Using diodes on a relay for spike protection
Hi folks. On the attached schematic, there's a diode on the coil of the relay, to protect the system from the coil spike when the relay opens.
Does it matter which way the 1N4004 diode is "pointing"? i.e., is a clamping diode always stopping positive from reaching negative, or vice-versa, or doesn't it matter?
Would it hurt/help/not matter to also include another diode "pointing" in the opposite direction across the same wires? (Since the diodes are so cheap, and some of the electronic components so expensive.)
Would I need another diode on the Timer Relay? If so, between which wires, and in what direction? ("A1" and "A2" are constant, with a trigger signal coming in on "S" to kick off the timer.)
With those diode(s) in those locations, would all the components of the system be adequately protected?
Relays:Thanks for your help/thoughts!
Your diode as shown will short the 12v supply when the access control NO connection closes. This will either fry the diode to a crisp, cause the 12v supply to shut down, blow a fuse in the 12v supply, or cause other undesired events to occur.
The diode must be installed so that the cathode (end with the band) is towards the more positive supply to the coil (reverse biased).
The "spike" occurs when the current to the coil is suddenly turned off. The magnetic field around the coil collapses, which "tries" to keep the current flowing in the same direction as it was when it was powered. This can cause very high peak reverse voltages unless there is a reverse-EMF diode (sometimes called a "flywheel" diode) installed. The diode provides a path for current back through the diode.
A single diode per coil is usually sufficient.
Sometimes a Zener diode is used in series with a standard rectifier diode. This causes the current to decay more quickly. In this case, the standard diode's cathode still goes towards the more positive supply, but the Zener's cathode goes towards the more negative.
There are also TVS diodes: http://en.wikipedia.org/wiki/Transie...pression_diode
The timer module is electronic; it doesn't need external diodes.
However, you are switching the current to the 120v/24v transformer's primary. This will cause similar spikes when power is disconnected. You may wish to include TVS suppression across the inputs/outputs of the transformer.
If you have a question, please start a thread/topic. I do not provide gratis assistance via PM nor E-mail, as that would violate the intent of this Board, which is sharing knowledge ... and deprives you of other knowledgeable input.
|The Following User Says Thank You to SgtWookie For This Useful Post:|
Hi Sgt. Thanks for the quick and informative reply.
A few questions --
In my application, do you think I would need the Zener diode you mentioned?
Without a TVS diode on the transformer, what components would be at risk? Would the spike remain on the primary side (and thus feed the spike back to the electrical panel)? (Which should be able to handle it without incident?) Or would the whole transformer or its secondary side (and the door strike) be at risk also?
If I do really need a TVS diode on the transformer, do you know what model # would be a good choice? (Primary is a 120VAC line, transformer's secondary is 24VAC, 20 Watts). I looked at some spec sheets, but swiftly got in way over my head...
|diodes, protection, relay, spike|
|Related Site Pages|
|Worksheet||Thyristor application circuits|
|Textbook||AC voltmeters and ammeters : Ac Metering Circuits|
|Worksheet||Basic electromagnetic relays|
|Textbook||The Silicon-Controlled Rectifier (SCR) : Thyristors|
|Textbook||Protective relays : Electromechanical Relays|
|Textbook||Vacuum tube audio amplifier : Discrete Semiconductor Circuits|
|Textbook||Commutating diode : Discrete Semiconductor Circuits|
|Textbook||Zener diodes : Diodes And Rectifiers|
|Textbook||Inductor commutating circuits : Diodes And Rectifiers|
|Thread||Thread Starter||Forum||Replies||Last Post|
|Unwanted voltage on open relay contacts.||NM2008||The Projects Forum||10||05-09-2009 01:07 PM|
|Is there a formula to calculate Protection diodes for relay coils?||Feign||General Electronics Chat||9||03-31-2009 12:07 PM|
|Relay contact protection.||martian||The Projects Forum||1||09-05-2008 12:13 PM|
|EFI Fuel Relay Problem||akajee||The Projects Forum||4||01-26-2008 08:18 PM|
|2 Protection diodes needed in different directions across motor = short circuit?||thomast||The Projects Forum||3||10-31-2006 10:31 AM|