:On Mar 9, 10:45 am, "michael nikolaou" : wrote: :> The motor i'm driving is operated every minute for 5 seconds so i get :> 24*60=1440 transients per day. :> I will actually speed thing up for testing and see what will happen after 5 :> years of operation. :> I imagine it will be enough .So your suggestions will have iummediate :> actions. :> Now regarding the 12VDC and isolation pls check my post to Ross . I know :> it's ancient story but the fact is :> that the new relays are extremely low power so somebody could check the :> minimum solution . :> Can somebody use a TVS unidirectional zener in shunt with the 12V relay :> inductor to protect spikes originating from the wiper as you mentioned :> since the zener-diode should create a similar behaviour ? :> The 1.5PKE seems fine though and you have my thanks for that. : : Testing without first doing calculations is a bad design practice. 5 :year testing is only to confirm those calculations; not to see if a :circuit works. : : Varistors are not intended for repeated spikes. Manufacturer :datasheets demonstrate the concept. With use, they degrade. :Therefore varistor type devices once used for protection are now :routinely replaced with semiconductor type devices. Avalanche diodes :(ie Transorb, Transil, etc) do not degrade. Either it works the first :day or it does not work at all.
What do you mean by "repeated"? Is it once every second or once per minute or whatever? Provided that the varistor is selected to have the right voltage and current surge rating for the task it will not fail as you suggest.
: : Varistors must never fail as implied by "lawsuits", etc. A failed :(degraded) varistor must have no visual indications of that failure. :Datasheets make that obvious. Varistors are intended for limited :events. With each use, varistors degrade - as datasheets define with :numbers. Any varistor that fails by creating a visual indication :leaves the circuit designer liable for a lawsuit. A completely :degraded varistor has no indication of its failed - a normal event :defined by the size and number of 'spikes'. And varistors are :typically used where rare, not normal, and excessive currents might :exist.
What you failed to state is that varistors degrade when they repeatedly have to absorb surges "beyond" their specified ratings of voltage and current. Provided that the transients absorb do not produce excessive surge currents they will continue to limit transients for many years. It all depends on the correct device selection. Transorbs which are not appropriately selected for the task will fail just as easily as varistors. It is just that when they do fail it is usually on the first occurrence and the destructive effect is more violent. I have seen a transorb blown to pieces just as I have seen a varistor when subjected to ecessive voltage and current. I have also seen varistors continue operating in 240Vac operated environments for 30 years without failing.
: : Current pulses induced by the wiper (armature or whatever) typically :would be completely ignored by a transorb across the coil. Appreciate :the two different types of current. A transorb across the coil may :see no current (or voltage) as that current passes out the on one or :both wires.
This statement makes absolutely no sense at all. What are the "two different types of current" you refer to? The fact that you use the term "wiper" in relation to electromagnetic relays indicates you have little knowledge or experience with them. A transorb or varistor across a relay coil will "always" see the voltage produced by the back emf as the relay is de-energised - that is the whole purpose of using such devices. The degree of current produced by the back emf is dependent upon the magnitude of the voltage transient produced by the back emf and the coil resistance through which this current must pass.
: : This is another reason for buffering the coil with a transistor. :Transient current conducts through transistor to ground AND the large :CB or drain to gate voltage in series with a resistor buffers the :microprocessor.
In the case of the OP's 12V dc relay we know that this is controlled by a solid state device. As such, it must include some appropriately rated relay driver transistor whether this is discrete or in an IC package. Your talk of producing ground currents as a result of surges produced by relay coils is meaningless. Provided that a separate ground scheme is maintained purely for the relay circuits there will be no deleterious effect on the sensitive digital control circuits. It is all about appropriate printed circuit design and layout technique. Transorbs are more appropriate to low voltage dc operation where solid state control is used.
: : Most modems do this slightly different. Rather than grounding the :coil with an NPN transistor, the coil is powered from the V+ side with :a PNP transistor. Even better. Therefore both types of transients :(discharging coil and transient from relay contacts) are kept from the :uprocessor and always have a direct connection to ground.
??? what the heck are you on about with this statement? It has absolutely nothing to do with the OP's question.
: : 12 volt zener on the coil does nothing useful. That diode must be :nonconductive to powering current from the microprocessor. And it :must be 0.7 volts conductive to discharge the coil. Using a 12 volt :zener does nothing useful and adds costs. Better is to enhance :buffering on the base or gate connection to the uprocessor.
What utter crap. For starters, if the relay supply is 12V regulated, it makes no sense to use a 12V zener across the relay coil. You have also fallen into the trap of thinking that the zener must not be conductive to the normally applied votage polarity. This is totally incorrect. The purpose of the zener is to limit the back emf produced by the relay coil when it is de-energised. If you know anything at all it is that the back emf produced by the coil is ALWAYS of OPPOSITE polarity to that of the applied operating voltage. Consequently, in order to limit the back emf the zener MUST be conducting to the NORMAL operating polarity and only when subjected to the reverse polarity back emf does the zener "break-over" into normal zener mode. If connected as you suggest the result is no different to using a standard diode across the coil.
If the relay manufacturers (eg.Tyco, Siemens and others) suggest that a standard silicon diode MUST be used in conjunction with a zener diode for this application, then I would suggest that you are not as smart as you make out.
: : Meanwhile, some MOSFETs are perfect because the coil's shunting :diode is already part of the MOSFET.
Yes, but this is not relevant to the OP's question. Trying to demonstrate your knowledge of MOSFETS which include an integral diode is immaterial.
: : Unmentioned is the reason for spikes from the motor. Best is always :to eliminate the reason for spikes rather than suppress them - as :discussed previously. And not just for circuit stability. Doing so :also increases life expectancies elsewhere, reduces EMI, and other :reasons. Best is to not create that harmful energy in the first :place.
Oh great. And just how do you suggest that you eliminate spikes produced by a motor? Transients produced by motor windings are just part of the deal - you can't just "design them out" of the equation. As long as there are inductive windings in a motor there will be transient spikes - end of story. So you are stuck with having to suppress or moderate these spikes in exactly the same way as suppressing the back emf from relays. BTW, varistors are commonly used to perform this function in electric motors as well. Even motors rated at hundreds of horsepower have their winding transients suppressed by the common varistor. Admittedly these are much bigger than the disc types we commonly encounter. It would be logical to assume that in this particular application the varistors would be subject to repeated and constant transients, but do we hear of frequent failure of electric motors due to the failure of varistors which have to endure these transients? I would suggest not. It is all about selecting the right varistor for the job and it will rarely, if ever, fail.