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Reed Switch Loves Resistor

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Rob Spiegel
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Simple solution, poorly communicated.
Rob Spiegel   11/6/2013 8:01:59 AM
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Such a simple solution, so poorly communicated -- or not communicated. 

tekochip
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Platinum
Re: Simple solution, poorly communicated.
tekochip   11/6/2013 8:35:20 AM
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Poorly communicated until today.
 
Great Sherlock Ohms story; a problem, a solution, and simple simple theory to follow it up.  Thanks for sharing the story.


Rob Spiegel
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Blogger
Re: Simple solution, poorly communicated.
Rob Spiegel   11/6/2013 8:45:24 AM
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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.

Amclaussen
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Platinum
Re: Simple solution, poorly communicated. ---Simple?
Amclaussen   12/6/2013 7:29:04 PM
Dear Rob: Life has taught me things only appear to be that simple.

One of my favorite Murphy's Law statements (as I understand it), deals with comments (or thoughts) like yours... but Murphy was right: "When you seem to understand a simple thing, you are just appearing to fully understand it!

As you will see in many of the comments below, "simple" diode fixes aren't going to cut it. This story will appear simple enough to elicit "obvious" fixes, but there is always more if you look closely!

Dear tekochip, please read my statement about simple things... it applies equally well to seemingly "simple theory". Cheers, Amclaussen.

P.D.: I have dedicated a whole wall at my home shop to Murphy's corollaries, it helps me A LOT every day when I'm ready utter some profanity when building something and, as Murphy used to say, things got wrong!!! It is a white painted wall, and I add a new statement as soon as it happens to me... it has now about seven feet of writtings, yesterday I added the last one: "It happens that, during the reassembly of a part that requires a multitude of bolts, you happen to exclaim (at bolt number 15 of 16) "well, I've almost finished...", and then you reach for that last bolt to put it, when, suddenly, you realize that you forgot to place the gasket (and some gaskets look like smiling faces, to add insult to injury!).

Rob Spiegel
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Re: Simple solution, poorly communicated. ---Simple?
Rob Spiegel   12/9/2013 12:02:54 PM
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Thanks for the comments, Amclussen. They make a lot of sense.

OLD_CURMUDGEON
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Platinum
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.

David Laing
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Iron
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. 

Electron Wrangler
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Silver
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.

Amclaussen
User Rank
Platinum
Re: That is OK, but there is a better way!
Amclaussen   12/6/2013 6:10:56 PM
Good answer, Electron Wrangler!...

But I like the Zener plus std. diode more: Using a Zener with an standard diode, back to back. That is the best recommendation I've tried, and was taken from an excellent Application Note published by Tyco Electronics Corp. (document 13C3264, Printed U.S.A., IH/12-00)

It says: "The Zener voltage value is chosen to limit the coil switch voltage to a level acceptable to the switch rating. This affords the best compromise both to coil switch protection and relay switching performance, and should be employed to assure maximum relay performance and reliability while providing protection to the control circuit from coil induced voltages."

While too many websites just repeat and repeat the single reverse diode across the coil approach as the one to use, the Tyco recommendation is the correct, inexpensive, easy one.  In my most recentapplication, I had to add a more robust relay to a radiator fan circuit using 12VDC in one of my modified cars, where I was replacing the original radiator with a larger one that has two electric fans, therefore I decided to use a beefier relay, which has a larger coil. So, I preferred to play it safe and was ready to use a simple diode across the relay's coil terminals, but happily found that application note at last minute. I became curious and tested both arrangements, plus other ones using diverse values of resistors. (some automotive relays even include small size resistors inside, placed across the coil). A trip to an electronic parts store (similar to a 'Rat-Shack') provided me with the parts on saturday's night, and the car was ready to run (quite literally) next morning!.

As the application note from Tyco stated, their fix was the very best, and that's why I'm telling all my friends about it!. Amclaussen.

bauerle
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Iron
Re: GOOD Advice??????
bauerle   11/7/2013 10:41:35 AM
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_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...

OLD_CURMUDGEON
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Platinum
Re: GOOD Advice??????
OLD_CURMUDGEON   11/7/2013 12:26:16 PM
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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.

William K.
User Rank
Platinum
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.

timbalionguy
User Rank
Gold
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!

RBedell
User Rank
Gold
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.

D. Sherman
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Silver
Re: Alternate solution
D. Sherman   11/19/2013 8:59:45 AM
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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.

Amclaussen
User Rank
Platinum
Re: Alternate solution: CALL THE MYTHBUSTERS!
Amclaussen   12/6/2013 6:39:12 PM
In regard to D.SHERMAN;

The simple diode "fix" has been repeated and repeated to become a myth!

As I suggested above (in response to Electron Wrangler), the problem with the reverse diode across the coil is that it slows the opening of the relay contacts, frequently reducing the contacts life -you end up protecting the switch sacrificing the relay contacts!-

Please see the excellent Application Note 13C3264 from Tyco. It will explain in detail all the related aspects involved in relay behavior and how the simple diode, mythified by all the people that use the easy web "Copy & Paste" knowledge dissemination attempt (without really performing a complete and through web search), has become an established Myth (call the Mythbusters NOW!). Best wishes to all. Amclaussen.

Cabe Atwell
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Blogger
Re: Alternate solution
Cabe Atwell   1/20/2014 5:42:01 PM
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It seems the simplest of solutions are often overlooked. Great story!

Critic
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Platinum
The Best Fix
Critic   11/22/2013 4:58:53 PM
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Adding a series resistor will certainly limit the current, which helps prevent damage to the reed switch in two ways:

1. With an inductive load, lower di/dt means lower induced voltage, hence reduced or eliminated contact arcing

2. Limted current reduces contact heating

However, adding a series resistor also limits the current capability of the switch at a given voltage.

The type of contact protection that should be used depends on the nature of the current:  dc or ac.  If we assume an inductive load (typical of relays), for dc, a diode across the relay coil to effectively short-circuit induced voltage (but not forward voltage used to energize the coil) works well, without the need to limit current using a resistor.  For ac, a properly-designed series RC circuit (snubber) in parallel with the reed switch reduces high-frequency high voltages across the contacts, but has negligible effect at the fundamental frequency.

tmoreau
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Iron
Cable Capacitance? AC relay coil in-rush?
tmoreau   11/22/2013 5:07:33 PM
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Nobody mentions that the reed switch and relay might potentially be located very far apart for each other; and thus connected by a cable, which if very long then it could be a source of considerable capacitance.  The fact the the series 100 ohms worked, suggests it's not related to any lack of relay coil transient suppresion, but indeed due to in-rush current upon reed switch closure.  If the in-rush is not from the relay coil (magnetizing current), which I'm having a hard time seeing how that's possible if the coil current isn't reversing (we are not told if it's an AC or DC circuit); then another possible source of high in-rush current is cable capacitance.


I suspect the reed switch is being operated at line voltage AC, because 100 ohms is a rather large amount of resitance to be putting in series with any common 12/14VDC relay coil, and have the circuit still operate.  There is potentially some in-rush current associated with the coil of a true AC relay (by true AC relay I mean an armature made from iron laminations, not a relay made by using diodes in conjunction with a solid iron armature constructed exactly like a dc relay).  In-rush current from inductive ac equipment has to due with magnetization, which is at it's worst if you opened the circuit at the peak current of the ac cycle, then close the circuit at the reverse polarity peak in the ac cycle.

 

 

cookiejar
User Rank
Gold
What about inductive kickback?
cookiejar   12/6/2013 6:17:22 PM
You say you were driving relays through reed switches.  Your description and solution doesn't quite make sense to me. 

A relay coil is an inductive load.  An inductor won't allow quick changes in current just like a capacitor won't allow quick changes in voltage.  A relay coil by itself will not show a current spike on activation, but a relatively gradual increase in current.  But, when the circuit is open the inductance of the coil will try to keep the current flowing and can reach a very high voltage in its effort.  I've seen over 10,000 volts on a relay coil when the circuit is opened (on a C. P. Clare mercury-wetted reed relay - 15 Henry coil).  The resultant arcing is what burns contacts and couples into circuits causing erratic circuit behavior. 
The easy solution is to put a diode across the coil to allow the current to circulate until it dies.  The diode direction is opposite to activation current.  This will cause a delay in the the relay releasing.  Using a zener instead of a diode will shorten the delay.  There are proprietary relay suppressors for critical timing applications like the homing of stepping switches, specific to the relay models.  (I'm showing my age).

If you have put a capacitor across the relay coil to suppress the kickback, then that capacitor will certainly cause a current surge when your reed switch closes and will burn its contacts.  In that case a series resistor will reduce the surge current on contact.  Upon the contact opening, the capacitor and coil inductance will cause a ringing at their resonant frequency, whose amplitude may still be high enough to damage contacts.  Poke around with a high speed storage scope - you might be amazed at what you see.  CAUTION do not blow your scope with 10,000 volt spikes.   Contact bounce waveforms can give an indication of contact health.


Amclaussen
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Platinum
Re: What about inductive kickback?
Amclaussen   12/6/2013 7:02:03 PM
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Good points, cookiejar!, and Yes, your post reveals your age, but in the good sense that you reflect your true experience, that truly comes with age, doesn't it? (now I'm showing MY age!).

BTW, the Tyco Application Note shows (very clearly) those spikes. as I was using an automotive, 12VDC, 40A relay, I didn't risk my old, school type scope!

JD_Daily
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Iron
Reed Switch Ratings
JD_Daily   12/6/2013 10:32:43 PM
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Reed type level switches do not have an inductive/capacitive load rating.  The ratings are in watts. For longest life a solid state relay should be used to switch power to relay coils, contactor coils, etc. Even with spike supression they should be applied at a small fraction of their rating to insure long life. 

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