I'm wiring some computer fans behind the fridge in my camper-van to help increase ventilation. As part of this circuit, I have a temperature controlling microcontroller plus relay and a series of SPDT switches to control various aspects of this circuit. Originally, I had simply planned to put all the switches and the microcontroller relay on the hot +12V side of the circuit, but it occurs to be that I could put all this on the ground side of the circuit as well.

This is a vehicle, so the body is tied to ground, but I'm not using that fact for this circuit (i.e., nothing in the circuit attaches to the frame, it's all explicitly wired even for the ground return).

So is there any practical difference between say this circuit:


simulate this circuit – Schematic created using CircuitLab

... and this one, which is identical except that the switches and relays are all on the ground side.


simulate this circuit

One potential advantage, I suppose, of putting all the control on the ground side is that keeping the hot side simply (a single return wire) decreases the chance of a short to frame, since all the switches and related wiring are at ground potential.

  • \$\begingroup\$ Is the metal on the fan housing grounded? \$\endgroup\$
    – Andy aka
    May 13 '16 at 16:47
  • \$\begingroup\$ The fan is all plastic and has no grounding. \$\endgroup\$
    – BeeOnRope
    May 13 '16 at 17:01
  • \$\begingroup\$ With option 2, when the circuit is "off" the fan terminals are hot. That could be a safety issue during maintenance or testing. \$\endgroup\$
    – The Photon
    May 13 '16 at 17:04
  • \$\begingroup\$ I'd just wire it so it disconnects whichever lead isn't chassis, that way, if you short one of the fans to chassis when the master switch is off, you won't get any spectacular fireworks as power can't flow from one terminal, through chassis and back to the main battery. other than that, 12V is pretty safe, (and if it's already fused) it'd be pretty hard to cause any major mishaps regardless of whether it was high or low side switching - that's more of an issue with mains stuff \$\endgroup\$
    – Sam
    May 13 '16 at 23:35
  • \$\begingroup\$ Right, well the all-plastic fan with a handful of internal components seems less likely to short than the various switches and the relay, which all have exposed metal components. Keep in mind that the normal mode for this circuit is to be "on" pretty much all the time, so the potential for short circuits when running is also a concern. Perhaps a hybrid approach is best, where there is the one master switch on the hot side, and then all the other switches and the relay are on the ground side? \$\endgroup\$
    – BeeOnRope
    May 14 '16 at 1:07

The problem with switched negative (or neutral in the case of AC) circuits is that the load is "live" when switched off. In normal use this shouldn't problems but during maintenance, for example, it can cause some confusion and danger. There are plenty of precedents for your approach in auto circuits where circuits are switched to ground: e.g., oil-warning lamp, interior lighting door switches, etc.

The negative switching arrangement is popular in electronic control in the form of NPN open-collector transistor outputs for driving relays and motors. Very often the ability to switch higher voltages than the controlling circuit is the primary factor in choosing this approach.


simulate this circuit – Schematic created using CircuitLab

Figure 1. Negative side switching (always "live") and positive side switching (always grounded).

Taking a simple example of changing a light bulb, the negative switching circuit leaves the lampholder live whereas the positive switching circuit leaves it dead. At 12 V it doesn't matter too much unless there's a risk of shorting the live terminal to earth.

As good practice, I would be inclined to wire switches in the positive line.

  • \$\begingroup\$ Wait a day or so before accepting my answer so you don't discourage other replies. You may always get a better one! \$\endgroup\$
    – Transistor
    May 13 '16 at 19:35
  • \$\begingroup\$ I'm on it. I'm an EE newb but not a stack overflow one :) \$\endgroup\$
    – BeeOnRope
    May 13 '16 at 20:22
  • \$\begingroup\$ Thanks for the answer - I put the switches on the hot side. \$\endgroup\$
    – BeeOnRope
    Jun 2 '16 at 20:33

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