How to Choose a Reed Switch: 4 Stages of Type and Contacts, Magnet and Operating Distance, Load and Electrical, and Environment and Mounting

To choose a reed switch, first decide the type and contacts (reed or electronic; normally open, normally closed or changeover); then match the magnet and operating distance (sensitivity, magnet direction, operating range); next check the load and electrical conditions (voltage, current, inductive loads and inrush current, cable length); and then deal with environment and mounting (the glass tube’s sensitivity to shock, nearby magnetic fields, mounting position). This article focuses on position detection on cylinders and mechanisms; coil-driven reed relays are covered just in the supplement below. It is divided into 4 stages:

• Stage ① Type and contacts: reed or electronic, NO, NC or changeover, wiring.
• Stage ② Magnet and operating distance: sensitivity, magnet direction, operating range and hysteresis.
• Stage ③ Load and electrical: voltage and current, inductive and lamp loads, protection, cable length and polarity.
• Stage ④ Environment and mounting: glass tube and shock, nearby fields and iron filings, life, checking on the machine.

This article starts from the point where you have decided to detect position with a magnet and a switch. This describes the general case; individual cases still need their own assessment.

Figure 1. Four stages (illustration, not to scale): ① type and contacts (reed or electronic; NO, NC, changeover; 2-wire or 3-wire) → ② magnet and distance (lower ampere-turns operate more easily; place the magnet as the data sheet shows; mount at the center of the operating range, allowing for hysteresis) → ③ load and electrical (within the switch rating; protect against inrush from inductive and lamp loads; protection box for long cables) → ④ environment and mounting (don’t drop it or pull the leads; space cylinders apart; check operation on the machine and adjust).

Stage ①: Type and contacts: reed or electronic first, then NO or NC

• First the use: position detection at a cylinder’s stroke ends (the piston must carry a magnetic ring), or a standalone sensor detecting a magnet on a mechanism.
• Type: a reed switch has two reeds sealed in a glass tube that close in a magnetic field; an electronic type uses a magnetic element plus a circuit output. They differ in leakage current, shock resistance and operating time (Figure 2).
• Contacts: normally open (closes in a magnetic field; common), normally closed (opens in a field), changeover (break before make).
• For electronic types, also check 2-wire or 3-wire; confirm the controller-side wiring first.

Figure 2. Reed vs. electronic (illustration): reed: reeds sealed in glass, mechanical contacts, no leakage current, more sensitive to shock (the glass breaks under stress). Electronic: magnetic element plus circuit, circuit output, leakage current and voltage drop on 2-wire types, more shock-resistant, polarity matters on types with an indicator lamp.

Sources for the above: a fluid power publication’s cylinder design guide (2026), an online encyclopedia entry, and an electronic component distributor’s guide (2026). A pneumatic component maker’s sensing switch catalog (undated) says reed types detect the field with a reed switch and electronic types with a magnetoresistive element; operating time is 1.2 ms for reed and 1 ms or less for electronic; shock resistance is 300 m/s² for reed and 1,000 m/s² for electronic. Another catalog from the same maker lists leakage current as none for reed, 10 μA or less for 3-wire electronic and 1 mA or less for 2-wire.

How JIAHON can help:

• A site product search (13 October 2026) for “reed” found 8 reed switch product pages, 6 of which are used in the stages below. For specifications not on the product pages, please ask.
• Reed switch series (multiple models): the page lists reed types with contacts (reeds close, mechanical contacts conduct) and contactless solid-state types (electronic elements sense the field), and says to confirm the cylinder sensor slot type and size, lead direction, lead length, 2-wire or 3-wire output and operating voltage when selecting.
• Electronic magnetic switch (NPN 3-wire): electronic, no mechanical contacts, transistor output; the page’s own comparison says reed types are 2-wire mechanical contacts, while 3-wire electronic types need a separate supply and a PLC input matching NPN or PNP; maximum switching frequency 1,000 Hz electronic vs. 200 Hz reed. 3 models (2 body cross-sections, 2 m and 3 m leads).
• Reed switch series (for level gauges): not for cylinders; signal elements mounted on the outer tube of bypass level gauges, sensing the float magnet; 4 models, single-pole normally open, 3 of which list maximum contact ratings (10 W, 10 W, 50 W). Use on cylinders is not on the page; please ask.

Stage ②: Magnet and operating distance: sensitivity, direction and range

• Sensitivity: expressed in ampere-turns (AT); the lower the number, the more easily the switch operates; for example, reed switch sensitivity can be specified at 18 to 22 AT.
• Magnet: the magnet’s type, strength and position decide the distance it operates at; on cylinders, the reed switch senses the magnet (magnetic ring) on the piston, so first confirm the cylinder has one.
• Direction: the magnet’s direction relative to the switch is an important consideration; place it as the data sheet shows.
• Operating range and hysteresis: operate and release points are not at the same place; for example, hysteresis of 2 mm or less for reed and 1 mm or less for electronic; the operating range changes with the surroundings.
• Mounting position: mount at the center of the operating range, not at its edge; after mounting, check operation on the machine and adjust.

Sources: the distributor’s guide (AT and magnet choice, some magnets especially strong or with deeper penetration); a component maker’s application note (copyright 2008; magnet direction); a reed component maker’s white paper (undated; operate point measured as switch-to-magnet distance; 18 to 22 AT); the pneumatic maker’s catalogs (hysteresis, center mounting, edge mounting unstable, check on the machine).

How JIAHON can help:

• Reed switch (3-wire, 5 to 30 V): reed technology; typical response sensitivity 3 mT, typical over-range distance 10 mm, repeatability 0.1 mT or less; fits T-slot, square rod, tie rod, round and dovetail cylinders.
• Reed switch series (two models): both reed type, normally open; the square model has 0.2 mm repeatability and 0.5 ms or less response; the round model’s values for these are not on the page, please ask.
• Reed switch (2-wire, 5 to 120 V): standard and high-sensitivity grades with the same electrical specifications, differing in the field strength threshold; choose high sensitivity for cylinders with weaker magnets.

Stage ③: Load and electrical: enough voltage and current, and protection for inductive and lamp loads

• Check ratings: the load’s voltage and current must not exceed the switch rating; reed types are mostly for low current. For example, one reed type allows a maximum load current of 50 mA at 24 V or less, 40 mA at 48 V or less, and 20 mA at 100 V or less.
• Inductive, lamp and capacitive loads: inrush current at closing stresses the contacts; lamp inrush may be 10 to 15 times rated current. With such loads, add a protection circuit.
• Protection box and cable length: for example, use a contact protection box for inductive loads and for cables 5 m or longer, with no more than 1 m between the box and the switch.
• 2-wire electronic types have leakage current: for example, 1 mA or less; the switch also has a voltage drop, for example 4 V per switch on 2-wire types, which must be counted when two are in series.
• Types with an indicator lamp: for example, 24 V DC lamp types have polarity; take care not to reverse them.

A relay maker’s technical note (undated) is the source for the inrush figures and adds that at opening, watch how fast voltage appears; the reed component maker’s white paper says reed switches can switch loads from a few μW to 120 W; the pneumatic maker’s catalogs also say not to use loads that generate surge voltages.

How JIAHON can help:

• Reed switch (2-wire, 5 to 120 V): 2-wire, load voltage DC or AC 5 to 120 V, maximum switching current 100 mA, contact rating 10 W, normally open, internal voltage drop 3.5 V or less, response frequency 200 Hz. Whether it is reed or electronic construction is not on the page; please ask.
• Electronic magnetic switch (NPN 3-wire): DC 5 to 30 V, maximum switching current 200 mA (30 V), maximum contact rating 6 W, internal current consumption 5 mA or less, residual voltage 0.7 V or less; reverse polarity and surge absorption protection.
• Reed switch (3-wire, 5 to 30 V): supply 5 to 30 V (AC or DC), 3-wire, normally open output, switching frequency 400 Hz.
• Reed switch series (two models): square model DC 12 to 27 V; round model AC or DC 5 to 30 V, maximum output current 500 mA (standard) and 80 mA (with mounting element).

Stage ④: Environment and mounting: the glass tube dislikes shock, and nearby fields should be avoided

• Glass tube: a reed switch’s glass breaks under mechanical stress; don’t pull hard on the leads in handling and wiring.
• Shock: for example, shock resistance data of 300 m/s² for reed and 1,000 m/s² for electronic; don’t drop or strike it.
• Nearby fields: space cylinders apart when side by side, for example at least 40 mm; magnetic shield plates or tape can reduce interference; avoid iron filings building up or magnetic materials close by.
• Temperature, moisture and dust: check the ranges on the data sheet; reed switches are sealed, and sources say they can be used in damp and dusty environments.
• Life: sources say several million operations or more; the larger the load, the shorter the life.

The distributor’s guide puts life in millions to billions of operations, depending mainly on switching voltage; the white paper gives −60 to 155 °C, a shock limit of 500 g and 1 million to 100 million operations.

How JIAHON can help:

• Reed switch (3-wire, 5 to 30 V): IP65 and IP67, shock 30 G (11 ms), vibration 10 to 55 Hz; PUR cable 0.3 m, usable in cable chains per the page.
• Electronic magnetic switch (NPN 3-wire): these 3 are standard types, IP64, −10 to 70 °C, shock 30 G; waterproof and oil-resistant types are separate, please ask.
• Reed switch series (two models): square model −20 to 70 °C, IP65; round model −40 to 70 °C, IP65 and IP68, oil-resistant, drag-chain PUR cable with 5 million bending cycles (75 mm bend radius).
• Reed switch (2-wire, 5 to 120 V): IP67, 2 m lead, red LED status indicator.

When you need to match a type, please prepare six items: cylinder or mechanism, reed or electronic, NO, NC or changeover, load voltage, current and type, magnet or ring position and distance, and site temperature and surrounding fields. You are welcome to contact JIAHON.

Supplement: reed relays vs. reed switches, and which specification columns to read

A reed relay drives a reed switch with an electromagnetic coil: energizing the coil creates a field that closes the reeds directly, with no separate armature. The reed switches above act when a magnet on a piston or mechanism comes near; a reed relay acts when a circuit energizes its coil, so its data sheet adds the coil figures.

Values depend on the model; follow its data sheet. For protecting inductive and lamp loads, see Stage ③. Sources: Wikipedia “Reed relay” (definition); Electronics Notes “Understanding Reed Relay Specifications and Parameters”; MEDER electronic “H Series High Voltage Reed Relays” data sheet (column names).

Cylinder position detection on SMT equipment

Printers, placement machines, conveyors and board loaders on an SMT line contain many small cylinders: stopper lifting, board clamping and support pin lifting all run on cylinders. The usual way for the controller to know whether a cylinder “has arrived” is a reed switch on the cylinder body sensing the piston’s magnetic ring.

Three situations come up often in line maintenance:

• The switch failed: replace with the same model. Check the cylinder slot type first (slots differ between brands and bores), then 2-wire or 3-wire, NPN or PNP, matching the existing wiring.
• The signal comes and goes. Usually the mounting position has shifted or the fixing screw is loose; where the machine vibrates heavily, retighten regularly and record it on the maintenance sheet.
• Positions that operate very often. For fast reciprocating stoppers and lifters, consider an electronic (contactless) magnetic switch, whose life is not limited by contact wear (see Stage ①).

During equipment maintenance, list each machine’s reed switch models and positions as a spare parts list, so failed switches can be replaced directly without waiting for parts. Related JIAHON pages: “SMC reed switch series” and “AirTAC DMS electronic magnetic switch.”

SMC, AirTAC and MEDER are trademarks of their respective owners. JIAHON is not the authorized distributor of SMC, AirTAC or MEDER in Taiwan.

Sources and limits

Sources: an online encyclopedia entry (multi-author); an electronic component distributor’s product knowledge page (published and modified 8 September 2026); a component maker’s application note (copyright 2008); a relay maker’s technical note (undated; document number includes “2003”); a reed component maker’s white paper (undated); two pneumatic component maker’s cylinder sensing switch catalogs (undated); a fluid power publication’s cylinder design guide (26 January 2026). No standard texts or JIAHON product data sheets were read.

Limits: sensitivity, hysteresis, rated current, inrush multiples, shock, spacing, temperature and life are each source’s examples for one product or situation, not selection thresholds. On changeover construction, the encyclopedia describes one reed moving between NO and NC contacts while the distributor describes three reeds; the main text follows the distributor. Product details in “How JIAHON can help” were read from product page text on 13 October 2026, not verified by this article; follow the product pages and ask for anything not listed.

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