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I have a project which I am using an octal driver/inverter (DARLINGTON ARRAYS) to allow a micro controller to control 4 electromechanical relays. I have been experiencing relay failure and driver/inverter failure which I believe is due to back EMF. I have attached a simplified drawing to show the layout as it is today. I need to figure the proper way to suppress the back EMF. Adding to the problem is

  • All parts are external from one another. The logic board is separate from the power supply as are the relays. Therefore I have harness making the physical connections.
  • The power supply is not on constant (driven by logic board) which means common free wheeling diode reference is not constant either.
  • The octal driver/inverter outputs are all independent from one another, it's up to the microcontroller as to which outputs are low at any given moment.

Can anyone help answer the following questions to help me best make the correct decision?

  • Is the only acceptable placement of the suppressor across the relay coil (inductor)?
  • Is placing a diode in parallel with the output from the driver / inverter to +24 an option? Therefore, diode anode to output (low when "on") and cathode to +24VDC.
  • Is the diode the best choice? MOV better?

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post a link to the drawing and someone will edit it in –  markrages Apr 15 '13 at 16:50
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While you're editing the post, remove the signature (as it clearly states in the FAQ) and don't ask for reputation. You get reputation by earning it. Specifically asking for it has the opposite effect. –  Olin Lathrop Apr 15 '13 at 17:01
    
Thanks for the heads up Olin –  becjasl Apr 15 '13 at 17:18
    
In your question you stated that the freewheeling diode reference was not constant. There is no real requirement that this be constant. As long as you place the diode in reverse bias across the relay coil, as Olin has pointed out, it will clamp any reverse kicback right there in place. –  Michael Karas Apr 15 '13 at 17:30
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I think it will be most folks strong belief that it is the lack of the diode that is causing problems. We can't help you easily find where to put the diode(s) so you have an option - don't fit them or do fit them. –  Andy aka Apr 15 '13 at 18:00
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2 Answers

Make sure that you use Schottky diode. Standard diodes may not turn on fast enough to protect your drivers.

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Actually, it's usually the turn off time that is the big difference. Most silicon diodes turn on fast enough, the problem happens when you try to turn the relay back while the inductor is still discharging thru the diode. Shottkys will be a lot faster in this situation, greatly reducing the time of the short thru the diode and switch. –  Olin Lathrop Apr 16 '13 at 13:05
    
Thanks for the tip on the Shottky diode, guys. I will look into this. –  becjasl Apr 16 '13 at 13:23
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The best place for the kickback diode is right accross the relay coil, in reverse polarity of course. You can also put diodes on the board by each driver from the output to the power supply if it is on the board. If the relay power supply is not on board with the driver chips, then there is no useful place to connect kickback suppression diodes there.

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Olin, thank you for the response. I am struggling with the 2nd option you have laid out above (diodes on board by each driver). As I understand the back EMF, as the relay (inductor) is deenergized, it will go positive where it was once negative and negative where it was once positive. Would it be true to state that by placing the diode on the board to suppress the positive spike we'd be neglecting the negative spike? Would this by the reason the diodes across the coil is preferred? –  becjasl Apr 16 '13 at 13:21
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