The other day I was playing with a ULN2803a and a stepper motor. I spent ages trying to work out why it wasn't working and why i was reading ~14V as an output on the PIC I was using. So why did I have to connect the 0V of both supplies together?
Thanks in Advance,
Dean
Electronic – Why do 0v have to be connected when using two power supplies and ULN2803A
power supplytransistor-array
Related Solutions
They come from the mains, but they're isolated from it by a transformer. You're correct about the complete circuit, because that the condition to have a potential difference (just say voltage difference, that's also OK).
If you would take a transformer and measure the resistance between a contact on the primary and a contact on the secondary you'll see that the resistance is very high, almost infinite. There's no closed circuit. If you would measure the voltage between the pins of power supply A you would have a closed circuit: from one pin through the multimeter's resistance back to the other pin.
A closed circuit is the condition.
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However! Note that this is for DC. AC signals can pass though transformers and capacitors, which also would block DC.
The very same schematic was used for the original Uno32 from chipKIT. As it happened, someone managed to blow it up by applying 15V to the barrel jack. That was perfectly within the specs of the regulator, but the comparator ejected all the Magic Smoke™ and ceased to work.
After some diagnosis of their board I did the same calculations as you and came to the same conclusion. 15V, while within the limits of the regulator, overwhelmed the input of the op-amp.
I subsequently recommended a pair of clamping diodes on that input (now in Rev C of the Uno32), and since then there have been no reports of dead boards from too much input voltage.
(Note: I don't know if it's just a labelling problem in the PDF version of the schematic, but the VIN on the upper diode should actually be Vcc so it's clamped to the same supply voltage as the comparator.)
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Best Answer
Just because two different supplies both have a terminal labeled 'ground' doesn't necessarily mean that those nodes are at the same voltage. In fact, if you have two supplies that have isolated output, you could have almost any potential between their ground lugs, unless you strap them together. You could find a DC offset or even significant AC between the grounds of two isolated supplies. To establish a proper relative voltage levels within your system, each component needs to have a common voltage reference, and usually that's the ground node, so you connect them for that purpose.