Relay Coil Suppression with single Zener Diode across BJT 4

[JDW0]

In terms of relay coil voltage spike suppression, the following thread focuses on using a 1N4007 diode in series with a Zener across the relay coil:

thread248-291225

But there are times when you need to suppress coil spikes but cannot put components across the relay coil. In this case, a single Zener diode across the Collector and Emitter of a common-emitter BJT (which drives the low side of the relay coil), should be adequate to suppress the coil's voltage spike to the Zener voltage. And using a Vz = 2 x Vcc (with Vcc being the coil's positive voltage) would ensure the presence of the Zener does not slow the switching mechanism in any significant way.

My question is how to properly calculate the wattage / power rating of the Zener. While we could just use a multi-Watt Zener "for safety," that is a rather "shot-in-the-dark" method that ignores size and cost. A 0.5W 1N5252B 24V Zener costs much less than a 5W 1N5359B 24V Zener, and there is a large size difference as well. As such, it is advantageous to know if the smaller and lower cost 0.5W Zener would be a long-term reliable choice to suppress relay coil spikes.

Consider this circuit:

The OMRON G8NW-2 relay shown is a single device that houses 2 coils and 2 switches. Here is the datasheet:

https://components.omron.com/compon...5AE814485257201007DD4FE/$file/G8NW-2_0607.pdf

In the above circuit we are using an NPN BJT to switch the low side of both coils, with their high side connected together at an automotive +12V. (That 12V is filtered, so don't mind about Load Dumps.) The Zener voltage was chosen to be twice Vcc, which is 24V. The resistance of one relay coil is 225Ω at 20°C, but the datasheet shows a worst case of 180Ω at -40°C. Low resistance means more current flow. At 225Ω we have 0.64W and 53mA current, but at 180Ω we have 0.8W and 64mA current; and again, that is for only 1 of the 2 coils.

I built a test circuit that matches follows the above schematic, first without any suppression and then with the Zener across the collector and emitter. On a scope I measured relay coil spikes at the low side of the relay coil (at the Collector of the BJT) to be from 100V up to about 143V:

The transistor used has a Vcc=50Vmax specification. Vce (voltage across the Collector and Emitter) will obviously be lower than Vcc with the relay in the circuit, low side connected to the Collector. But even if Vce was spec'd at 50V, that's still only half of the measured voltage spikes coming off the relay coils. As such, we need to protect the transistor, and that's where our Zener across the Collector and Emitter comes in.

Assuming a safe maximum Vce of 40V, it takes between 150us and 175us for the spike to decay to that level (varies by Vcc voltage level):

And with the 0.5W 24V Zener connected across the transistor, we see this on the scope:

But what is the minimum reliable wattage rating we should choose for the 24V Zener? That's my question. These are short-term voltage spikes, not continuous high voltage. And again, there are times when you need to reduce both cost and size, making a random selection of a high-wattage Zener impractical. I used a 1N5252B 24V 0.5W Zener across the transistor with the OMRON relay shown in the schematic for my measurements above. I've even used that same 0.5W Zener in a bench-top test circuit with 2 larger automotive relays which draw 122mA each. I only tested for a matter of minutes, not hours or days or weeks. But the 0.5W 1N5252B did not blow. (And actually, even without any suppression at all, for my short-term bench testing, the high voltage did not fry the transistor.) So I am curious if a 0.5W rated 24V Zener across the transistor would be a long-term reliable choice for relay coil spike suppression in this particular application.

I would appreciate hearing your thoughts.

 

[Skogsgurra]

JDWO
re: your comment 19 Jun 17 09:39 to my post 19 Jun 17 08:59

I must ask; is this a personal wind-mill that you are fighting? I gave you a perfectly working alternative and you still think that you need to complicate things a lot more than necessary.

A simple free-wheeling diode is what is used in most cases. And the delay it causes is very seldom a problem.

If it is a problem, then a series resistor can be used to reduce delay to less than half of that caused by a single diode. And certainly as much as a zener does. If your application is so very special that you need to keep a discussion like this for a ridiculously period of time, then please tell us what the application is. And if it is as exotic as you seem to think that it is, then ten cents or a dollar can't be difficult to justify.

But, actually, I don't think that your problem is real. Not even close to real.



Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[JDW0]

MacGyverS2000, thank you for sharing your thoughts. Most people don't read product documentation, and while that is technically not the fault of a product manufacturer, it is prudent to give due consideration to that reality in a given design. And as to "wanting guarantees for everything under the sun," I would say that is a pretty extreme exaggeration. I am exploring information written in datasheets, basically. Some datasheets say not to use a single diode across the coil, as I've already stated earlier in this thread. That alone should lead any "thinker" among us to explore other options. And many have chimed in with their own preferred solutions. But I added something to the mix by saying there are times when an open-collector output dictates you cannot apply suppression across a relay coil. And that is why I have not merely said to Mr. Skogsgurra, "Yes, a diode + relay across the coil is perfect!" Because, again, I am pondering the case where I cannot put any suppression across a coil nor expect someone else to do so outside the circuit.

Skogsgurra, it matters little to me if you "think the problem is real." It's actually rather amusing for you to say that in light of the fact we are exploring suppression options for kick back spikes -- something very real and confirmable on the bench. We are talking about datasheets because engineers use that information to design. If the interpretation is wrong or a datasheet lacks details, then any number of things could happen.

I appreciate your advice and certainly think your diode and resistor in series across a relay coil is a very good solution IF AND ONLY IF one can put suppression across the coil, which again, as I have repeatedly said is NOT ALWAYS THE CASE. I'll give you an example...

Vehicle Security Systems have existed for decades. Many of them have provided what is called a "GWA output" (Ground-When-Armed) which is nothing more than a 1A-capable open-collector output from a BJT, most of the time with no protection or perhaps a PTC fuse. That output is used by the security system installer to connect any many of things that can be controlled by Ground. They might connect an LED scanner on the dashboard to that GWA output, which would illuminate only when the alarm is Armed. They might alternatively or even at the same time, connect an external 12V relay to act as a starter kill switch, preventing a theft from entering the vehicle and starting the car even if they have the key, so long as the security system is Armed (and the GWA output is proving Ground to the relay). There might be other devices attached to that GWA line as well.

That is just one example. With that said, it is interesting that the unprotected transistors driving unsuppressed relays on these security systems are not failing left and right. One could argue that few users are connecting relays externally, or perhaps they are adding a diode suppressor (I honestly don't think so though), or the transistor is merely robust enough to be overstressed beyond the Vce spec in its datasheet mainly because the kickback voltage spike from the relay coil is so short in duration and infrequent.

But regardless of all that, it is still prudent to analyze datasheet information and consider the implications of relay coil voltage spikes and how to properly suppress those spikes for a give design. A 1N4007 across the coil does that job nicely, but only in cases where one has access to the relay coil AND on top of that some relay datasheets put notes about TACK-WELDING of the relay contacts, which is perhaps one reason why you yourself use a diode + resistor across the coil. But again (repeating myself countless times here), there are times when you want to protect your transistor but have no access to the relay to put suppression across it's coil, hence the discussion in this thread about a Vcc*2 Zener across the transistor, and other such suppression methods.

LionelHutz, you basically are referring to the third circuit in this example (D1 & D2):

Using something like a 1N4007 (1000V) for D2 and a part like a BZD27B10P-M (10V, 0.8W) Zener for D1 (cathode facing the Collector of the controlling transistor), going under the design assumption that Vcc (relay coil voltage) is 12V. And in your second example you provided a suggestion for a 24V system. I agree those are very reasonable suppression solutions when you have access to the relay coil. But in terms of your suggestion that I shouldn't be "concerned about putting the 10V, 0.8W zener in a circuit to protect a relay coil that sees a low duty cycle," someone will surely come along and remind us of that "death by a thousand cuts." And I don't mock them for having said that. However, I still feel that certain clarifications of diode datasheet information is prudent before making a design decision based on you "assume" it is actually saying. Like I said earlier in this thread, there is some vagueness as to the meaning of "repetitive" or "non-repetitive" in relation to actual current passed through a diode, the duration, and how warm the diode gets as a result. Again, if you look at my scope measurements earlier in this thread, you will see we are talking about a very short spike. The duration of the spike, until a safe Vce point of 40V is about 175us at 14.4V, and the duration of the portion of the spike above 100V is less than 25us in duration. (Again, one can glean all this from my scope measurements posted earlier.) So while I agree that death can happen by a thousand cuts, if the cuts are barely breaking the skin, death surely is not imminent.

In my own testing, even a 0.5W 24V Zener across a BJT works works to suppress coil spikes in repeated tests (hours) without the Zener failing. And my decision to use a 1.0W 24V Zener adds more safety margin to what I already know. And while I could go berserk and just use a 3W Zener, as I said, there are cost considerations to some designs that may prohibit that added cost, even if some argue it is "but pennies."

Anyway, I sincerely appreciate all of the helpful insight you gentlemen have shared in this thread. It has helped me to consider a wider set of variables. THANK YOU.

 

[IRstuff]

You wrote a lot, but nowhere in that writing did you actually say WHY you couldn't install an FWD across the relay. The fact that people haven't and the fact that the transistors have died isn't a reason why.

If the OC transistor has a collector breakdown voltage of, say, 30V, that might be sufficient to kill the spikes anyway, particularly if the relay current is barely 100 mA. That's not necessarily going to immediately kill the transistor, and if the owner re-sells the system soon enough, they may get away with it.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

faq731-376 forum1529 Entire Forum list

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[JDW0]

Actually, I have repeatedly said "why" throughout this entire thread. To repeat...

There are some designs that provide only an open-collector output (a switchable ground) which may or may not be used to control an external relay. In the case where an external relay is attached (i.e., not mounted on the same PCB where you find the open-collector transistor), that relay may or may not have coil suppression. It is most prudent to assume (a) that external relay does NOT have suppression and (b) it won't be added by the person adding the relay. As such, the question then becomes, "How shall I best protect my NPN transistor, not knowing if someone will attach an unsuppressed relay to it?"

As I have said repeatedly in this thread, one method I am rather warm to is to use a Vcc*2 Zener across the Collector and Emitter of the said transistor. The wattage rating of that Zener has been the heart of the discussion in my mind, but as I said, even with a 0.5W Zener (24V) I am not seeing the Zener fry in repeated testing.

Yet another way to protect the transistor is to simply use a high voltage transistor such as a BST39 which has a Vce of 350V, more than adequate to handle coil kickback. But as I said in my previous post, a lot of products like vehicle security systems have for decades used an open collector transistor with no special protection, to which an unsuppressed relay is attached. Using a BST39 would just offer more of a "guarantee" (if I can dare use that term) that a coil spike would never harm the transistor.

 

[MacGyverS2000]

In the end, the problem breaks down like this:
1) You provide a "reasonable" (to be defined by you) amount of circuit protection, state what that protection is in the manual, and it's up to the end-user to provide any necessary further protection.
2) Provide no extra circuit protection and let the chips fall where they may.

If a user fails to read the manual, well, sucks to be them. This is particularly true for case #1 above.

I'll state again... you appear to want a unicorn solution. You want to protect against all things a user might put on a specific line, and that's simply not reasonable/feasible. If you have an OC pin, then state in the manual the line is not to be used for relay control. Have a line specifically for relays if it makes you feel safer, but you're free to place limitations in the manual on that, as well.

I have torn apart many of the alarm systems on the market (DEI makes the majority of them these days). They are pretty damn simple. There are some over-voltage protection clamp diodes on the majority of the input lines, but in the end they connect directly to the processor pins (which, BTW, is rarely more than a basic Microchip PIC or a bottom-of-the-barrel Atmel ARM). Only a few are connected to driver circuits for more power, and the outputs of those are generally left unprotected.

But stop trying to prevent the circuit from the world... my laptop's manual says "Do not immerse in water", so I don't take it into the bath with me. Problem solved.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[LionelHutz]

Seriously? The car alarm is NOT an example. A car alarm has a 12V power connection to power it which means you certainly CAN put a protection device between the 12V and the collector of the transistor.

We also have relays on the circuit board with simple diodes across the coils and the relays contacts survive quite fine while operating large inductive AC loads.

 

[Skogsgurra]

JDW0
Were you trying to insult me when you said: "It's actually rather amusing for you to say that in light of the fact we are exploring suppression options for kick back spikes -- something very real and confirmable on the bench. We are talking about datasheets because engineers use that information to design. If the interpretation is wrong or a datasheet lacks details, then any number of things could happen"?

Your problem with the diode and slow release is what I meant. That should be obvious to most engineers that has dealt with real engineering and not being overly cautious and mis-interpreting badly supported rumours on the web.

Anyhow, I have been young and inexperienced myself. So I know how one can get a fixed idea in one's head and that such ideas can grow into real hinders. I therefore did a few tests with an OMRON 24 V DC relay (sorry, no 12 V). The comments in the recording should be clear enough. But I add some of them below:
1. The test shows coil voltage (brown traces) with a diode and also with a diode plus a 1200 ohms resistor (twice coil resistance)
2. The black traces are contact position. The movement is around 1 mm. A little less, actually.
3. The orange and yellow traces show NO and NC contacts and how they conduct.
4. The release speed is totally independent on if you use a single diode, diode plus resistors, zener, RC or MOV.
5. All that changes is delay. And that is of no concern in this application, if I understood you correctly.

I would normally bill you $150,000 (like IR), but I didn't use that much time on this. So 15 kUSD is OK with me. Where shall I send the bill?

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[Skogsgurra]

As I said before, this is part of a book on The Automation engineer and Reality that I have been working on for quite a few years. If it is possible to attach a pdf, you can read it here:

[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1499114042/tips/Relay_delay_okey_1_jqjqbj.pdf[/url]

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.