Sticking reed switch

[Wood Kayaker]

Using 3A reed switch in 12v Rule bilge pump circuit. Under tests the reed switch sticks closed 4 out of 5 times.
12v LiFePO4 battery, reed switch, bilge pump. That's all. Have inserted diode across the switch, no change.
What am I missing? Should have been a simple circuit.
Thanks for any help.

This was to replace a relay/reed switch combo.
All fits in a sea kayak.

 

[Pommie]

All the read switches I've ever seen will not handle 3A. That is why they used a relay in the first place.

Mike.

 

[Wood Kayaker]

Thanks for reading Mike. 3 years ago I could not find a 3A switch, so used a relay. Now they are available.
Ordered from Digikey; Littlefuse part # DRS 50-42-48, DK part # F1025-ND.

 

[Pommie]

In that case the current could be generating a magnetic field which is holding the read switch closed. Any circuit carrying a current will try to straighten out when energized - so if your switch has to open "inwards" it will be fighting the magnetic field. Hope that makes sense.

Mike.

 

[alec_t]

The start-up current of a bilge pump could be several times the normal running current, and that could be welding the contacts. What is the pump rating?

 

[dknguyen]

That 3A might be for a resistive load, not an inductive one.

 

[Wood Kayaker]

Pump is rated 1.6 - 1.8 Amps, depending which page of factory data you are reading. Could not find start up draw, but the recommended fuse size is 2.5A.
I may not fully understand Pommie's answer; is he stating that rotating the switch 90* - 180* may move the switch out of its own magnetic field?

At this point thinking I will go back to using a relay/reed sw combo. It did work but the relay was the weak link in a salt water environment. May paint the relay and cb with a 5 minute epoxy to seal. Run time is under 2.5 minutes; frequency is once per month. Don't think heat would be an issue.

 

[alec_t]

A SSR (solid state relay) might be the answer.

 

[dknguyen]

Wouldn't hermetically sealed relays solve that problem?

 

[Wood Kayaker]

"That 3A might be for a resistive load, not an inductive one. "
That's going over my head. Switch is rated 3A switching, 6A running. Could the induction load be greater than the 3A switch can handle?

 

[dknguyen]

"That 3A might be for a resistive load, not an inductive one. "
That's going over my head. Switch is rated 3A switching, 6A running. Could the induction load be greater than the 3A switch can handle?

It's not about the current it carries as much as what happens when the switch opens to interrupt the current. Inductive loads make extra big arcs that can weld things. It's electricity's equivalent of water hammer. It's not enough for the valve to be able to hold back the water pressure. It has to be strong enough to survive the water hammer when it closes. That's the reason you have the diode (a small temporary bypass valve to relieve the water hammer a bit) but maybe it's not enough if the relay itself can't handle it.

EDIT: Your diode should be across the motor, not the relay primary (or equivalently, motor AND battery). It's not doing it's job as is. That's probably why it's failing. Try it like that and you might be able to stick with the reed relay.

 

[Wood Kayaker]

"Wouldn't hermetically sealed relays solve that problem? "
Like that thought and for about $1.50 a piece cost less than the epoxy.

 

[Wood Kayaker]

Thanks for the education dknguyen. Will relocate diode and test - tomorrow