Even though the current draw is low on a reed relay, It is the voltage spike that hurts the driver circuit. A 10 cent diode is cheap insuranse for a $49.00 Stamp.
I'm sure you know it, but you have 2 issues here.· If you're driving the reed relay coil with a stamp you will need to protect the stamp output with some form of flyback mitigation such as a diode across the reed relay coil.· The second, less known issue is that you will have to protect the reed relay contacts themselves if they are driving an·inductive load.· They have a bad habit of welding themselves together.
Sid,
In response to your PM, I like to put suppression diodes in parallel with the inductive load.· I also put them as close to the load as possible.· I've attached a sketch for your reference.
Best of luck!
Thanks for the sketch.· I'm driving the relay with a transistor and have the diode connected acros the transistor, cathode to collector and anode to emitter.
Interesting, I have asimilar application where I use a Digi Key reed relay which has a diode built in across the relay coil, so I guess I am fine there.
But I am switching a 110VAC aquarium pump (coil vibrates a pump mechanism) with the relay and now, after reading Hulks remark, I am wondering what to do on the AC side of the relay.
Is this a concern at all, a 5 watt pump coil on a 2 amp reed contact??
Uwe,
I don't have direct experience with reed relays in AC circuits, but I think if you search the internet for "snubber" circuits you can give your relay a fighting chance. You may also choose to risk it. If the contacts weld how much harm will it cause?
I tried it while I was waiting for a reply and indeed the reed contacts stuck together. Even though I used a bleeder capacitor across the relay.
I might have to change the type of relay, I chose these because of size, but reliability is more important.
I wonder if reed relays are particularly prone to sticking because the return force is soo small.
I wonder if regular size mechanical relays are not a better choice in my application since the harm of welded contacts means having to fly to Germany and fix the problem, so this is important.
Tell you what Uwe, use a reed type and I'll fly to Germany!
You've hit on the root cause! Every time the contacts open/close/bounce, asperities are created which are variations of a microscopic weld. Return spring forces must be strong enough to break these welds when power is removed.
Those things are so prone to welding that I avoid them like plague. If you decide to use the regular kind you may have to interface with a driver transistor, but your reliability will go way up.
Good Luck
Newzed
Posts: 2,503
Comments
yes.
If the load is even slightly inductive, use diode protection to be safe.
73
spence
k4kep
Even though the current draw is low on a reed relay, It is the voltage spike that hurts the driver circuit. A 10 cent diode is cheap insuranse for a $49.00 Stamp.
Alan Bradford
Plasma Technologies
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Basic stamp Newbie
In response to your PM, I like to put suppression diodes in parallel with the inductive load.· I also put them as close to the load as possible.· I've attached a sketch for your reference.
Best of luck!
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Basic stamp Newbie
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Sid Weaver
NEW! 4 MB EEPROM
http://hometown.aol.com/newzed/page4.html
·
But I am switching a 110VAC aquarium pump (coil vibrates a pump mechanism) with the relay and now, after reading Hulks remark, I am wondering what to do on the AC side of the relay.
Is this a concern at all, a 5 watt pump coil on a 2 amp reed contact??
Uwe
I don't have direct experience with reed relays in AC circuits, but I think if you search the internet for "snubber" circuits you can give your relay a fighting chance. You may also choose to risk it. If the contacts weld how much harm will it cause?
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Basic stamp Newbie
I might have to change the type of relay, I chose these because of size, but reliability is more important.
I wonder if reed relays are particularly prone to sticking because the return force is soo small.
I wonder if regular size mechanical relays are not a better choice in my application since the harm of welded contacts means having to fly to Germany and fix the problem, so this is important.
Uwe
You've hit on the root cause! Every time the contacts open/close/bounce, asperities are created which are variations of a microscopic weld. Return spring forces must be strong enough to break these welds when power is removed.
Those things are so prone to welding that I avoid them like plague. If you decide to use the regular kind you may have to interface with a driver transistor, but your reliability will go way up.
Good Luck
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Basic stamp Newbie