relay peak current

  • Thread starter Johan Vereecken
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Johan Vereecken

We suspect a 230V 10A relay (which controls a heating element) is causing peak currents to transmitters. Everything that has to be grounded has been grounded. What is the best method to avoid those peak currents. One proposal was to put a diode over the relay. Is this appropriate ? Any hints or other sollutions? Thanks in advance Johan
You need to provide a bit more information about your application. As a general case, there are a number of arc and inductive kick suppression methods including the diodes you suggest. A book that has a chapter on this is: "Noise Reduction Techniques in Electronic Systems" by Henry Ott(Publisher John Wiley & Sons, Inc. ISBN 0-471-85068-3) One common method is the RC network. Electrocube at makes RC networks and has an application note in their online catalog for RC networks. Bill Mostia =========================================== William(Bill) L. Mostia, Jr. PE Independent I &E Consultant WLM Engineering Co. P.O. Box 1129 Kemah, TX 77565 [email protected] 281-334-3169 These opinions are my own and are offered on the basis of Caveat Emptor.

kalpak dabir

If you mean that the back emf of the relay switching off is causing problems then yes a reverse diode across the coil will solve the problem, this problem occurs if the xmitters and the relay have the same supply. If it is happening due to the load(heater)which in this case is resistive, then it points to bad wiring layout(looping) or inadequate wire gauge.
Responding to Johan Vereecken's query: You haven't stated if the relay coil is ac or dc. The most likely cause of the voltage transient occurs when the relay deenergizes. In rare instances an ac coil's inrush current can be be the cause. Mount a transient suppressor in parallel with the coil. If coil is ac, use a Metal Oxide Varistor (MOV) as the surge suppressor to "clip" the disturbance. The MOV should be rated higher than the peak value (ca 300V) of the coil's rated voltage. Pick a mid-range surge (joules) rating, e.g, CO-7, if you select a Panasonic, or like, MOV. If the coil is dc, either a MOV or a 1-Amp diode can be used as the surge suppressor. The MOV or diode should be rated higher than 230 V, say 300 V. I have found the diode to be more effective. If a diode, pay careful attention to polarity. Remember, both devices have a finite life-expectancy regarding transient absorption. Plan on scheduled replacement every two to five years. The number selected is based on the number of expected coil operations per year. Regards, Phil Corso, PE (Boca Raton, FL)
Hello Johan When the contacts of the relay open a small arc may happen - sending a high voltage spike back to the instrument - burning it out. For very little money you can use a supressor circuit as shown on our web page. We supply specs for both AC and DC voltages. At the Almeg web page - Go to the 2nd page - scroll down till you see FAQ. click. Go to Level Switch Protection. click. Go to "What can the Level Switch Control" Go to the pdf file - and at the bottom of page 1, you will see a simple circuit, and description, for both AC and DC. Thanks Bob Hogg

Johan Bengtsson

Just a small comment, the diode is really working in the forward direction in this application and should not really be suffering from this. I would
expect life to be whatever it would be in any other normal use for that diode as long as it is rated for a current at least equal to the coil current.

/Johan Bengtsson

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Available from Motorola is a device called a "transorb", it is made just for your purpose, I am sure that there are other manufactures that make similar diodes.



Hakan Ozevin

Diode will help in DC case, but you will get the best result from RC+Varistor combination in AC and DC.