RC Snubber for headlight cut out switch - Stromtrooper Forum : Suzuki V-Strom Motorcycle Forums
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post #1 of 36 Old 12-03-2014, 01:51 PM Thread Starter
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RC Snubber for headlight cut out switch

All,

My background is Mechanical not Electrical Engineering so a question for the EEs in the crowd. Would adding an RC Snubber parallel to the headlight cutout contacts in the starter switch extend the life of the contacts and prevent early failure? What would be good R and C values for the Snubber circuit? How would you calculate those values based on headlight loads? Since the load is resistive and not inductive can you just use a capacitor to protect the contacts? Do you think the contacts are mostly damaged during opening or closing the switch? (my guess is on close when the dV is highest).

I think this might be a cheap and easy mod for those of us that don't want headlight/starter switch issues but don't want/need the headlight relay fix.

Here is a random example found online;
504M02QA100 Cornell Dubilier | Mouser

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post #2 of 36 Old 12-03-2014, 08:27 PM
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Cool what should snubber be?

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Originally Posted by dmfdmf View Post
All,

My background is Mechanical not Electrical Engineering so a question for the EEs in the crowd. Would adding an RC Snubber parallel to the headlight cutout contacts in the starter switch extend the life of the contacts and prevent early failure? What would be good R and C values for the Snubber circuit? How would you calculate those values based on headlight loads? Since the load is resistive and not inductive can you just use a capacitor to protect the contacts? Do you think the contacts are mostly damaged during opening or closing the switch? (my guess is on close when the dV is highest).

I think this might be a cheap and easy mod for those of us that don't want headlight/starter switch issues but don't want/need the headlight relay fix.

Here is a random example found online;
504M02QA100 Cornell Dubilier | Mouser
(I write as a practicing electronics designer.)

A snubber of appropriate design would do much to extend switch life. To design it would require knowing the switched load current at turn-off (Isw) and the inductance of the current loop being switched, (Lsw). The resistance should be something near 24V/Isw and the capacitance near Lsw/(24V/Isw)^2 . (Units are Amperes, Ohms, Farads and Henries.)

What damages switches is arc formation where the arc energy is mainly set by the energy stored in the switched loop inductance just prior to turn-off, U = Isw^2 * Lsw / 2. Without the inductance, switches would last much longer. There is also some arc formation during turn-on, involving the inrush current for incandescent lamp loads, due to contact bouncing, but I think that is less of an issue and the same snubber would serve to reduce arc energy then also.
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post #3 of 36 Old 12-04-2014, 11:51 PM Thread Starter
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Originally Posted by Trepidator View Post
(I write as a practicing electronics designer.)
Sorry, I wasn't trying to exclude anyone with the EE comment, you probably know a lot more electronics than I do. I am way too familiar with the Mysterious Blue Smoke. I like to stick to F=MA.

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A snubber of appropriate design would do much to extend switch life. To design it would require knowing the switched load current at turn-off (Isw) and the inductance of the current loop being switched, (Lsw). The resistance should be something near 24V/Isw and the capacitance near Lsw/(24V/Isw)^2 . (Units are Amperes, Ohms, Farads and Henries.)
I was afraid it was this complicated and getting those numbers would be near impossible except for Suzuki or a fully equipped test lab like UL.

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What damages switches is arc formation where the arc energy is mainly set by the energy stored in the switched loop inductance just prior to turn-off, U = Isw^2 * Lsw / 2. Without the inductance, switches would last much longer.
Yes, I've been poking around the internets and got that too. What is interesting (and I'll explain below) is that this Starter/Headlight switch problem is almost a mirror image of the contact arcing damage when turning off an inductive load for which these snubbers are designed to minimize.

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There is also some arc formation during turn-on, involving the inrush current for incandescent lamp loads, due to contact bouncing, but I think that is less of an issue and the same snubber would serve to reduce arc energy then also.
Exactly what I was thinking. In fact I was originally thinking that maybe just a high Ohm shunt resistor might drop the voltage and thus keep the arcing down since the headlights are not an inductive load.

In the typical case (in which snubbers are used) the opening of contacts causes the collapsing magnetic fields in the inductor to spike the voltage and dump a bunch of energy across the barely open contacts that cause the damage. The snubber just gives that dissipating inductor energy a place to go instead of across the contacts.

What I realized is that the headlight contacts are not seeing an inductive LOAD on open but an inductive (EMF) SOURCE on closing. That was what I meant by this being the mirror image of the typical snubber use case. When you push the starter button the headlight cut off contacts are probably fine (probably even spec'd for that load plus margin) but when the starter button is released the motor is now spinning up and I'd bet that the magneto/stator generator is dumping a lot of energy into the system that is not yet being shunted by the R/R (too slow). Moreover, I am guessing that the field energy is really high when a magneto is starting because the coils in the stator have not saturated yet and back EMF is still building. This is all happening about the time that the headlight contacts close. Once the startup transient fades everything calms down and the output is per spec at steady state operation but the contacts have probably already arced and then seated.

So, my conclusion is that a snubber across the headlight contacts should help with that failure mode. It certainly won't hurt anything and it would be a cheap and easy mod. My only question is what R & C snubber to use but I think almost any high R, C combo will work because the time constant of the snubber absorbing energy is related to R times C. As long as the time constant is on the order of 2-10 seconds then the snubber will take the brunt of the generator start up transient and the headlight contacts can close without damage.

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post #4 of 36 Old 12-05-2014, 03:55 PM
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Cool A motorcycle headlamp switch snubber

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Originally Posted by dmfdmf View Post
I was afraid it was this complicated and getting those numbers would be near impossible except for Suzuki or a fully equipped test lab like UL.
Rather than setting up instruments and pulling connectors to measure the wiring harness inductance, I have estimated it at 3 microHenries. Much of this is wire self-inductance, not dependent on the exact path of the current loop and so mainly dependent on the length and wire diameter. So this is going to be close enough for a snubber design.

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Originally Posted by dmfdmf View Post
In the typical case (in which snubbers are used) the opening of contacts causes the collapsing magnetic fields in the inductor to spike the voltage and dump a bunch of energy across the barely open contacts that cause the damage. The snubber just gives that dissipating inductor energy a place to go instead of across the contacts.

What I realized is that the headlight contacts are not seeing an inductive LOAD on open but an inductive (EMF) SOURCE on closing. That was what I meant by this being the mirror image of the typical snubber use case. When you push the starter button the headlight cut off contacts are probably fine (probably even spec'd for that load plus margin) but when the starter button is released the motor is now spinning up and I'd bet that the magneto/stator generator is dumping a lot of energy into the system that is not yet being shunted by the R/R (too slow). Moreover, I am guessing that the field energy is really high when a magneto is starting because the coils in the stator have not saturated yet and back EMF is still building. This is all happening about the time that the headlight contacts close. Once the startup transient fades everything calms down and the output is per spec at steady state operation but the contacts have probably already arced and then seated.

So, my conclusion is that a snubber across the headlight contacts should help with that failure mode. It certainly won't hurt anything and it would be a cheap and easy mod. My only question is what R & C snubber to use but I think almost any high R, C combo will work because the time constant of the snubber absorbing energy is related to R times C. As long as the time constant is on the order of 2-10 seconds then the snubber will take the brunt of the generator start up transient and the headlight contacts can close without damage.
I'm not going to pick apart your reasoning, or justify mine here. Perhaps, if authority matters, it will suffice to say that people pay me good money to render advice on problems such as this.

The load and wiring harness are in series, so it is a fine distinction, as far as the switch is concerned, to treat load inductance and wiring inductance separately. The inductive energy is mainly in the magnetic field surrounding the wire rather than inside a motor or solenoid, so your "mirror image" idea makes some sense. However, the inductance is much smaller than what is found in the usual "inductive" load. This means that a snubber sold for inductive load switching is unlikely to be close to optimal.

Based on that 3uH inductance, I would put a series R-C across the switch contacts, with 1 uF capacitance and 2.4 Ohms resistance. This is for a pair of 60W headlight lamps. This keeps the voltage across the switch limited to about 22V, yet adds little to the turn-on stress. (I simulated the circuit to confirm snubbing performance.) A 25V capacitor and a 1/4W resistor should be quite adequate.

Last edited by Trepidator; 12-05-2014 at 04:16 PM.
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post #5 of 36 Old 12-06-2014, 12:22 PM
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Cool snub ignition switch too

Looking at the schematic, it appears that another snubber, (with the same 1 uF and 2.4 Ohm series components), should be placed across the orange and red wires at the ignition switch. There is a contact carrying headlight current there which is stressed similarly to the NC contact in start switch.
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post #6 of 36 Old 12-06-2014, 05:37 PM
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There is a short and simple answer:
Headlight is not an inductive load. Then you get no benefit installing a snubber.
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post #7 of 36 Old 12-06-2014, 07:24 PM
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Cool Why snub a resistive load?

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Originally Posted by fortp View Post
There is a short and simple answer:
Headlight is not an inductive load. Then you get no benefit installing a snubber.
If that were true, then all the fuss about getting a relay to switch the headlights instead of the thumb-operated start switch or the ignition switch would be pointless. A corollary would be that the contacts switching the headlights should last for hundreds of thousands of cycles. But they do not.

The fact is that when either contact closes there is about 100 Amps of current flow while the filaments are first heating. There is enough wire in the switched current loop to add a few microHenries of inductance to the whole circuit. When that gets interrupted, as the contacts bounce, 10+ milliJoule arcs are formed, in the very small arc volume. Diverting most of that energy into a snubber will reduce pitting and metal transfer at the contact surfaces.
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post #8 of 36 Old 12-07-2014, 02:37 PM Thread Starter
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Quote:
Originally Posted by Trepidator View Post
Looking at the schematic, it appears that another snubber, (with the same 1 uF and 2.4 Ohm series components), should be placed across the orange and red wires at the ignition switch. There is a contact carrying headlight current there which is stressed similarly to the NC contact in start switch.
I was thinking about the ignition contacts as well. In fact my new-to-me K7 has two keys because the ignition had to be replaced because the headlight contacts fried. My plan now is to steal graywolf's trick of holding the starter button in while turning on the ignition to spare the headlight contacts. This only works on bikes with the clutch cutoff switch, so its not a strategy for everyone.

As for the headlight cut off switch, I am in the middle of a winter rebuild project and planned on inspecting and servicing the switch anyway. I will probably install a snubber but not sure on the type.

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post #9 of 36 Old 12-07-2014, 03:05 PM Thread Starter
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Originally Posted by Trepidator View Post
If that were true, then all the fuss about getting a relay to switch the headlights instead of the thumb-operated start switch or the ignition switch would be pointless. A corollary would be that the contacts switching the headlights should last for hundreds of thousands of cycles. But they do not.
I agree something is wrong with the design and certainly the relay kits do fix the problem (with other benefits). I am planning on servicing my starter switch anyway and would rather insert a $6 snubber than a $70 relay setup.

Quote:
The fact is that when either contact closes there is about 100 Amps of current flow while the filaments are first heating.
I don't know if it makes a difference but when you turn on the ignition the headlights turn on so they aren't cold when the headlight cut out turns them off then back on in about ~1sec or so. I believe that the cold filament inrush current is what typically damages the headlight contacts in the ignition switch. My plan to protect the ignition switch is to turn on the ignition with the starter button pushed in and the clutch cutout switch preventing it from turning over (certain models only, credit to GW).

Quote:
There is enough wire in the switched current loop to add a few microHenries of inductance to the whole circuit. When that gets interrupted, as the contacts bounce, 10+ milliJoule arcs are formed, in the very small arc volume. Diverting most of that energy into a snubber will reduce pitting and metal transfer at the contact surfaces.
I don't think it is the inductive energy in the wiring that is damaging the contacts. I think the extra unaccounted for (in the design) energy is the generator start up transient that the R/R cannot dampen completely. This is my only extra assumption but I think it is real.

Cutting off inductive loads means the energy of the inductor is dumped across the contacts causing damage. The snubber just gives that energy a place to go besides the contacts. This is exactly analogous to the transient energy of a starting generator dumping across closing contacts. In both cases a snubber would give that energy a place to go instead of across the contacts, I just need to know what values of R & C to use.

Trepidator, I really appreciate your comments and expertise. Please feel free to shoot a hole in my theory so I can move on to other ideas. Thanks.

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post #10 of 36 Old 12-07-2014, 03:55 PM
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Cool snubber cost, benefit

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Originally Posted by dmfdmf View Post
I agree something is wrong with the design and certainly the relay kits do fix the problem (with other benefits). I am planning on servicing my starter switch anyway and would rather insert a $6 snubber than a $70 relay setup.
The labor cost (or value) swamps the parts cost. The 2.4 Ohm resistor and 1 uF capacitor cost 65 cents per snubber.

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Originally Posted by dmfdmf View Post
I don't know if it makes a difference but when you turn on the ignition the headlights turn on so they aren't cold when the headlight cut out turns them off then back on in about ~1sec or so. I believe that the cold filament inrush current is what typically damages the headlight contacts in the ignition switch. My plan to protect the ignition switch is to turn on the ignition with the starter button pushed in and the clutch cutout switch preventing it from turning over (certain models only, credit to GW).
I think you overestimate the degree to which the filaments stay hot during a start. A good measure of how quickly they cool is how quickly they become non-light-emitting. That looks to be below a hundred mS to me.

The clutch cutout switch does not carry headlight current in any schematic I have seen. It is in series with the coil of the starter relay.

Quote:
Originally Posted by dmfdmf View Post
I don't think it is the inductive energy in the wiring that is damaging the contacts. I think the extra unaccounted for (in the design) energy is the generator start up transient that the R/R cannot dampen completely. This is my only extra assumption but I think it is real.
The only energy worthy of concern for switch contacts is that which exists because current is flowing and which must vanish when the current ceases. The battery buffers anything like that which the generator exhibits. The battery is highly capacitive, and there are other, more explicit capacitors on the 12V bus, at the computer at least. So the only inductance I see needing some snubbing is in the wiring harness.

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Originally Posted by dmfdmf View Post
Cutting off inductive loads means the energy of the inductor is dumped across the contacts causing damage. The snubber just gives that energy a place to go besides the contacts. This is exactly analogous to the transient energy of a starting generator dumping across closing contacts. In both cases a snubber would give that energy a place to go instead of across the contacts, I just need to know what values of R & C to use.
I agree with that first sentence, provided that "loads" includes all inductance in series with the switch. Technically, the inductive energy is first going into the arc between the contacts. It only gets into the contacts as heat or metal ion motion. Nevertheless, the snubber provides an alternative destination for most of that energy.

When the contacts close and stay closed, there is no mechanism for damaging them other than excessively high current flowing through too little contact area. This is because the voltage across the contacts is low when they are closed, and so the power dissipated there (which is the product of voltage and current) is necessarily low.

It is contact bounce, where there is not simply a simple closing but rather a closing followed a few mS later by a brief opening, which can damage contacts. It is a rare mechanical switch that does not have 1 or 2 bounces upon closing. And it is at that event that the 10-15 X inrush current is flowing, which translates to 100-225 X as much inductively stored energy which will go into an arc if not a snubber.

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Originally Posted by dmfdmf View Post
Trepidator, I really appreciate your comments and expertise. Please feel free to shoot a hole in my theory so I can move on to other ideas. Thanks.
Perhaps this will count as partial penance for my contribution to the mercifully killed thread on climatology.
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