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Unlike solid-state devices, relays often (in my limited experience) do not come with maximum peak current draw, only maximum continuous current draws. In general, how well do relays take pulses of current over their rated continuous current draws?

Is this different between 'traditional' relays (like this one: http://shop.ciseco.co.uk/jqc-3f-t73-5v-pcb-relay/) and reed relays?

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The maximum current rating considers both current carrying capacity, and arcing during disconnects, which is also why you have separate AC and DC ratings (arcing is less of a problem with AC, as the arc would be interrupted during the zero crossing).

Short pulses that exceed the DC, but not the AC rating should be fine, but if you turn off at an inopportune time, that may shorten the lifespan a bit.

Exceeding the AC rating means warming up the conductors, especially at the point where the moving parts touch as that is going to be the narrowest point of the connection. If you weld the connectors together, the relay may fail closed.

If this is about inrush current after turning on, you can probably also add an inductor in series after the relay to give it a slow start, but keep in mind the inductor needs a flyback diode (so this can be used for DC only).

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  • \$\begingroup\$ I don't see an AC current rating on reed relays, like these ones: mouser.com/ds/2/357/105A_117SIP_107DIP_171DIP-6475.pdf . Are reed relays different? \$\endgroup\$ – dpdt Apr 13 '16 at 21:16
  • \$\begingroup\$ They're not that different, but they usually only control very small currents. I'd probably use them only to switch a transistor that switches a regular relay. \$\endgroup\$ – Simon Richter Apr 13 '16 at 21:26
  • \$\begingroup\$ Small currents? The reed relays I linked to are rated for 3A. 3A isn't small. \$\endgroup\$ – dpdt Apr 13 '16 at 23:29
  • \$\begingroup\$ Right, but typical usage is for smaller currents, and there is a limit up to which reed relays can be manufactured, because the contacts need to be flexible. The vacuum avoids arcing, so the DC and AC limits are both given by current carrying capacity. \$\endgroup\$ – Simon Richter Apr 14 '16 at 14:14

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