Fast snubber circuit for relay coil

[pwnell]

I have a 24VDC (coil) relay that has a built in LED + resistor in parallel with the coil:



Since I cannot modify this (it is inside the relay package), if I want to use the diode + zener diode circuit to speed up the relay switch time, can I add a diode and zener in parallel?  Would it be effective?

[langwadt]

it'll be fastest without the diodes

[Zero999]

Or put the zener diode across the switching device.

[Kleinstein]

The LEDs usually have a quite limited reverse voltage. So one kind of needs the diode and a zener of not too high a voltage (e.g. 4 V max) to stay below some 5 V as a typical limit to the LEDs.  The relay may include an additional diode anti parallel to the LED - than one may get away without the extra external diodes.

The diode in parallel to the coil is usually there to limit the inductibe kickback voltage to protect the driver and here also the LED. The diode make the relay a little slower to turn off.

[wraper]

can I add a diode and zener in parallel?  Would it be effective?

For what?
Circuit below makes no sense, snubber/LED should be in opposite direction. Also are you sure there is no diode inside the relay already? If there is none you'd likely kill the LED on first removal of power without external low voltage snubber as LEDs usually are only good to about 5-6V of reverse voltage (unless it has reverse voltage protection not shown). I suggest saying part number of the relay.

[jfiresto]

Here is something to read if you are wondering about a reverse transient and the LED.

[pwnell]

It is this relay (with LED):

JQX-13F(D) DC 24V Coil

https://www.handsontec.com/dataspecs/JQX-13F.pdf

[Zero999]

can I add a diode and zener in parallel?  Would it be effective?

For what?
Circuit below makes no sense, snubber/LED should be in opposite direction. Also are you sure there is no diode inside the relay already? If there is none you'd likely kill the LED on first removal of power without external low voltage snubber as LEDs usually are only good to about 5-6V of reverse voltage (unless it has reverse voltage protection not shown). I suggest saying part number of the relay.

There's nothing wrong with that circuit.

D4 is the snubber, which is in the opposite direction as the LED. D3 is just to prevent the zener from conducting in the forward direction.

[wraper]

can I add a diode and zener in parallel?  Would it be effective?

For what?
Circuit below makes no sense, snubber/LED should be in opposite direction. Also are you sure there is no diode inside the relay already? If there is none you'd likely kill the LED on first removal of power without external low voltage snubber as LEDs usually are only good to about 5-6V of reverse voltage (unless it has reverse voltage protection not shown). I suggest saying part number of the relay.

There's nothing wrong with that circuit.

D4 is the snubber, which is in the opposite direction as the LED. D3 is just to prevent the zener from conducting in the forward direction.

Diode only conducts in direction where snubbing is not needed. In fact you will blow up zener diode if its clamping voltage is lower than relay coil voltage.

[wraper]

It should be like this. EDIT: but then if LED in not internally protected from reverse voltage, any snubber other than simple diode will not clamp the voltage low enough to be safe for the LED. It probably will survive if it's say <24V zener and inefficient LED made by old technology as they have much higher actual reverse breakdown than 5-6V specified in datasheets. Modern high efficiency LEDs likely will not survive it though. The only type of snubber that is safe for this type of relay is a simple diode. If you want faster release, use a relay without a LED or one that specifies that LED is protected against reverse voltage or it works in either polarity (antiparallel LED) so you can safely use snubber like below.

[Zero999]

Oh, I missed he fact the LED was connected backwards. 

Reverse volage is no a problem, for old GaP based LEDs which will non-destructively avalanche and is most likely what's used as an indicator inside the relay. Newer InGaN LEDs are more of a problem and can be destroyed by over voltage.

[MarkT]

That reminds me of a suspicion I have - that every device which hasn't had its reverse voltage properly characterized has "-5V" in its datasheet - many BJTs Vbe and LEDs have this suspiciously round 5V specification for reverse voltage, which I take to mean "we don't really know" or perhaps "its not a tightly controlled parameter but samples survived a quick test with 5V"

[wasedadoc]

That reminds me of a suspicion I have - that every device which hasn't had its reverse voltage properly characterized has "-5V" in its datasheet - many BJTs Vbe and LEDs have this suspiciously round 5V specification for reverse voltage, which I take to mean "we don't really know" or perhaps "its not a tightly controlled parameter but samples survived a quick test with 5V"

Try building a conventional 2 transistor multivibrator and varying the supply voltage between 4 and 10 volts.  Somewhere in that range you will find that the frequency ceases to remain fairly constant around the theoretical rate given by the cross coupling capacitors and the base resistors. Frequency will increase as supply volts increase when the reverse biased base-emitter junctions act as zeners providing additional current to the capacitors. The transistors are not damaged and expected operation returns when you go back to lower voltages.

[wraper]

That reminds me of a suspicion I have - that every device which hasn't had its reverse voltage properly characterized has "-5V" in its datasheet - many BJTs Vbe and LEDs have this suspiciously round 5V specification for reverse voltage, which I take to mean "we don't really know" or perhaps "its not a tightly controlled parameter but samples survived a quick test with 5V"

For BJT it's actually very close to that. I've seen BJT V_EBO used as zener diode. IIRC you get about 6.5V breakdown voltage for most of them.

[Zero999]

BJTs don't tolerate reverse bias very well, unless the current is very low i.e. the leakage through a reverse biased diode.

GaP LEDs are generally unharmed by reverse voltage, so long as the power dissipation isn't excessive. I've connected LEDs to the mains, with nothing other than a current limiting resistor and they were fine.

[SteveThackery]

I'm sorry if I'm being thick, but going back to the original question, surely adding any current-carrying device in parallel with the coil will slow down the relay release time.

Won't it?

[T3sl4co1l]

I'm sorry if I'm being thick, but going back to the original question, surely adding any current-carrying device in parallel with the coil will slow down the relay release time.

Won't it?

The angle of responses, when this question was asked of EE.SE, was to consider the performance/survival of the transistor itself, and optimize settling time within that constraint.

Otherwise you're just avalanching the poor thing and it will eventually fail (or overheat if switched too often).

Other than that, yep, it won't be faster than a true open circuit, but the skin effect of the armature itself (who knows how it's constructed, maybe it's solid, maybe it's laminated, but there's a cutoff in there whatever the case) will have a minimum decay in some ~ms, so it probably doesn't matter beyond the zener+diode or S-D clamp zener cases, which grant unlimited survival, and well-defined power dissipation limits.

Tim

[SteveThackery]

Other than that, yep, it won't be faster than a true open circuit, but the skin effect of the armature itself (who knows how it's constructed, maybe it's solid, maybe it's laminated, but there's a cutoff in there whatever the case) will have a minimum decay in some ~ms, so it probably doesn't matter beyond the zener+diode or S-D clamp zener cases, which grant unlimited survival, and well-defined power dissipation limits.

The OP asked a very simple question: will his suggested circuit work to speed up the relay release time. The answer is:

No, it will slow it down.

The thread has wandered off completely, and is going on about back emf spikes. But that is not what the OP was asking about. Every circuit proposed in this thread will leave the relay release time unaffected or will increase it.

The only two people in this thread to get it right are @langwadt and me.

[T3sl4co1l]

Well, yeah? Thread drift occurs; this is a nearly unmoderated forum, we're just here to shoot the shit.  Many of us do take questions seriously, but once the question has been answered, what's left to discuss?  Context drifts, that's just how it goes, if it's not to your taste, put the thread on the ignore list, whatever is fine.

If one expects a direct, more tightly moderated question-answer format, I would suggest Stack Exchange or the like.

Or, at the very least, make such strict expectations clear at the outset -- not that we are at all required to honor such expectation, but some may take the hint, play as inline moderators, etc. as a result.  But, without any real moderating power (not even suggested edits, let alone the power to flag/move/edit/delete posts), such activity likely clutters the thread that much more.  And in any case, OP here didn't raise such an expectation, so it seems odd to assert that later on.

Speaking of which, ah here it is, the question I was thinking of before: https://electronics.stackexchange.com/questions/719405/best-method-to-dampen-inductive-spiking-from-pulsing-coil-with-mosfet not quite the same, but a very similar central point.

Tim

[jfiresto]

... The OP asked a very simple question: will his suggested circuit work to speed up the relay release time. The answer is:

No, it will slow it down.

The thread has wandered off completely, and is going on about back emf spikes. But that is not what the OP was asking about. Every circuit proposed in this thread will leave the relay release time unaffected or will increase it.

The only two people in this thread to get it right are @langwadt and me.

That depends on your meaning of getting it right. It is one thing to answer the question, it is another to recognize what the problems are and are not. I would think that someone, here, would be more interested in solving the former, and noting the latter, in case the latter become problems some day.

[SteveThackery]

That depends on your meaning of getting it right. It is one thing to answer the question, it is another to recognize what the problems are and are not.

The correct answer was given in post number 2 by @langwadt. It was completely ignored by everyone, including the OP.

Yes, threads drift (I'm as guilty as anyone), but this drift is different: the proposed solutions - literally from post 3 onwards - all make the relay release lag worse.  In fact, most of the discussion suggests that the contributers hadn't even read the original question past the word "snubber" in its title.The thread drift was 180 degrees in the wrong direction.

Nobody acknowledged that #2 was the correct answer. That's why I felt it important to point it out.

[SteveThackery]

Well, yeah? Thread drift occurs; this is a nearly unmoderated forum, we're just here to shoot the shit.  Many of us do take questions seriously, but once the question has been answered, what's left to discuss?  Context drifts, that's just how it goes, if it's not to your taste, put the thread on the ignore list, whatever is fine.

Totally understand and agree. My concern is specific to this thread. It wasn't clear that post #2 was the answer and everything after that is "shooting the shit". Post #3 begins with "Or...", like it's an alternative solution to that given in post #2. It isn't. It should have been challenged because it does the very opposite of what the OP was asking, and what #2 was explaining. (My fault - I didn't challenge it either.)

[voltsandjolts]

Perhaps the OP wants to switch the relay contacts faster to reduce arcing at the contacts? If that is the case, then I suggest a snubber at the coil side, to protect the switching element, and another snubber at the contact side, to protect the contacts.

[SteveThackery]

Perhaps the OP wants to switch the relay contacts faster to reduce arcing at the contacts? If that is the case, then I suggest a snubber at the coil side, to protect the switching element....

How will that make the relay release faster?

Perhaps the OP wants to switch the relay contacts faster to reduce arcing at the contacts?

Perhaps. The reason wasn't given, so we are speculating.

[voltsandjolts]

Perhaps the OP wants to switch the relay contacts faster to reduce arcing at the contacts? If that is the case, then I suggest a snubber at the coil side, to protect the switching element....

How will that make the relay release faster?

It doesn't. Like I said, it protects the switching element.

Perhaps the OP wants to switch the relay contacts faster to reduce arcing at the contacts?

Perhaps. The reason wasn't given, so we are speculating.

Yup. Like so many newbie threads not enough information about the overall application is provided, so we don't even know if the OP is asking the right question. Hence the guess work, all in an effort to help them, and simultaneously irritate you it seems.