I would like to ask a question related with what the title says.

In the past I have built systems which use two voltages. Say 12V and 5V. For that I used either two power sources, or one power source that gives both voltages. Of course the ground is joined.

I have also built systems of 5V only which are USB powered, meaning I dont use any power source, just the USB gives the 5V that my microprocessor needs.

My question is, what happens if I want to built a system in which the 5V part is given by the USB but the 12V by a power source. I am building this right now but, is there no problem that the grounds are joined?? I dont want to burn anything ....

 

If the 12 V supply is a stand alone unit, there will be no issues.

 

As post #2 says, its OK.
I do not know what your project is.?
But be aware that you may get unexpected operation if the sequence of applying the 5V[USB] supply and the external 12V supply is critical.

E

 

If the 12 V supply is a stand alone unit, there will be no issues.

What do you mean "stand alone". The unit provides 12V and 5V and works without problems. But I was planning of not using this 5V, and use the one provided by the USB, but of course the grounds will be common...
or maybe I should just use the power unit and not connect the Vcc pin of the USB?

As post #2 says, its OK.
I do not know what your project is.?
But be aware that you may get unexpected operation if the sequence of applying the 5V[USB] supply and the external 12V supply is critical.

E

which one should I apply first?
or what happen if I connect the USB and is working and then turn on the 12V...

 

I would like to ask a question related with what the title says.

In the past I have built systems which use two voltages. Say 12V and 5V. For that I used either two power sources, or one power source that gives both voltages. Of course the ground is joined.

I have also built systems of 5V only which are USB powered, meaning I dont use any power source, just the USB gives the 5V that my microprocessor needs.

My question is, what happens if I want to built a system in which the 5V part is given by the USB but the 12V by a power source. I am building this right now but, is there no problem that the grounds are joined?? I dont want to burn anything ....

As per posts #2 and #3 -- no problem -- Be very, very, careful, however, to avoid inadvertent connection of the +12V and +5V rails -- lest you express the 'magic smoke' from the system providing the USB supply...

Best Regards
HP

 

What do you mean "stand alone". The unit provides 12V and 5V and works without problems. But I was planning of not using this 5V, and use the one provided by the USB, but of course the grounds will be common...

With this new input, my answer is "No, there will be no issues"

 

maybe I should just use the power unit and not connect the Vcc pin of the USB?

Sounds like a plan!

 

which one should I apply first?
or what happen if I connect the USB and is working and then turn on the 12V...

What is the projects function.?

 

What is the projects function.?

Well originally I built (I think it is documented in the Project section of this forum) a system that through a PIC (using 5V) controls a series of LEDs (using 12V). To do this I used a transistor (Base connected to 5V, the rest to 12V). It works well.

Now I am adding USB functionalities to it (it is after all a 18F2550) so I am connecting the USB connection D- and D+ to the respective pins and the USB ground to ground. Now I was thinking of connecting the Vcc pin to the system 5V and letting the USB provides this.

But I am afraid it can have bad consequences so maybe I ll leave the Vcc pin not connected.....
I will use jumper pins to let my options open...

The project I was thinking will connect to the PC and then after turning on the 12V can control the LED series...

 

If you are just using the 12V to power LED's and not say robotic actuators etc, then it should be OK to power the logic from USB and then switch On the external 12V.

I have a link option for USB 5V or external 5V, which I use when debugging a circuit for the first time, using external 5V.

 

What do you mean "stand alone". The unit provides 12V and 5V and works without problems. But I was planning of not using this 5V, and use the one provided by the USB, but of course the grounds will be common...
or maybe I should just use the power unit and not connect the Vcc pin of the USB?


...

A typical wall wart would be "stand alone" - in most countries the regulations require it to be double insulated, usually that means if it has the old type iron-cored transformer; the primary and secondary each have their own completely separate bobbin, usually the primary will also have a clip on plastic shield so if there's a catastrophic failure that disintegrates the primary turns - they can't droop down and touch the secondary.

An earthed unit may have the GND terminal tied to the mains safety earth, in most cases this isn't a problem, but there is the possibility of stray current in the earth lead of one device or the other setting up a potential difference - its what can cause earth loop hum in audio gear.

Most installations these days have earth leakage breakers on the panel next to the electricity meter, so this problem should become increasingly rare.