.
They have mixed low and high voltage ground and to keep their size small, no isolation transformers or optocouplers are used ..
However, my advice is that whenever it is possible always use proper isolation ..
Regards,
IanP

With DC power supplies this is usually not a problem and sometimes a requirement. Keep in mind though with the proliferation of switch mode power supplies simply connecting the ground of two power supplies may cause strange side effects as it's a way for the ripple to interact inbetween units when they're under curent draw. As far as 5 volts vice 240V's... Common sense should kick in at some point. A 240 V power supply with normal wireing while under load is going to draw some pretty impressive pulse wattage and gonna screw with the 5 volt side pretty badly. Meanwhile if you're drawing high current on the 5 volt side it may likewise cause voltage fluctuations on the 240v side. No wireing is a perfect ground so things will always flluctuate, it's mostly a matter of how much current you're drawing. All sorts of unwantted reactions could take place. For common electronics applications it's generally safe to use a common ground for things like multiple power supplies in the range of -12 to +12 volts or so, the less current you're dealing with the better. What worries me is it sounds like you're using rectified mains voltage and logic voltage totally un issolated from each other.... bad idea. Lots of smoke and melted metal possibilities =)

isolation is very necessary & ur appllications will run properly only if u have connected the grounds of two or more circuits for which the 'voltage level' to recognize the ground is same otherwise u can make damage

hi:idea:
To isolate a digital source From a Uc you can use OPTO coupller.
To isolate a sine wave you can a transformer easily.
To isolate an analoge source , use this method


................VCO ............OPTO.............filter or PLL.....................
analoge -------->Freq. ---------->Freq. ------------->analoge
................................................................................................

Honeybee said:

HI

Can the grounds of two different power supplies each supplying different voltages be connected?

If not, why?

And if they can...

what about the theory of isolation? For instance in the case where a microprocessor operating at 5V is connected to a device using 240V, the have to be isolated from each other?

Click to expand...

Hi,
I think there is some confusion talking about ground here. If you have more than one voltage in your design, you will normally not talk about ground but of 0V. If your two powers are both positive supplies, lets say +5V and +12V, you connect the minus of both powers together and call it 0V.
If your two powers are opposites, like +5V and -5V, you will connect the positive terminal from one power to the negative terminal of the other, and call it 0V.
Normally you then connect the 0V to the ground, if to anything at all. If the ground is protective, it may only be connectd to the metalic case of the mechanical construction. The voltages inside will then float relative to the ground. This always depend on the type of equipment in use. Geound loops is a big issue in many systems.

If your 240V are AC, you will not normally connect this together with a low voltage DC power system. For DC it is normal to use the same reference 0V if the high voltage is i galvanic connection with/ referenced to the rest of the circuit.

TOK