While using dc 12 power supply, If i connect only positive line to my electronic board and do not connect negative ,what actually happens ?


Nothing. You need a closed circuit with a voltage difference for electrical current to flow.

  • \$\begingroup\$ It's amazing how similar are the answer and I swear I didn't copy! \$\endgroup\$ – clabacchio Mar 16 '12 at 10:51

Whilst I agree with the other answers, I will point-out one exception. If your electronic board has outputs (or inputs) which are connected to another board which is connected to the negative terminal of your power supply, you might damage your electronic board. This is because current can find a path from the power supply positive to the negative terminal via the signal paths which were not intended to carry supply current.


I will add one more note of caution. When I first learned the fact that electricity required a closed path to flow, I figured that I could unwire a 240V mains plug without switching it off first. I thought "If I only touch one terminal no current will flow, right?".

Wrong. I was young and thought I was clever.

I can still remember my sense of disbelief as I was thrown across the room. I had of course failed to understand that the human body, and the ground on which it stands will also conduct current, especially where high voltages are present.

  • 2
    \$\begingroup\$ Oscilloscope probes might create another path that closes the supply circuit, given both the supply has an output connected to protective earth (PE) and the scope is a class I type of equipment. Nasty effects may happen. (Analogy, stolen from NPR's car talk, www.cartalk.com: If you need a long jump start cable and have only one pair of cables, you can connect both cables together, connect both batteries' + terminals with the extended cable and park the cars so that the bumpers touch and create the - connection. Works with old cars only because you need metal bumpers.) \$\endgroup\$ – zebonaut Mar 16 '12 at 14:28
  • \$\begingroup\$ I think that the OP is far from using instruments and/or multiple boards :) \$\endgroup\$ – clabacchio Mar 16 '12 at 14:37
  • \$\begingroup\$ @zebonaut - I have blown-up line drivers in the past by connecting a 'scope probe after my power negative had fallen off :( \$\endgroup\$ – MikeJ-UK Mar 16 '12 at 14:40
  • \$\begingroup\$ You've really traveled across a room without touching the ground? :D \$\endgroup\$ – clabacchio Mar 16 '12 at 14:48
  • \$\begingroup\$ @clabacchio - Oh yes :{} \$\endgroup\$ – MikeJ-UK Mar 16 '12 at 14:51

"The sound of one hand clapping".

This is usually intended as a joke answer when given BUT in this case there is some useful analogy.

Ask yourself: If you clap your hands together but only use one hand, what actually happens?


"Not connecting" one lead is the same as having a switch in that lead.

Ask yourself: If you have a circuit with a switch in the circuit and you turn off the switch (create an open circuit) what actually happens?


Charge is conserved. It cannot appear or disappear, which is why there can be no current in a disconnected wire. There will still be a 12 V voltage drop over the source, but since there is no current, it will not supply any power to the circuit. It is equivalent to hooking up 0 V source to the circuit, obviously doing nothing, and a -12 V terminal at the other end, which is disconnected, therefore also doing nothing.

  • \$\begingroup\$ If one touches an object which has a negative charge to one end of a copper wire whose other end is disconnected, some electrons will flow through all parts of the wire except the very end (more electrons will flow through the part near the charge source). The number of electrons which will flow into the wire is tiny compared to the number that could flow through the wire continually if there were a closed circuit, but that doesn't mean that most of the wire wouldn't, briefly, have a small amount of current flowing through it. \$\endgroup\$ – supercat Mar 16 '12 at 15:00
  • \$\begingroup\$ Absolutely right. Moreover, even after the negative charge has redistributed itself, there will be still be microscopic currents everywhere in the material as charged particles are kept in motion by a non-zero thermal energy. We could go even deeper and discuss these motions based on quantum physics. That would hardly be circuit level anymore though. \$\endgroup\$ – Marcks Thomas Mar 16 '12 at 21:09

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