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Switching the negative for relays

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Jason277

Member
Location
Longmont, Colorado
Occupation
Electrician
Why is the negative usually switched instead of the positive for 24v-12v DC relays for control wiring at VFDs, across line and soft start motor controls? Every pre-fab cabinet has me running my negatives to a dry contact. Why not switch positives?
 
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GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
When the negative is also the grounded common conductor, it makes sense to use dry contact or solid state switching which just has to ground one side of the controlled circuit to complete the circuit. That allows for more than one power source to be switched by the same array of control devices with a common grounded bus.
 
synchro

synchro

Senior Member
Location
Chicago, IL
Occupation
EE
The polarity of pulling a load to ground instead of to a positive supply is very preferable when using solid state devices. This is because N-channel MOSFETs, NPN transistors, and common IGBTs can be used which are cheaper, more prevalent, and higher performance than their complementary versions (P-channel, PNP, etc.) that are needed to switch a positive voltage for a load. This is because of the higher "mobility" of N-type vs. P-type silicon among other factors.
 
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