How do I go about dimensioning a diode to suppress turn off spikes on a relay coil and what considerations are there apart from coil current.

I have a 12v 100amp power relay that I wish to use a diode across the coil of to minimise interference from inductive spikes fom the coil when the relay is turned off.

I do not yet know the coil current rating due to supplier intransigence, but a generic answer will assist me in using the correct diode in due course.


The following relates to interference suppression caused by the relay coil.
The relay contacts may also need 'suppression' - see at end.

The maximum current that will flow in a protection diode is the current flowing in the coil at switch off. So a diode rated at Icoil max will be more than adequate. If the relay is de-operated only occasionally a diode rated such that turn off current is somewhere between the diode's continuous and surge current ratings may suffice.

This Digikey search lists relays rated for 100A DC or more at 12V.
The maxumum coild current is 3.3A for a Panasonic 300A switchingrelay
and under 500 mA for a superb OMRON GPEA-1 relay rated to switch 100A at 120 VDC, and 60A at 400 VDC.

An inductry standard 1N400x diode is rated at 1A continuous, 10A for 1+ seconds and 30A one off surge (needs a nice strong cup of tea and a sit-down before repeating). So a 1N400x duode would work without question for the 500 mA coil current relay and almost certainly for the 3.3A coil current relay.

A spike suppression diode should be mounted as close electrically and physically to the coil as is reasonably possible. Ideally across the relay or socket terminals. This minimises reradiating loop area for EMC and minimises inductance in the diode current path which might otherwise allow very short period spikes.


Relay contacts and switching mechanism for a properly rated relay switching DC will usually be designed to allow a minimum of arcing at the contacts. Voltage spikes will still occur if any inductive load is present and a suppressor to deal with this may be needed. This may be a di

Relay contact snubbers - image links and text links

  • \$\begingroup\$ One other factor is how fast you need the contacts to open. The relay coil is an inductor and V=dI/dt. The low ~ 1V drop across a diode means the coil current will ramp down somewhat slowly. Adding a series resistor to the diode generates a bigger negative voltage, but the contacts will open faster. \$\endgroup\$ – Evan Nov 12 '16 at 3:52
  • \$\begingroup\$ @Evan - yes, needs to be considered in some cases. Very approximately, as Energy stored = 1/2.L.i^2 and as dissipation = VI.t and as V~= 1, 1/2.L.i^2 = i.t so t ~~~= L.i/2. ie decay time is proportional to L and to i. \$\endgroup\$ – Russell McMahon Nov 12 '16 at 7:00

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