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.
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!).
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 13C3264from 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.
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.
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.
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.
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.
Fifty-six-year-old Pasquale Russo has been doing metalwork for more than 30 years in a tiny southern Italy village. Many craftsmen like him brought with them fabrication skills when they came from the Old World to America.
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