M12 Proximity Sensors: Dimensions, Sensing Range and Termination

M12 is the size you specify when an M18 will not physically fit. Tool changers, small pneumatic actuators, indexing tables and anywhere a bracket has to sit inside a guard. It gives up sensing distance in exchange for the space it saves, and that trade is the whole decision.

Thread and dimensions

M12 proximity sensors use a fine thread, M12 x 1, not the coarse M12 x 1.75 of a general fastener. As with M18, the fine pitch exists so the locknuts give you fine adjustment of the sensing gap.

Order replacement locknuts as M12 x 1 explicitly. A hardware shop will hand you coarse by default and it will not run on the barrel.

Standard barrel lengths run around 50 to 60 mm for the cable version and longer for the connector version, because the M12 connector adds roughly 10 mm to the back end. If you are designing the bracket before the sensor arrives, allow for the connector and the mating cordset bend radius behind it, not just the barrel.

Sensing distance: 2 mm or 4 mm

Our D30C inductive range offers M12 in two variants:

• 2 mm, shielded. The face can be mounted flush in a metal bracket. Surrounding metal does not trigger it.
• 4 mm, unshielded. Double the distance, but it needs a clear zone around the face and cannot sit flush in metal.

Those are rated distances against a standard mild steel target. Design to around 70 to 80 percent of the rated figure, so 1.4 to 1.6 mm on the 2 mm part and 2.8 to 3.2 mm on the 4 mm. A sensor set at the limit of its range passes the commissioning test and goes intermittent the first hot week of May.

Then correct for your target material. Stainless steel, aluminium and brass all read short, and on an M12 the margin is already small enough that a reduction factor can take you below the gap you built. Our reduction factor table has the arithmetic.

Mounting clearance

This is where M12 catches people. The clearances scale with the body, so they are tighter than the M18 figures you may be used to:

• Shielded (2 mm): the face may be flush with surrounding metal. Keep the opposing metal, anything the sensor is not meant to see, at least three times the rated distance away.
• Unshielded (4 mm): the face must stand proud of the bracket, and you need a clear annular zone around it roughly three times the barrel diameter.
• Side by side: two M12 sensors facing the same way need a centre to centre spacing of at least twice the barrel diameter, or they will interfere with each other.
• Facing each other: at least six times the rated sensing distance.

If you cannot hold the unshielded clearance, use the shielded 2 mm part and move the bracket closer. That is almost always the better answer to a crowded installation.

Output and wiring

M12 is available as PNP or NPN, normally open or normally closed, three wire with short circuit protection. In India most new panels are PNP sourcing, because that is what Siemens, Allen Bradley and Mitsubishi input cards expect by default. NPN still turns up on older Japanese machinery and on retrofits where the existing card is sinking.

Check the card before you order. Our guide to PNP against NPN covers how to tell which you have, and two, three and four wire wiring covers the terminations.

There is also an AC two wire M12 in the A20C range, 2 mm and 4 mm, 200 mA, for direct switching of contactors and older relay logic where there is no DC supply in the panel.

Cable or M12 connector

Confusingly, an M12 sensor can have an M12 connector, and the two M12s are unrelated. One is the barrel thread, the other is the connector standard.

Specify the connector version where the sensor is removed for changeovers, because a fixed cable dragged through a machine frame fails at the gland eventually. Specify fixed cable, cut to the length you actually need, where the sensor is fitted once and left. We cover the trade in more detail in choosing the termination.

Ordering an M12

The code reads left to right. D30C-1204-PM-3S is an M12 body, 4 mm sensing, PNP, normally open, three wire with short circuit protection. Change 1204 to 1202 for the 2 mm shielded version, PM to NM for NPN, or PM to PB for PNP normally closed. The full ordering code guide decodes every position.

M12 is also available as a NAMUR version in the D20C range at 4 mm, for intrinsically safe loops, and as a D33C speed and direction sensor at 30 mA for underspeed monitoring.

When M12 is the wrong choice

If you have the room for M18, take it. Four millimetres of sensing distance is not much margin once you account for target material, thermal drift and a bracket that moves slightly under load. M12 is for when the space genuinely is not there, not for saving a little money on a sensor that then needs setting twice a year.

Going the other way, if you need more standoff than M12 gives, look at M30 at 10 or 15 mm, or the extended range parts.