# When sparks fly, are spikes sure to follow?

When two positive electrical circuits contact each other, and one is 14 volts dc, and the other is 10 volts dc, do voltage spikes occur at the moment of contact, and what would the magnitude be of those voltage spikes?

I would say the short answer to your question is there will be some pulses on the line when connected. Assuming that you are referring to connecting two batteries I would say that these pulses will be pretty close to the battery voltage. When you get into a circuit with high inductance or capacitance then the answer gets more complicated.

In a situation like this “spikes” occur as contact is made then broken. Just touching two wires together can cause a dozen make/break contacts before a constant and consistent connection is made. This typically happens every time you turn on a switch also, and in that situation is known as “switch bounce”.

Magnitude in your example would typically max at 14 volts.

The voltage of the “spike” could be 14 volts or 24 volts depending on how they were connected. Spikes and sparks are not exactly the same thing.

Generally, no. Simply completing a circuit, regardless of the net voltage, does not necessarily cause either sparks or spikes. That is why home light switches last 25 years or more.

This is a very complicated question. It may well depend on the inductance and capacitance in the circuit. Attaching a battery cable to the battery often is accompanied by sparking, chiefly because the alternator is in the system. The alternator has coils that naturally provide a large inductance, and there is very limited capacitance.

We have all seen sparks fly when an automotive battery is disconnected or reconnected, and we note there is no further damage to automotive systems. Let this observation be your guide.

Generally, no. Simply completing a circuit, regardless of the net voltage, does not necessarily cause either sparks or spikes.

Are you sure you want to stick with that statement?
I have watched high voltage contactors arc between the contacts long before they actually come in contact with each other. And they arc repeatedly as the switch bounce diminishes. The transients produced by these operating is astounding. Even low power relays vaoprize contact material with each operation. It’s just less pronounced as the power level gets lower.

You mentioned the LCR part of the equation as to whether or not transients will occur. The circuit load versus contact area is another factor. Battery terminals will arc significantly more if there is an immediate demand due to some load being left on. It “arcs” because you cannot get an instantaneous connection with a sufficient cross section of the materials to prevent vaporization.

Many circuits produce high voltage transients for these reasons but they last miniscule amount of time so the overall power is low. Radiated and conducted “noise” can be very high from something as simple as connecting your battery terminals. High voltage transients can cause puncture failures in semiconductors when they have enough potential and duration to exceed the barrier resistance. But most of the electronics have provisions for this expected phenomenon like shielding, ferrite beads and/or special diodes (e.g. tranzorbs) as a few examples to safely shield from or dissipate the energy.

Positive with respect to what? Do they have the same ground? As detailed by others, it depends on the circuit.

I agree with others that it’s highly dependent in the circuit.

Sparks and spikes are two different phenomenon.

A spark happens when the space between two conductors becomes small enough that the voltage (which is actually the potential between the two conductors, one relative to the other) is strong enough to overcome the resistance between the contacts and free electrons move through the elements in the air (the particulates actually) jumping from atom to atom between the conductors until equilibrium is achieved or full contact is made. It can also happen if the voltage (the difference between the aggregate of free electrons on one electrode and the aggregate of free electrons on the other) becomes high enough to push free electrons through the resistance of the gap. This is also called a discharge or a spark (if visable).

If there is some continued source of electrons, such as a charged battery or a generator, then current continues to flow. If not, voltage drops (how fast depends pn circuit resistance) until equilibrium is reached.

A spike is a sudden jump in voltage to a specific point due to the aforementioned discharge. That will not exceed the maximum difference in potential between the two points, and how it discharges (peak voltage reached and discharge rate) will depend on alot of factors in the circuits themselves. It gets messy, bacause ignition systems use induction to “stack” many spikes induced from individual collapsing coils to achieve high voltages out of low input voltages, creating enough potential to discharge through a relatively large gap.

Batteries are actually a form of capacitor, and capacitors a form of battery. Both store electrons on one plate and can have a relative deficiency on the other, with a dielectric in between. Both can be designed to discharge at various rates across the dielectric, and both have some unavoidable leakage across the dielectric. Both will blow their dielectrics if forced with enough voltage applied from an outside source.

So there you are. It’s all about the difference in free electrons between one and the other. And resistances to electron flow. And capacitors. We won’t get into diodes and bridge rectifiers and such.

What was the question again?

One thing that most or all of the replies have missed is that the difference in potential is 4 volts dc. Another is that the power supply current may be large or small or somewhere in between. Any arcing that you might see is a result of metal vaporization at the point of contact which initially is very small and therefore subject to overheating more or less or none depending on the available supply current. If you are dealing with a resistive load, the resulting arc voltage will be very small; in the neighborhood of 4 volts. An inductive load, however, can generate a high transient (spike) voltage during the time that the contact gap is open during one of the material vaporization events (during what is called contact bounce) with the potential for generating hundreds of volts. Air at room temperature and pressure has a dielectric constant of about 60,000 volts per inch so if the temporary air gap at the area of contact is .001", then you might see 60 volts or less, depending on how much ionized gas is in the gap.

I assume since you talk of 10 and 14 volts DC that you are really asking about the possibility of dammaging you car electronics when you connect the jumper cables to a low battery - short answer - if you do it correctly - you will be just fine.

Electronic circuits require vrey “clean” power - that is they won’t work correctly if the voltage is bouncing around all over the place. However, automotive power does just that - so the electronics are outfitted with circuits on the power inputs that “clean up” the voltage. The can smooth out some pretty severe spikes and I doubt you can create big enough spikes with you jumper cables if you are connecting them with the correct polarity.

HOWEVER - if you connect the jumper cables in reverse polarity - all bets are off and you should bring out steaks to BBQ on your now fried electronics!

A lot of GREAT responses to my little question. I appreciate them all.
I thought that someone might purpose actually using an oscilloscope on this to see what the voltage spikes (or, some say voltage “surges”) would, actually, be “When Two Voltages Suddenly Meet!”.
Yes, there are surge suppression devices in electronic circuits; but, one wonders if they are robust enough to handle the current surges which are resultant of the voltage surges. Most circuit protective devices do have a punch-through limit. If these limits are exceeded, the rest of the circuitry is subject to damage. What values are the voltage spikes, and are they damaging to the electronics? Most respondents said, “…shouldn’t be.”.
I realize that I wasn’t specific on the makeup of the circuitry for the two circuits. I tried to avoid making the scope of the question too broad, or too narrow. From the responses, it seems to have been an OK compromise. The respondents were, of course, free to expand or contract the scope as they deemed fit.
Now,if someone were to pull out, and dust off their O’scopes…

Spikes and surges are hard to predict without knowing some details about the two circuits that are being connected. The method by which you connect them matters a lot as well. I wonder how may automotive folks own oscilloscopes - or have even ever used one… it’s not a typical garage tool and most mechanics I know - even the really good ones - don’t use or have one.

But since you brought it up… Some things to consider is at what frequencies are you interested in looking at? I can take the 8 Ghz DSO (which is now an old tool) and see things on the order of fractions of nanoseconds - or I can dust off the trusty Tec 475 and be happy ignoring what is going on at the nanosecond level. One thing to remember though is that it’s not the magnitude of the spike that matters - it’s the energy it will deliver. This is the “area under the curve” concept you recall from your college calculus classes (if you took them). So a voltage spike of hundreds or thousands of volts that is only a few picoseconds wide at the base will likely be harmless to your automotive components while a 50 volt spike that lasts 50 ms will not be harmless. You need the 8G (or faster) scope to see the former the 475 can’t see that fast.

About the circuits - if you are connecting RC loads together - you probably won’t see any voltage spikes - and depending on R component, the current surges would likely be small. Again - depends on what the “other” circuit is. If you were connecting an RL circuit you may see voltage spikes, but the current surge would be very low or non existant. LC circuits can get interesting - and this is where you really need to know what’s going on in each circuit to predict the behaviour.

So anyway… sounds like you are administering a test here and don’t really have a problem to solve!

I didn’t want to limit the problem solving to one specific problem; but, leave it open to a family of problems. Since this is an automotive forum, the question, naturally, has applications to electronic circuits in general, and to automotive circuits in particular.
I was hoping that someone who is actively involved in designing, and protecting automotive electronics, could tell us what the measured dangers are, and what the measured design limits are, and what the protection limits are in the vehicle electronics. If we don’t know what to protect from what dangers, how can we protect them (the electronics)?
OP hellokit

When you open an inductive circuit, what happens is similar to the pressure surge known as “water hammer” when you shut a water faucet real fast. Once electric current gets going in an inductive circuit, it doesn’t want to stop.
The “mother of all inductive circuits” is a scrap metal lifting magnet. I tested one of them using a DC welder, about 90 volts. Making the circuit was totally uneventful but when I pulled the electrode away, the resulting arc was nearly a foot long. People ran over to see if I was OK. These magnets usually get 250 volts DC and draw about 80 amps. The contactors that control them have arc blowout magnets and large power resistors to shunt the current to until a reverse voltage contactor can come in to kill the magnet so it doesn’t take forever to drop the load.

“Once electric current gets going in an inductive circuit, it doesn’t want to stop.”

That is EXACTLY the function of an inductor. It resists changes in the flow of current.

You are lucky you weren’t killed or severly injured - an inductor of that size, excited with that much current, is capable of generating tens or hundreds of thousands of volts in its search for a circuit path for the current.

You would need to supply more specific information. I will assume you are jump starting a “dead” battery from a running vehicle. In this case, the regulator on the 14v system compensates for the added load of the low battery and the low battery simply begins to charge. Any voltage spike would be minimal. Of course, you would make sure the ignition is off on the 10v system when making your connections thus isolating most of the system from even this minimal spike.

I’m an electrical engineer but do not design automotive circuits or products. I can say with relative confidence that “sparking” your battery connection from a charger or a jumper cable connected to another car will not create spikes and surges that are beyond what you automotive electronics are designed to withstand. Keep in mind that designers know that these things happen and would never intentionally design something that would be dammaged by that. The big danger is reversing the polarity - this is not typically protected against.

“Are you sure you want to stick with that statement?
I have watched high voltage contactors arc between the contacts long before they actually come in contact with each other. And they arc repeatedly as the switch bounce diminishes. The transients produced by these operating is astounding. Even low power relays vaoprize contact material with each operation. It’s just less pronounced as the power level gets lower.”

Were you looking at AC or DC circuits? What kind of power source. In a simple DC circuit with a 12 volt battery source you are not going to get a spike over 12 volts. Un-regulated non-battery powered or AC systems you might get a spike over the nominal voltage.

Mountainbike is right. You have a net 4 volts available here to jump the gap, which, since it’s air, has infinite resistance. So how can any voltage jump the gap? Well, even though there’s no current at this point (open circuit), there can be voltage, which by itself can ionize the air (change it into a different substance, basically; one with non-infinite resistance). The lower the net voltage;

(14 minus 10 = 4); the closer the 2 contacts have to be for the ionization to take place. (As a counter-example an Amtrack overhead train wire with what- 100,000V or whatever could probably jump 3 feet.) OK, so the 4v jumps the gap when it’s maybe several thousandths of an inch across. (That’s a guess.) Now the current starts to flow but it takes a few milliseconds for it to get to its max. The magnetic field

that accompanies any flow of current through a conductor takes about as long to build to its max also. So now we have relative motion between a magnetic field and a conductor, which creates a momentary current flow, but how strong can it be? It’s amperage (current,
basically) that melts fuses and conductors, isn’t it? Now if you make and break a circuit real fast over and over you could get way

excessive current, I believe, but I don’t really know why. Maybe the engineers could answer that one.

That sounded pretty much like an engineer answer to me.