NPN vs PNP

sticking with one type is good

European machines are more likely to use sink logic

to me sink logic is more likely to cause confusion

I've seen equipment that had sink logic installed on some of the AC circuits

That'll get your attention when it's time to trouble shoot

My preference is to switch +v and have 0v grounded. I had a discussion paper some time back that went through all the pro's and con's with both NPN and PNP inputs.

My way of thinking is that if I have a wiring fault in the field I would rather see a fuse trip rather than a false trigger. It would all depend on your requirements and risk management. And like Garthh mentioned, troubleshooting is 'trickier' with NPN 0v grounded system.

Just to add another point, if you dont have a special requirement to float the control supply then ground th 0v. Floating supplies can be troublesome, and dangerous.

Cheers,

Tooz.

I have to disagree with some comments here, in a CR4 friendly manner of course!!

My personal preference is always for NPN because its usually far easier to provide a positive switching signal, usually via a resistor/transistor from the plus of the power supply, than to easily supply a proper negative level signal for PNP, that usually must dip below the minus of the power supply, though not always..........

It also helps my thinking as switching with a "NOT" signal (as is a low level/negative active signal is correctly described) is easily forgotten when problems arise and in the heat of the moment.....(been there done that and "NOT" got the T-Shirt!!).

If fault finding with a standard Multimeter, with NPN logic you connect the negative lead to negative of the supply as usual and search with the positive probe for signals - as usual......

For PNP, you usually need to connect the multimeter to the positive side of the PS and use the negative lead to search for signals (do remember that no/missing signal will often look the same as 0 volts signal otherwise, though they are NOT the same, if you connect the multimeter to the minus or ground of the supply as usual)....its a "thinking" problem....at least for me....so I always try and think "positively", (awful pun, sorry!!!) though at various times I have used PNP as there was nothing better available.....

At the end of the day the OP must go with what he finds easier to design for.....

I hope these comments help....

Generally on industrial equipment in USA the - [minus] is tied to the case ground & so is the white [neutral] wire

If you are trouble shooting with a wiggy, one probe get stuck to ground & you merrily poke about with the other, clack, clack, clack. you can feel when things are hot, through the meter & may assume that just because there is voltage present components are active

an electrical, industrial sensibility [relay logic]

Andy's is describing the more careful electronic sensibility, where there may be not logical elements present & as he points out NPN is allows transistor level solutions

Neither right or wrong

my initial comments are based on GG's description of the projects use as automation equipment in a machine tool environment

though it is always entertaining to see a trainee tech encounter negative logic in an industrial environment for the 1st time

Your last line is SOOO correct and true!!