How does a series resistor inside a spark plug rduce EMF noise? - Page 1
Forum section: electronics / general

author: Beamin
author_rank: Super Contributor
timestamp: February 08, 2021, 09:33:30 pm
content: Wouldnt a capacitor work better? Plus you want the spark as hot as possible and a resistor would lower that. I have no idea what the ohms are if its like 10 ohm or 100k, I dont have one to test. I know putting a ceramic cap across your alternator will reduce noise but a resistor in the plug? Plus its going to get super hot and change value.

author: Mazo
author_rank: Regular Contributor
timestamp: February 08, 2021, 09:42:10 pm
content: Reduces the edge rates(think R-C),and dampens oscillations,in some cases might make the burn time longer at the expense of weaker spark(will that reduce sputtering of the spark plug electrodes is something I don't know) .Just for reference BP7ES and BPR7ES are essentialy the same plug,the difference being the R version has a 5K resistor inside.

author: Circlotron
author_rank: Super Contributor
timestamp: February 08, 2021, 11:26:34 pm
content: Depending on in-cylinder conditions on that particular cycle, the plug gap might take say 20kV to jump the gap. Once the arc occurs the voltage across the gap drops to maybe 100 volts in severalnanosecondsfor a duration of several nanoseconds while a couple of hundred amps flow (true arc) as the stray capacitance is discharged. After this, depending on the ignition coil parameters, a glow discharge will occur for 1mS or so with a current of 50-100mA running linearly down to zero, with a voltage across the gap of about 1500V. The sudden drop in voltage from 20kV down to 100V in nanoseconds makes the plug wires radiate noise, along with the sudden current pulse. The resistor reduces this rate of fall of voltage and also reduces the peak current.

author: amyk
author_rank: Super Contributor
timestamp: February 09, 2021, 01:09:18 am
content: A popular performance upgrade when points ignition was still the norm was copper-cored spark plug wires instead of the OEM carbon-cored ones.

author: gnuarm
author_rank: Super Contributor
timestamp: February 09, 2021, 06:48:47 am
content: Quote from: Beamin on February 08, 2021, 09:33:30 pmThe resistance is high, but not seriously high. Remember, the voltage on the plug is high, but the current is very low. The spark is actually an oscillation with many cycles of current, not unlike a bell ringing. The spark only needs to fire once and the first arc is the hottest spark. The cycles die away after that. But not fast enough to reduce the RFI adequately.The resistor takes a bit of energy from each cycle to make the oscillations die away more quickly without significantly impacting the initial spark. This hugely reduces the RFI from the spark. The only other way to mitigate the RFI is to use braid to enclose all the high voltage wires from the coil to the spark plug. But these days, aren't most coils on top of the spark plugs? I guess they still use one wire per coil to reach the other plug the coil fires.The capacitor on the points is on the low voltage circuit. It was originally added not to reduce RFI, but to preserve the life of the points. The oscillations in the primary circuit are a much higher current and erode the points. With the cap in place the back EMF from the coil is reduced and so the arcing is much reduced as well.

author: Stray Electron
author_rank: Super Contributor
timestamp: February 09, 2021, 08:04:48 pm
content: Quote from: amyk on February 09, 2021, 01:09:18 amWas there, did that, many, many times. But it wasn't so much for performance as it was to increase reliability. Carbon cored wires frequently failed and lead to poor ignition and replacing then with copper wires eliminated that problem. But replacing all of the ignition leads with copper wires caused too much ignition noise so we usually kept a carbon cored wire in the ignition coil to distributor lead and used copper cored wires in the distributor to spark plug wires. That gave good reliability with a minimum of ignition noise.

author: silverback
author_rank: Contributor
timestamp: February 09, 2021, 08:47:14 pm
content: Have a read of this from 1947

author: S. Petrukhin
author_rank: Super Contributor
timestamp: February 11, 2021, 11:05:52 am
content: The resistor limits the current, the lower the current, the less radiation in the general case.

author: Circlotron
author_rank: Super Contributor
timestamp: February 11, 2021, 11:31:45 am
content: Quote from: S. Petrukhin on February 11, 2021, 11:05:52 amYes and no.It does limit the current for the first few nanoseconds while the HV wiring stray capacitance is discharging, but for most of the spark duration, 1-2ms, the current is unchanged because the ignition coil secondary in a normal inductive discharge ignition behaves as a current source, not a voltage source. The coil simply puts out more voltage to make the current the same as it would have been without the extra resistance. This extra voltage causes the current to run down a bit quicker though so the spark duration is reduced a little.

author: langwadt
author_rank: Super Contributor
timestamp: February 11, 2021, 11:56:31 am
content: Quote from: gnuarm on February 09, 2021, 06:48:47 amwith coil-on-plug each plug has it's own coil, there's no high voltage wires

author: S. Petrukhin
author_rank: Super Contributor
timestamp: February 11, 2021, 12:02:07 pm
content: Quote from: Circlotron on February 11, 2021, 11:31:45 amI do not argue and I am not sure that I am right, just have this assumption: at the beginning of the discharge of the coil, the self-induction EMF transfers the charge to the electrode, while the resistance of the resistor is negligibly small relative to the resistance of the air gap. The resistance of all the circuits is very high, the current is negligible. But, when a breakdown occurs, a conductive plasma is formed, which has a low resistance and ringing (it is flounce by choosing different paths). There is a fluctuation of the current already felt, which causes the conductor to emit interference. This is where the resistor comes into play, which converts most of this current into heat, and the heat is low-frequency and does not make noise in the ether.

author: gnuarm
author_rank: Super Contributor
timestamp: February 11, 2021, 02:34:22 pm
content: Quote from: langwadt on February 11, 2021, 11:56:31 amIn four stroke engines each cylinder fires once every two revolutions of the crank. One coil can fire the spark plugs in two opposing cylinders on each revolution of the crank. One will ignite the air-fuel mixture and the other fires with an empty cylinder at the end of the exhaust stroke. This uses half as many coils (a significant savings) at the cost of a bit of wire.My V6 T100 did this. Each of three coils was on one bank of the engine and the wire ran over to the other bank. Man I miss that truck. :-(

author: langwadt
author_rank: Super Contributor
timestamp: February 11, 2021, 02:57:13 pm
content: Quote from: gnuarm on February 11, 2021, 02:34:22 pmalso know as "wasted spark"Quote from: gnuarm on February 11, 2021, 02:34:22 pmyou are right, that one uses a weird combination of coil on plug and wasted spark. usually it is either a coil on each plug or a seperate coil pack with outputs for each plug.guess it saves a bit of wire and space compared to a coilpack

author: S. Petrukhin
author_rank: Super Contributor
timestamp: February 11, 2021, 02:58:30 pm
content: Quote from: gnuarm on February 11, 2021, 02:34:22 pmMy Opel Omega is so arranged and has served me for 22 years.

author: HighVoltage
author_rank: Super Contributor
timestamp: February 11, 2021, 07:10:22 pm
content: The reflection of the discharge at the spark plug electrodes is damped at the series resistor.This can be observed by measuring before and after the resistor, with a special test spark plug but it is not easy.The first phase of the spark discharge is called "breakdown phase" and can easily reach 2000A within 10 ns.The EMF radiation from this discharge can be very large and the resistor is damping it.

author: SilverSolder
author_rank: Super Contributor
timestamp: February 11, 2021, 11:43:52 pm
content: Quote from: gnuarm on February 09, 2021, 06:48:47 amThe capacitor actually builds up a counter-EMF as it charges - making the current in the primary stop even faster, and generating a stronger spark. If the capacitor is taken out, the engine may not run at all... (Ask me how I know!)

author: GlennSprigg
author_rank: Super Contributor
timestamp: February 12, 2021, 12:10:30 pm
content: This is a 'little bit' off topic, but there is 'another' Resistor integration that is worthy of note!!A lot of older cars, (mainly Japanese etc), had a Resistor in series with the Coil, (low voltage).The coils are actually rated at about 6v or something, and when the engine is running, the 12v supply isdropped to what is really needed. The reason for this, is that while the ignition key is cranking the enginetoinitiallystart it, the system shorts out the resistor, putting over-voltage on the coil, to assist starting!Often, when that's not working, they can be very sluggish to start. A problem I've found a few times!

author: Gyro
author_rank: Super Contributor
timestamp: February 12, 2021, 12:18:00 pm
content: Thatresistor was often implemented in the form of a resistive wire (heated blanket style). It was easy to miss and would get replaced by an ordinary wire if it went open circuit - with predictable results!.I had a VW Golf (US: Fox or Rabbit) that had such a system: It fired up when I used the starter but immediately died when I released it. I finally traced that to a dry joint in the (PCB) Fuse box.Edit: That fuse box ended up with two or three spade crimped jumpers between various unused relay sockets to bypass cooked solder joints, it really was a crappy design.

author: langwadt
author_rank: Super Contributor
timestamp: February 12, 2021, 12:38:05 pm
content: Quote from: GlennSprigg on February 12, 2021, 12:10:30 pmthat is also because the voltage can drop quite a lot while running the starter. ECU often use LDOs to make 5V even thought that wouldn't seem neccessary from 12V

author: gnuarm
author_rank: Super Contributor
timestamp: February 12, 2021, 07:58:36 pm
content: Quote from: GlennSprigg on February 12, 2021, 12:10:30 pmYou must be an old guy... like me... The ballast resistor went away with the distributor when fully electronic ignition was adopted. They charge up capacitors to fire the spark and it is plenty hot these days. In the old days cars actually barely ran. Get some cracks in the high voltage wires and they stop working on damp days. Cold out so your battery voltage is low and the engine turns over but won't fire up. Now with all the improvements cars have a lot more margin and run under nearly all conditions and don't need as much attention as they used to.

author: langwadt
author_rank: Super Contributor
timestamp: February 12, 2021, 08:14:58 pm
content: Quote from: gnuarm on February 12, 2021, 07:58:36 pmI think most cars use inductive discharge not capacitive discharge ignition. But the the ignition coil primary is only a few hundred milliOhm so the computer can just increase the dwell time at low voltage to get the needed energy

author: DavidAlfa
author_rank: Super Contributor
timestamp: February 15, 2021, 12:34:03 pm
content: Quote from: Beamin on February 08, 2021, 09:33:30 pmFrom my point of view:Without the resistor, once the spark jumps, the wire capacitance would cause a very short but high current pulse, generating a lot of EMI due the oscillations at the cable resonance frequency (depending on its LC characteristics), this could cause the voltage to reach even higher levels than it originally had and cause damage the driver. Not to mention the noise taken by all nearby electronics.That's where the resistor comes to help. It limits the peak current, greatly avoiding the oscillations, but lets the voltage rise as fast as it's generated, so the spark will work the same.A capacitor would filter the spike, reducing the peak voltage, requiring longer pulses for the spark to jump.Once it jumped, the current would be higher, worsening the EMI worse and shortening the life of the electrodes.You can see it yourself very easily. Take a small capacitor (ex. 10uF 16v) and attach a 1m cable to it.Connect the oscilloscope in 10x mode at the capacitor pins. Charge the cap.And check the difference when directly shorting the wires or when doing so through 0.5ohm resistor.See the difference when shorting out di

author: SilverSolder
author_rank: Super Contributor
timestamp: February 15, 2021, 02:33:44 pm
content: So the resistance is effectively lowering the Q of the spark plug circuit, reducing its ability to resonate?