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D. Sherman
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Re: Alternate solution
D. Sherman   11/19/2013 8:59:45 AM
A reed switch as a relay driver is fine if you simply treat it like you do when you use a transistor (or its IC relay-driver equivalent) to drive a relay. You always protect the transistor with a reverse diode across the coil. That alone might actually protect the reed switch. If not, bear in mind that transistors are inherently current-limiting, and safe to be used that way so long as their instantaneous power and current ratings aren't exceeded.

But back to the reverse diode. There's nothing in this article to indicate that anyone tried to determine whether the switches were failing when closing versus when opening. The tech wrongly guessed that because relays and motors are both inductive, the both draw an inrush surge. This isn't true. Inductors draw no current initially, but the current ramps up gradually after the voltage is applied. Inductors are the "nicest" load of all to turn "on". Motors draw inrush current because of mechanical inertia, back-EMF, etc, which I won't try to explain here.

When an inductive load is switched off, however, the current tries to keep flowing, which, without an alternative path, increases the voltage across the switching device until either the contacts arc or the transistor C-B junction breaks down. A series resistor probably helps dissipate some of this energy and probably helps dampen any ringing due to stray capacitance, but a reverse diode across the relay coil would provide a much better fix.

This "fix" is actually a good illustration of the common mistake of futzing around and finding something that appears to work, without understanding what the problem really is. Such fixes tend to fail when the circumstances change a bit.

User Rank
Alternate solution
RBedell   11/8/2013 6:55:56 PM
Personally, I would never use a reed switch to operate a relay or any other inductive device (or capacitive for that matter).  The current/voltage surges would be damaging.  Adding a resistor in series limits application.  Unless AC voltage has to be used, a transistor controlled by the reed switch would solve many issues.  If AC is required then the reed switch could control a triac.

William K.
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Re: GOOD Advice??????
William K.   11/7/2013 8:09:24 PM
We used reed type float switches as a low-oil cutoff in hydraulic poer units, and the secret that I found was to have a higher current rated reed in the float switch. The low current reeds were only good to drive the input of a PLC, not a relay. So one step up was what I used to drive the small interlocking relay, which also monitored the oil temperature switch, and often the filter monitor switch as well. That intrinsically safe module was a great interface, but it made kmore sense to me to replace the $17 float switch with the $25 float switch, instead of purchasing the $55 module and using the $17 switch. But at all times we had to be certain that we never came close to running at the contacts rated load, since that would only work for a few operations, then fail open. Few= perhaps 10 total.

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Common sense
timbalionguy   11/7/2013 2:20:42 PM
You'd think this would be engineering 101! Common sense woul seem to dictate you don't put any kind of reactive load on a fragile reed switch!

Many years ago, as my career in electronics was just getting started, I got called by a friend to a church who was having a problem with their electronic organ. It was a rainy, stormy night as I got to work on this instrument, alone, in the church.

There were no manuals (no puns intended) for this instrument, so I kind of had to reverse engineer what was going on. I forget what the exact problem was, but it seemed to have something to do with a bank of reed relays in the voicing filter section of the instrument.

In any case, I was taking some voltage measurements around a suspect relay when my probe slipped. It momentarily shorted the +12 volt rail through a reed relay contact, to ground. There was a noticeable spark when this happened. But amazingly after that, the problem disappeared. As far as I know, it never returned. The contacts of this reed relay had somehow welded themselves together (which is amazing, as this circuit had very little energy in it), and the momentary short cleared the weld. To this day, this remains one of the best 'seat-of-the=pants' repairs I have ever made. And it was one of two closely related events that began a lifelong love of the pipe organ and its literature!

Part of the fun of that night ended up being able to play the bits of Bach's Toccata and Fuge in D minor that I knew, alone, in a big church space, during a thunderstorm!

User Rank
Re: GOOD Advice??????
OLD_CURMUDGEON   11/7/2013 12:26:16 PM
Two things...

#1)  No doubt the reed switch is controlling the action of a coil (inductive) component.  Maybe it's the coil of a larger-sized contactor.  The article was NOT specific in that regard.  Therefore there could very well be a sizable back emf due to the collapsing field of that coil.

#2)  I suggest you look at a recent article in EDN magazine.  One of BOB PEASE'S last contributions which had been reprinted there was a discussion about a circuit which addresses this topic specifically.  In this article, he has described a couple of circuits to deal with this topic, including the SUSTAINING power needed to keep a solenoid (relay coil) energized in the steady state.

p.s.  In case you're unaware of Bob Pease, he is acknowledged to be one of THE MOST brilliant analog engineers of the 20th Century, having had pre-eminent positions in several major electronics components corporations, including NATIONAL SEMICONDUCTOR, etal.

Electron Wrangler
User Rank
Re: GOOD Advice??????
Electron Wrangler   11/7/2013 11:27:32 AM
While the diode prevents the voltage spike when the coil is de-energized, it allows current to flow, and can significantly extend the drop out time.  Use a zener to limt the voltage to a safe level.  The drop out time will be longer than with no suppresion, but less than with a regular diode.  For AC use back to back zeners rated higher than the p-p voltage.

User Rank
Re: GOOD Advice??????
bauerle   11/7/2013 10:41:35 AM
_I_ doubt it - a relay is just an inductor in series with a resistor (coil resistance).  Applying a step voltage results in an exponential current that starts at zero and rises to V/Rcoil with a time constant of Lcoil/Rcoil.

If the load were a motor, transformer, or cap bank, I could believe there's an in-rush current: http://en.wikipedia.org/wiki/Inrush_current

I agree with the post that said the contact damage is probably due to the arc from when the relay is de-energized; what I can't explain is how a series resistor would help this, as the arc should still jump the reed switch contacts when they open...

David Laing
User Rank
Re: GOOD Advice??????
David Laing   11/7/2013 9:51:18 AM
A relay coil works almost the same as an ignition coil did in automobile engines. As the connection is opened by the distributor, a spark would be created in the spark plug. In this case, when the reed switch opens, that is when the damage would be created. Putting a diode across the relay coil absorbs the back EMF. 

User Rank
GOOD Advice??????
OLD_CURMUDGEON   11/7/2013 9:13:33 AM
While on the face of it, this MAY be good advice for technicians, BUT there is a caveat.  While no one doubts the surge current of a relay coil @ initial conditions, the advice to insert a 100 to 200 Ohm resistor may not be correct.  One has to determine the "cold" d.c. resistance of the relay coil to make a more accurate determination of the proper resistor value.  In general, one would not want to have a resistor that IS GREATER THAN 80% of the nominal d.c. value of the coil.

Also, there are many relatively simple circuits (using transistors) which will limit the hold-in current once the relay is closed beyond the initial conditions.

Rob Spiegel
User Rank
Re: Simple solution, poorly communicated.
Rob Spiegel   11/6/2013 8:45:24 AM
I agree Tekochip. Some of the best Sherlock Ohms submissions are simple and short. The point is the ability to find a solution. Often, we harvest Sherlock Ohms entries from Made by Monkey submissions. Often, it takes a Sherlock Ohms to fix a product that was engineered by monkeys.

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