The Circuit Designer's Companion - diagramas.diagram...

Grounding and wiring 9PSUVB+PSB0V(B)VA+PSA0V(A)I 0V(3)I 0V(2) + I 0V(3)V SPCB3PCB2I 0V(2)shared wire, resistance R SFigure 1.6 Common power supply returninstead link the 0V rails together at PCB3 and PCB2, as shown in Figure 1.6?The supply return currents I 0V from both PSB/PCB3 and PSA/PCB2 now share thesame length of wire (or track, in a single-pcb system). This wire has a certain non-zeroimpedance, say for dc purposes it is R S . In the original circuit this was only carryingI 0V(2) and so the voltage developed across it wasV S = R S · I 0V(2)but, in the economy circuit,V S = R S · (I 0V(2) + I 0V(3) )This voltage is in series with the supply voltages to both boards and hence effectivelysubtracts from them.Putting some typical numbers into the equations,I 0V(3) = 1.2A with a VB+ of 24V because it is a high-power output board,I 0V(2) = 50mA with a VA+ of 3.3V because it is a microprocessor board with someCMOS logic on it.Now assume that, for various reasons, the power supply is some distance remote fromthe boards and you have without thinking connected it with 2m of 7/0.2mm equipmentwire, which will have a room temperature resistance of about 0.2Ω. The voltage V S willbeV S = 0.2 · (1.2 + 0.05) = 0.25Vwhich will drop the supply voltage at PCB2 to 3.05V, less than the lower limit ofoperation for 3.3V logic, before allowing for supply voltage tolerances and othervoltage drops. One wrong wiring connection can make your circuit operationborderline! Of course, the 0.25V is also subtracted from the 24V supply, but a reductionof about 1% on this supply is unlikely to affect operation.Varying loadsIf the 1.2A load on PCB3 is varying – say several high-current relays may be switchedat different times, ranging from all off to all on – then the V S drop at PCB2 would alsovary. This is very often worse than a static voltage drop because it introduces noise on