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Thread: Question about diode across relay coil

  1. #1
    Gaz
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    Question about diode across relay coil

    This may be a dumb question, but I was wondering if the diode typically used across a relay coil had to be physically soldered across the relay?

    Since the cathode of the diode goes to the power supply, and the anode goes to ground, is it okay to place the diode away from the relay itself and over at the power supply going straight to ground? It seems to me that this is electronically the same, but I'm not sure if my pseudo-science is on today. Thanks.

  2. #2
    Supporting Member loudthud's Avatar
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    The relay coil has significant inductance. When the relay is energized, typically by switching one side of the coil to ground, a magnetic field is created in the coil. When that one side of the coil is disconnected from ground, the inductance tries to maintain current flow as the magnetic field collapses. This causes the voltage at the now disconnected side of the coil to swing positive above the voltage that energized the relay. There are many variables but the voltage can easily swing 50 or 100 Volts above ground for a short time. If you were using a 40V transistor to energize the relay, it can be damaged by excessive voltage. The diode across the coil limits the voltage swing to about 0.7V.
    WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personel.

  3. #3
    Gaz
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    Thanks for the response, it helps my fundamental understanding of what the diode is actually doing. I'm still unclear about whether the diode can be placed from the relay's power supply (a small transformer in my case) to ground, or if it needs to be physically placed across the relay's contacts.

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    Supporting Member loudthud's Avatar
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    It has to be connected across the coil. If it just goes from from the power supply to ground, it's not going to protect the transistor. Try to measure the resistance of a choke. If you are touching the wires from the choke when you disconnect the ohm-meter, you'll feel the jolt.
    WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personel.

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    Supporting Member Jazz P Bass's Avatar
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    We are talking the speed of light.
    Whether or not the diode is physically attached to the coil or one foot away is irrelevant.

  6. #6
    Gaz
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    Right, but Loudthud seems to be suggesting that the physical placement does matter. I'll have to draw a little layout to show exactly how I'm doing it...

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    Gaz
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    Here's a schem/layout that represents the two different options.

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    kg
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    hey gaz

    speed of propagation hasn't anything to do with it.

    you want the diode close to the coil it's catching the flyback from.

    this is to keep the current loop small.

    flyback voltage can hit Q*Vsupply. Q is a function of the reactance vs. resistance of the coil and is intrinsic to its physical design.

    a small loop will best serve to contain the interference generated to the smallest possible area.

    hth
    ken

  9. #9
    Gaz
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    Thanks for the cogent explanation, Ken, I'll be moving those diodes over to the relays themselves.

    And I have to mention. There was a funny coincidence a while back when my new roommate and I somehow figured out we were both reading your website: I was studying tube amps, and he was downloading hard to find Subaru manuals!!! Haha!

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    Quote Originally Posted by Gaz View Post
    This may be a dumb question, but I was wondering if the diode typically used across a relay coil had to be physically soldered across the relay?
    The flyback diodes are typically used to protect driver transistors to engage the relay coil , like the old workhorse 2N2222.. I don't see any driver transistors in this print, and I'd don't think what "little" flyback voltage is going to hurt anything... But, if you want to use one, anywhere would be fine...

    -g

  11. #11
    Gaz
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    I was under the impression that they were necessary with any setup... maybe they're encouraged in non-transistor driven designs to prevent a popping noises ??? - never tried relay without them, so I couldn't say myself.

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    I'm just saying the where's and the why's of when they came from.. I use a 12 volt relay in my preamp channel switcher, and it works fine without it.. Not saying not having them is a bad idea. You make up your own mind.. Just understand why you will always see one across a relay coil, with using either a transistor or a fet driving the coil...

    -g

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    Noodle of Reality Steve Conner's Avatar
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    Gary uses a LED instead of a regular diode.

    As there is no transistor to kill, the only use of the diode is to snub the voltage spike that might otherwise couple into the signal wiring and cause a pop.
    "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

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    The diodes will also protect any arcing on your switches too. A few years ago I had switches failing on 12v ambulance lift systems because of arcing from relays and other coils. Ok so its probably not a major concern here but it does help.

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    kg
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    Quote Originally Posted by Gaz View Post
    Thanks for the cogent explanation, Ken, I'll be moving those diodes over to the relays themselves.
    it's just good housekeeping and circuit design... and it absolutely DOES reduce popping and switching noise.

    And I have to mention. There was a funny coincidence a while back when my new roommate and I somehow figured out we were both reading your website: I was studying tube amps, and he was downloading hard to find Subaru manuals!!! Haha!
    obviously i'm going to have to make it a pay site.

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    While you’re looking at relays you may want to consider solid state relays.

    No coils, no voltage flyback and no mechanical clicking.. just doing dpdt is a bit more tricky.

    It was thanks to Steve that I found these:
    Data Sheet - ASSR-401C and ASSR-402C Low C x R, Form A, Solid State Relay (Photo MOSFET) (400V/100

    Here are some others that could work
    http://uk.mouser.com/catalog/643/1850.pdf

    I attach flying leads and then use them on turret builds, they are small enough to do so.

  17. #17
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    Even if a mechanical switch is used , instead of a transistor, you need them.
    The voltage generated at turn off may easily be 10x the coil rated one, or even much more , and produce arcing and pitting at the switch contacts, not forgetting pop.

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    kg
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    Quote Originally Posted by guitarmike2107 View Post
    While you’re looking at relays you may want to consider solid state relays.

    No coils, no voltage flyback and no mechanical clicking.. just doing dpdt is a bit more tricky.

    It was thanks to Steve that I found these:
    Data Sheet - ASSR-401C and ASSR-402C Low C x R, Form A, Solid State Relay (Photo MOSFET) (400V/100

    Here are some others that could work
    http://uk.mouser.com/catalog/643/1850.pdf

    I attach flying leads and then use them on turret builds, they are small enough to do so.

    no clicking???


    how the hell am i supposed to troubleshoot?

    (only half joking.)

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    Quote Originally Posted by J M Fahey View Post
    Even if a mechanical switch is used , instead of a transistor, you need them.
    The voltage generated at turn off may easily be 10x the coil rated one, or even much more , and produce arcing and pitting at the switch contacts, not forgetting pop.
    and ah, bypassing the contacts would not abate some of the arcing ??

    -g

  20. #20
    Supporting Member Jazz P Bass's Avatar
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    (according to National Instruments, the flyback diode should be no greater than 18 inches from the inductor).
    Source: Wikipedia: flyback diode.
    I do agree thogh that the diode right at the coil is a common layout.
    Last edited by Jazz P Bass; 03-08-2011 at 03:07 AM.

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    ha ha, I was looking at a friends superdooper prosonical Fender recently, when you turned it on and off a number of relays were energized/released click click click click. Sounded weird actually

    That was a good deal though as I got a 6 pack and box of chocolates for not fixing his non broken amp.. he did learn a good lesson always check your cables first!

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    If you stick with the mechanical relay, try and get one with the suppression built in.
    If not, you may want to consider putting a resistor or zener diode in series with the flyback diode.
    Depends on how robust you want to build the amp of course

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    and ah, bypassing the contacts would not abate some of the arcing ??

    -g
    Yes, of course.
    A capacitor across the switch contact will work; it's good practice though to add a small resistor in series with it (say, 100r) to dampen the resonant tank made by it and the unspecified coil inductance.
    This turns the simple capacitor into a snubber , which is a fancy way to say that you add a lossy component (the resistor) to "eat" that energy pulse in a reasonable time.
    This is mandatory on AC circuits (say a line switch feeding a power transformer) because diodes can't be used. You can also use Varistors or back to back Zeners to absorb voltage peaks.
    As of the power of inductive circuits , whan I was building my "big" magnetizer my anxious partner wanted to try it, and hooked a Frankenstein movie type knife switch between the 6 diode, 380V, 3 phase rectifier and the monster magnetizer coil (think a fridge sized electromagnet), *without* the big diode I was buying in that very moment.
    He was trying to interrupt *DC*; all 380V 60A of it feeding that coil.
    He turned it "on" with no problem at all, but when he tried to turn it off, it would not.
    An impressive arc jumped the air between the switch contacts, happily keeping it "on" ... and burning everything within 10 inches.
    He had to *run* to the "street power cabinet" and pull the fuses there.
    After that, we wired the flyback diode *and* switched the 380V AC side with a Siemens 100A contactor, which still works faithfully after 20/25 years.
    We got tired of burning SCRs and the mechanical solution was best.
    My "small" magnetizer (monophasic 220V) is also AC switched and the bridge rectifier acts as a "free" clamping diode.

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    Quote Originally Posted by mooreamps View Post
    and ah, bypassing the contacts would not abate some of the arcing ??
    I'm sure you think you understand what you think JM might have meant if he had said what you thought he might have said when he said it, but I'm not sure I think you mean what you might have meant when you actually said it when I thought you said it.


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    I'm using a 5V relay for switching in an amp I recently built with no diode across it. I have been using it for months without any issues or pops.

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    Quote Originally Posted by R.G. View Post
    I'm sure you think you understand what you think JM might have meant

    Ya, what ever......

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    kg
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    Quote Originally Posted by GregS View Post
    I'm using a 5V relay for switching in an amp I recently built with no diode across it. I have been using it for months without any issues or pops.
    that may very well be true, but it doesn't mean there's no use in following best practices.

  28. #28
    Senior Member Enzo's Avatar
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    Amen, getting away with something doesn;t mean it is right.

    Gaz' original problem was not recognizing his circuit. The diode is NOT from power supply to ground, it is from power supply to the other end of the relay coil. It is only when the switch is closed that the anode of the diode connects to ground. But since the purpose of the diode is to shunt the coil's inductive spike when the current to it is interrupted, the diode must be across the coil not across the supply.

    When someone says the diode must be wired across the coil, that means the two ends of the coil are connected to the two ends of the diode. it doesn't specify that the diode be right on the relay. In other words, "across the coil" specifies electrical position, not physical position. ANother refernce above states the diode should be within 18 inches, fair enough, but that is a separate thing from electrical wiring.
    Education is what you're left with after you have forgotten what you have learned.

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    Quote Originally Posted by mooreamps
    Ya, what ever......
    Wow! Clever reply! Oh, agony! I'm mortally wounded! Cut to the quick by the serpent's tongue!

    Quote Originally Posted by GregS View Post
    I'm using a 5V relay for switching in an amp I recently built with no diode across it. I have been using it for months without any issues or pops.
    Yes, what they said.

    What that means is that something is absorbing the flyback energy. Sometimes a driver transistor can eat this kind of abuse, especially if it's at human speed, for lots of iterations without failing immediately. It may even last for a long (by human standards) time if it's not overheated by the avalanches. But it's not good for whatever drives the relay. Generally if what drives the relay is a transistor it lasts longer, as single transistors can take more abuse than the insides of an IC in many cases.

    Using a diode means the diode is always shunting the flyback spike back into the power supply, and there are no long term worries (or sudden failures) with the driver. This is why it's just good practice to do this and not count on getting by with with it for an unknown period of time. Good design practice means never (OK, hardly ever ) having to say you're sorry after shipping the design.

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    Quote Originally Posted by R.G. View Post
    Wow! Clever reply! Oh, agony! I'm mortally wounded! Cut to the quick by the serpent's tongue!
    Don't worry.. He'll live... :|

    -g
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    Quote Originally Posted by mooreamps View Post
    Don't worry.. He'll live... :|
    Thanks gary. I was worried for a minute there.

    Hey, I've been meaning to ask you - what happens when you load up an inductor and then interrupt the current, causing an arc in the switch if there's a great, big loop of wire with a big area that the current's flowing in?

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    Quote Originally Posted by R.G. View Post
    Thanks gary. I was worried for a minute there.

    Hey, I've been meaning to ask you - what happens when you load up an inductor and then interrupt the current, causing an arc in the switch if there's a great, big loop of wire with a big area that the current's flowing in?

    Well, you know the answer as well as I.. But for the sake of the others reading this , lets walk through it..

    When you apply a dc voltage to a "DC" coil, it will generate a magnetic field around it. It is this magnetic field that pulls the contacts inside the relay.. Then, when the dc voltage is removed, the coil will discharge it's energy and the magnetic field around it will collapse thus releasing it's pull on the relay contacts.. It is the collapsing magnetic field, also known as the inductive kick that is feed back into the circuit... Hense the flyback diode.. It is inserted reversed biased in the circuit when the voltage is applied. But becomes "forward biased" as a result of the polarity reversal of the magnetic field generated by the coil as the field collapses..

    But, inside these little 12 volt dc relays, I don't think the coils are big enough to generate much of an inductive kick. Well, they would spike a 2N2222, but I don't think they could hurt a fairly robust 12 vdc power supply... Still, having a flyback installed across a relay coil, driver transistor or not, is still probably a pretty good idea....


    -g
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    Quote Originally Posted by mooreamps View Post
    Well, you know the answer as well as I.. But for the sake of the others reading this , lets walk through it..

    When you apply a dc voltage to a "DC" coil, it will generate a magnetic field around it. It is this magnetic field that pulls the contacts inside the relay.. Then, when the dc voltage is removed, the coil will discharge it's energy and the magnetic field around it will collapse thus releasing it's pull on the relay contacts.. It is the collapsing magnetic field, also known as the inductive kick that is feed back into the circuit... Hense the flyback diode.. It is inserted reversed biased in the circuit when the voltage is applied. But becomes "forward biased" as a result of the polarity reversal of the magnetic field generated by the coil as the field collapses..

    But, inside these little 12 volt dc relays, I don't think the coils are big enough to generate much of an inductive kick. Well, they would spike a 2N2222, but I don't think they could hurt a fairly robust 12 vdc power supply... Still, having a flyback installed across a relay coil, driver transistor or not, is still probably a pretty good idea....
    Good walk. Yeah, I knew that part and I figured you did too.

    But I wondered whether you'd tumble to the setup where the voltage arcs the opening switch and generates a pulse of RF which is transmitted out of the loop antenna formed from the - and I quote - "great, big loop of wire". They used to call them spark gap transmitters.

    Sure, a random loop of wire doesn't transmit all that well, but for things either in or near that - and I quote - "great, big loop of wire", the RF is easy to pick up. And when that's inside some piece of amplifying equipment, it's really easy to induce RF clicking issues as the RF is detected by the first junction or nonlinear amplifying device (tubes qualify here) it hits. It's very dependent on the energies, the flyback voltage, the clamping voltage, and whatever frequencies the random loop happens to "tune" the arc pulse to. The random nature of the current loop means that the burst of RF isn't always all that detectable. But sometimes it is, especially if you are using high gain, high impedance things like tubes and FETs.

    One neat trick for suppressing the arc is to clamp it back into the power supply with a diode. After all, this isn't rocket science, is it?

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    Quote Originally Posted by kg View Post
    that may very well be true, but it doesn't mean there's no use in following best practices.
    The last time I went shopping for relays, I was finding that it's now almost universal to find a diode built into the package. The important thing to know about them is that polarity is important.

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    Quote Originally Posted by mooreamps View Post
    But, inside these little 12 volt dc relays, I don't think the coils are big enough to generate much of an inductive kick. Well, they would spike a 2N2222, but I don't think they could hurt a fairly robust 12 vdc power supply... Still, having a flyback installed across a relay coil, driver transistor or not, is still probably a pretty good idea....
    The diode is a good idea. I once repaired a little relay driver without a diode that was killing 150V transistors so I measured the flyback voltage with a 'scope and it was over 350V.

    Dave H.
    Last edited by Dave H; 03-08-2011 at 02:34 PM.

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