A20C AC Two-Wire Proximity Switches: Retrofitting Panels With No DC Supply

A great deal of working plant in India was built around AC contactor logic. There is no 24 V DC rail, no PLC input card, and no convenient place to get one — just a contactor coil that needs switching and a mechanical limit switch that has finally worn out.

Replacing that limit switch with a modern DC three-wire proximity sensor means adding a power supply, finding panel space, and running a new circuit. The A20C exists so you do not have to.

What a two-wire AC sensor is

It wires exactly like a mechanical switch: in series with the load, two wires, no separate supply. The sensor takes its operating power through the load itself. When it switches, current flows to the contactor coil; when it does not, only a small residual current flows.

That makes it a genuine drop-in replacement for a limit switch, a reed switch or a microswitch, with no panel modification at all. For retrofit work on ageing plant this is frequently the difference between a half-hour job and a rewiring project.

The A20C range

Eight body and sensing combinations, each available normally open or normally closed:

• M12 — 2 mm and 4 mm sensing
• M18 — 5 mm and 8 mm
• M30 — 10 mm and 15 mm
• M36 — 20 mm
• M50 — 30 mm

All are two-wire, rated 200 mA. The ordering code follows the house pattern:

A20C-1808-M-2 — M18 body, 8 mm sensing, normally open (M for make), two-wire. Substitute B for break to get the normally closed version.

The two things that catch people out

Two-wire AC sensors have two well-known characteristics. Neither is a fault, but both cause confusion when they are not expected.

Residual current. Because the sensor powers itself through the load, a small current flows even when the sensor is off. In most cases the contactor coil ignores it. With a very low-burden load — a solid-state relay input, an electronic timer, some PLC AC input cards — that residual current can be enough to hold the load partially on, or to make an input read permanently true.

The symptom is a contactor that hums, or an input that never clears. The fix is a bleeder resistor across the load to give the residual current somewhere to go. Check the minimum load current your device requires before ordering.

Voltage drop. The sensor drops a small voltage across itself when conducting, because it needs that to operate. With a single sensor this is immaterial. Put three in series for an interlock chain and the drops add, and the coil at the end may not see enough voltage to pull in reliably.

If you are chaining sensors, add up the drops and compare against the coil pull-in voltage. Where the sum is marginal, use DC sensors and a relay instead. Our note on series and parallel connection of proximity switches covers the arithmetic.

When to use AC two-wire, and when not to

Use it for direct replacement of a worn mechanical switch in an AC panel, where the load is a contactor or solenoid with a normal burden, and where you want to avoid adding a DC supply.

Do not use it where the load is electronic and low-burden, where several sensors must be chained in series, where fast switching is required, or where you are building new. On a new machine, specify DC three-wire from the D30C range — more output options, no residual current, no voltage drop, and better diagnostics.

Why this range still matters

Because a very large amount of Indian manufacturing capacity was installed before DC control became standard, and that plant still has to run. A machine from the 1990s with a failed limit switch does not need a control system upgrade. It needs a sensor that fits the wiring that is already there.

The A20C is made in Mumbai to IEC 60947-5-2, so obsolescence is not a concern in the way it is with an imported range that may be rationalised away.

Ordering

Send the body size the existing mounting takes, the sensing distance needed, whether the function is normally open or normally closed, the AC supply voltage, and what the sensor is switching — ideally with the coil rating. If you are replacing a mechanical switch, a photograph of the installation usually answers the mounting questions faster than measurements do.