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| How does a series resistor inside a spark plug rduce EMF noise? |
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| SilverSolder:
--- Quote from: gnuarm on February 09, 2021, 06:48:47 am ---[...] 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. --- End quote --- The 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!) |
| GlennSprigg:
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 is dropped to what is really needed. The reason for this, is that while the ignition key is cranking the engine to initially start 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! :-+ |
| Gyro:
That resistor 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. |
| langwadt:
--- Quote from: GlennSprigg on February 12, 2021, 12:10:30 pm ---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 is dropped to what is really needed. The reason for this, is that while the ignition key is cranking the engine to initially start 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! :-+ --- End quote --- that 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 |
| gnuarm:
--- Quote from: GlennSprigg on February 12, 2021, 12:10:30 pm ---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 is dropped to what is really needed. The reason for this, is that while the ignition key is cranking the engine to initially start 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! :-+ --- End quote --- You 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. |
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