AEDs sit idle for years and must work once
An automated external defibrillator detects shockable cardiac arrhythmias and delivers a controlled shock, and it is built to be operated by an untrained bystander rather than a clinician. That is why the devices are placed in airports, stations, offices, schools, gyms and homes: in a sudden cardiac arrest they are often the only available therapy before an ambulance arrives. The consequence for the design is unforgiving — a component that is rarely exercised still has to work on the one occasion it is needed.
What battery operation imposes on the design
A portable defibrillator runs from a battery and may stand idle for months or years, so every stage of the charging electronics is optimized for standby current. The same circuitry also has to be unusually reliable, because the device charges a capacitor bank to high voltage and then delivers that energy through the patient’s chest. A switch in the HV path has to hold several kilovolts, leak almost nothing while the device sleeps, and close on demand for the whole service life.
The relay’s position in the energy path
Inside the AED the DC supply charges a high-voltage capacitor bank, and switching elements route the stored energy either to the patient electrodes or to an internal safety-discharge path. SHR AUTOSENSOR high-voltage reed relays are used directly in this charging and routing chain.
Why the part fits this circuit
- High voltage in a small body. A small sealed contact gap holds and switches kilovolt levels, which keeps the AED light enough for wall mounting or an ambulance kit.
- Sealed contacts. With no exposure to air the contacts do not oxidize or collect contamination through years on a public wall.
- Very low leakage. High insulation resistance holds standby leakage to a minimum — a requirement that is unusually strict in a patient-connected device.
- Low coil power. The coil draws current only during the brief charge sequence, which protects battery reserve over long idle periods.
- Long shelf-and-cycle life. With no pivoting or sliding wear surface, the part matches the maintenance-free profile a public-access device needs.
A duty profile of standby, not cycling
The AED profile is the opposite of a production tester: almost the entire life is standby, with a single decisive event at the end of it. That rules out components whose parameters drift while idle — oxidizing contacts, lubricants that dry out, or semiconductor leakage that grows with age. A sealed reed contact cannot oxidize, has no wear mechanism at rest, and the coil is energized only during charging. The controller’s periodic self-test can exercise the relay at negligible battery cost to confirm readiness, and the same low leakage that protects the patient keeps the capacitor bank from slowly self-discharging between inspections. Across a service life of a decade or more, the charging circuit needs no relay-level maintenance.
Design takeaway
A defibrillator asks for high-voltage switching, near-zero standby current and high reliability in a small, maintenance-free package at the same time. Sealed HV reed relays meet those requirements in the charging and routing path, and the same reasoning carries over to electrosurgical generators and imaging systems where a patient-connected HV path has to hold its isolation for years between service calls.
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