Author Topic: Reed switch gets stuck closed (Read 16292 times)

hazuki · « **on:** July 17, 2013, 01:23:30 am »

Hi all. I have a project which involves using a magnetic reed switch to switch 11.1V lipo battery which powers a circuit. I am haviing an issue where the reed switch seems to get stuck closed even when the magnet is removed. I've tried multiple types of reed switches (https://www.sparkfun.com/products/8642 and http://www.digikey.com/product-detail/en/CT10-1540-G1/306-1124-2-ND/388961). I've even tried putting 5 reed switches in series to lessen the chance that the circuit is complete when the magnet is removed. Still the switch gets stuck closed.

The circuit that I'm powering is a voltage multiplier that ends up producing around 20kV. I've tried testing the reed switches by themselves (not in the circuit) and they seem to work just fine. Is there a chance that the HV circuit is keeping the reed switch closed somehow?

Looking for advice on how to stop this from happening.

Thank you!

David_AVD · « **Reply #1 on:** July 17, 2013, 01:34:37 am »

Both switches are rated for 500mA max switching current. How much current does your circuit draw, especially at start-up ?

Psi · « **Reply #2 on:** July 17, 2013, 01:39:22 am »

Yeah, a reed switch getting stuck connected is a really strong indication that you're pushing too much current through it and the metal is fusing together due to the contact arc.

As David said above, it could be startup/inrush current. (from a capacitor charging etc).

Mr Smiley · « **Reply #3 on:** July 17, 2013, 02:47:25 am »

Is there any chance of a locally generated magnetic field keeping the reed closed

hazuki · « **Reply #4 on:** July 17, 2013, 02:50:18 am »

I measured the max inrush current to be just under .2A. I don't think there are magnetic fields being generated strong enough to keep the switch closed. Although I can't be certain since I have no way of measuring the generated magnetic field.

c4757p · « **Reply #5 on:** July 17, 2013, 02:51:45 am »

$5 says inrush. How did you measure it? The inrush is often very quick, a DMM set to 'peak' or 'max' will miss it.

Also, the burden resistance may limit it.

Mr Smiley · « **Reply #6 on:** July 17, 2013, 02:56:37 am »

Have you a compass handy

Put it next to the reed so that when the magnet is placed next to the reed, the compass swings 90, then apply magnet with circuit powered and see if the compass still stays at 90, just an idea, maybe a bad one but worth a try.

digsys · « **Reply #7 on:** July 17, 2013, 02:59:55 am »

Temporarily, put a 10R resistor in series with the reed switch. That will at least smother the Inrush current and you can prove / disprove
that as a possibility. It will limit current to ~1A which should be fine for the reeds.

hazuki · « **Reply #8 on:** July 17, 2013, 03:20:00 am »

I measured the inrush current with a DMM using the max function.
I'll try a current limit resistor tomorrow. Thank you guys!

mikeselectricstuff · « **Reply #9 on:** July 17, 2013, 08:02:43 am »

Reed switches are not really designed to carry current - use the switch to control a MOSFET that does the actual switching.

poorchava · « **Reply #10 on:** July 17, 2013, 08:07:34 am »

Usually a reed switch needs a rather strong magnetic field. I mean the order of magnitude that is rather hard to generate by accident.

ignator · « **Reply #11 on:** July 17, 2013, 10:25:48 am »

I HATE reed relays. They caused nothing but problems in the test multiplexer I worked on. Parallel circuit traces could keep them energized, and very small current flowing through them would keep them closed. The vendors spec. never seemed to apply to how they actually worked. These were 5volt coil, 4 pin, SIP packages with integral coil clamping diode.The application was to test avionics lightning protection circuits. 10milliamp max current. I had to put lots of software around them to protect them from discharging 0.1microfarad capacitance that was charged during the TVS (P4KE Transorbs, and some MOVs 110volt max), clamping voltage tests.
You need to see just how much current your trying to interrupt. Do a test to see what the threshold is to interrupting what you believe is a very small current.

My solution was to turn off the power source to the UUT (unit under test), if there was a capacitor to discharge, I put a 10K resistor across it, delay, then clear all the relays, and set up for the next test.

Mr Smiley · « **Reply #12 on:** July 17, 2013, 12:40:08 pm »

Does the reed release when the battery is removed from circuit ?

ie, power up, switch on with reed, disconnect battery, is the reed now off ?

woodchips · « **Reply #13 on:** July 17, 2013, 05:03:15 pm »

In a telephone exchange the reed switches set the call up but the loop current was switched by a transistor after the reeds and closed, and turned off before they opened. The usual way of testing a reed was to put it above the coil in an analogue multimeter when it would be operated by the stray magnetic field from the meter movement. Tey are low power and very sensitive.

SeanB · « **Reply #14 on:** July 17, 2013, 06:21:52 pm »

I remember the ATE I used to work on, racks of cards containing magnetically shielded mercury wetted reed relays. There was a 4 hour test routine ( and a corresponding patch bay block and test lead set to plug in as well) to check all the switches and tell you the card and reed that was faulty and how. Most common was leakage which was easy to cure by tapping the card to get the mercury droplets back in position.

hazuki · « **Reply #15 on:** July 18, 2013, 12:07:48 am »

Thanks for the replies all. I hooked up a FET to do the switching so the reed switch carries very little current. The system seems to be working just fine!

I guess it was the current flowing through it that caused it to keep closed.

Many thanks again!

David_AVD · « **Reply #16 on:** July 18, 2013, 12:11:47 am »

Good to hear you solved the issue.

I hate it when people offer up solutions and the OP never bothers to confirm (or deny) it.

digsys · « **Reply #17 on:** July 18, 2013, 12:23:58 am »

Quote from: hazuki

I guess it was the current flowing through it that caused it to keep closed.

Did you try putting a 10R (to 20R) in series to prove / disprove that it was just the inrush spike and not the "main" current?
It's your call, but knowing WHY is good for learning and honing skills. It may be helpful for future problems.

hazuki · « **Reply #18 on:** July 18, 2013, 12:26:55 am »

I didn't get a chance to try the resistor. But I did notice that I wasn't able to get the thing to stick close when I was measuring the current with my DMM. The DMM might have been limiting the current a bit.

digsys · « **Reply #19 on:** July 18, 2013, 02:33:46 am »

Quote from: hazuki

I didn't get a chance to try the resistor. But I did notice that I wasn't able to get the thing to stick close when I was measuring the current with my DMM.
The DMM might have been limiting the current a bit.

Yup, that good enough. As others have said, reed switches can easily "stick" with minimal hold currents. A very short high current can be enough to
"weld" the contacts.


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Author Topic: Reed switch gets stuck closed (Read 16292 times)

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