What a relay contributes to a circuit
A relay lets one electrical domain control another: a small current switches a larger one, a low-voltage logic line controls a high-voltage rail, and a DC signal can close an AC loop. The part that design engineers usually care about most is the galvanic barrier between control and switched sides — without it, a fault on the load side can propagate straight back into the control electronics. That combination of level translation and isolation is why relays turn up across automatic control, remote switching, protection, instrumentation and industrial automation rather than in any single niche.
Where conventional electromechanical relays run out
A classic electromagnetic relay pulls an armature against a core with a coil. The principle is forgiving and the parts are robust, but the mechanics impose a floor on size and speed, and a pivot-and-contact assembly that moves on every cycle wears. As boards shrank and channel counts climbed, that floor became harder to live with. The reed relay was an answer to that specific set of constraints rather than a universal upgrade — for high-current mains switching an EMR or contactor is still the usual choice.
The reed capsule, sealed at manufacture
The reed switch dates to Bell Laboratories in the 1940s. Two ferromagnetic blades (the reeds) are sealed inside a glass capsule in a controlled atmosphere and close when an external field — from a coil or a magnet — pulls them together. Used bare it works as a proximity sensor; wound with a coil it becomes a relay. Sealing is the defining feature: the contacts never see ambient air, humidity or flux fumes, so there is no contact oxidation and no mechanism open to contamination. Against solid-state switches the trade runs the other way — a sealed mechanical contact tolerates surge and inrush that would stress a semiconductor, at the cost of moving parts.
Insulation and leakage at the bottom of the scale
Insulation resistance in a sealed reed contact reaches 1015 Ω, which puts leakage in the region of 10-15 A. That figure matters most where a switched node sits next to a human body: intracardiac leads, pacemaker pathways and patient-connected probes, where a leakage current measured in single microamps is already enough to disturb cardiac tissue. It also matters at the other end of the bench, in electrometer and ionization circuits where the signal itself is only a few femtoamperes.
Signals an air-exposed contact cannot hold
Because the contacts close in an inert, dry atmosphere, a reed relay switches signals down to femtoampere (10-15 A) currents and nanovolt (10-9 V) levels without the contact film and oxidation that offset an open-frame EMR. Lifetime follows from the same construction. A pivoted armature relay is typically worn out around a million operations as its hinge and contacts degrade; a reed contact has no hinge or sliding surface, and under signal-level loads it is rated toward one billion operations. The caveat is that “signal-level” does the work in that sentence — rated life drops as switched current and arcing rise, so the billion-operation figure should not be read across a power-switching duty.
Specification range of SHR AUTOSENSOR relay solutionss
- Mechanical life toward 109 operations at signal level
- Up to 5 poles in multi-pole constructions
- Contact forms: A (NO), B (NC), C (SPDT break-before-make), D (SPDT make-before-break) and E (bistable/latching)
- Contact resistance down to roughly 50 mΩ
- Insulation resistance up to 1015 Ω
- Switching to 10,000 V, with high-current carry versions
- Signals down to 10 nV and into the femtoampere range
- Signal bandwidth up to 6 GHz
- Operate time in the 100–300 µs range
- Operating temperature -55 °C to +100 °C
- Sealed operation in air, water, vacuum, oil, fuel and dusty environments
- Shock to 200 G; vibration to 30 G over 50–200 Hz
- Small outline with standard pin-outs
- Choice of package, capsule, coil resistance and optional magnetic shield
Where these parts are actually specified
In practice the parts land in two broad groups of designs. One needs a dense, fast, long-life signal switch: in-circuit and semiconductor test, multiplexed data acquisition, telecom and instrumentation. The other needs a small isolated contact that holds high voltage or survives a hostile medium: HV power supplies, medical equipment and safety-related automotive circuits. The common requirement is a switch with the isolation of a mechanical part and closer to solid-state speed, size and life — the point at which designers tend to reach for a reed relay rather than an EMR or a semiconductor switch.
About SHR AUTOSENSOR
SHR AUTOSENSOR TECH LIMITED (SHR AUTOSENSOR) manufactures high-voltage reed relays, mercury wetted relays and high-voltage DC contactors. Contact: [email protected] | +86 13761571029 | www.reed-relay.com