The value of having resistance in my ignition circuit ht .
Discussion
I've notice that the more resistance i add to the HT side of my ignition,the more 'fluffy'(not missing though) the engine note sounds.I was running resistive plugs and heatshields.If i remove all resistance i cant detect any 'fluffiness'.I now just use resistive plug heatsields(5k).Could i do away with all resistance and run non resistive shields without harming anything?I run P38 coil packs and an Omex 710 ecu on my mildly modded 5.0l Griff by the way.
aceparts_com said:
Then why do we have resistance in things like coaxial cable for antennas?
Now your getting into RF stuff. Your entering the realm of Voodoo and Black Magic it makes no sense. An RF transmitter, cable and antenna need to be impedance matched(the same) otherwise some of the power gets reflected back to the transmitter. One extreme circumstance is when the cable is unplugged and all the power gets reflected back to the transmitter blowing it up! A bit like short circuiting a dc power supply. See I told you it was Voodoo!steve_d said:
The thing you will be harming is yours and your neighbours TV reception. That I believe is the only reason you have resistive devices in the system.
Steve
As well as mucking up listening to Terry Wogan on Radio 2 (not such a bad thing you might think) it can interfere with delicate electronic devices like the ecu. There was a recall on the Lotus Elise to fit extra shielding around the ecu. It prevented them going up in flames every time they got near any airport ground radar! It was called the anti-woof bracket. So it's probably best to keep a bit of noise suppressing resistance in there!Steve
My 710 with Ford coil packs was more or less unusable without any HT resistance, it kept resetting. I think the resistance spreads the spark out and reduces the inductive pulse back into the LT side, this is what seemed to be doing the ECU in. If you're running wasted spark you should have a hell of a strong spark so I wouldn't expect a bit more resistance to cause fluffiness.
leorest said:
aceparts_com said:
Then why do we have resistance in things like coaxial cable for antennas?
Now your getting into RF stuff. Your entering the realm of Voodoo and Black Magic it makes no sense. An RF transmitter, cable and antenna need to be impedance matched(the same) otherwise some of the power gets reflected back to the transmitter. One extreme circumstance is when the cable is unplugged and all the power gets reflected back to the transmitter blowing it up! A bit like short circuiting a dc power supply. See I told you it was Voodoo!And to make it even weirder, the "resistance" isn't apparent until you get to sufficiently high frequencies, where the wavelength is sufficiently small in relation to the cable. If you measure a piece of antenna coax on a multimeter you'll read a short circuit, not 75 ohms.
The reason TV antenna coax has an impedance of 75 ohms is that it's pretty close to the impedance of a half-wave dipole, which is 73 ohms, and in VHF days TV antennas were usually either simple dipoles or dipoles with one director so would be a good match to the cable without having to use any additional matching devices.
Pigeon said:
And to make it even weirder, the "resistance" isn't apparent until you get to sufficiently high frequencies, where the wavelength is sufficiently small in relation to the cable. If you measure a piece of antenna coax on a multimeter you'll read a short circuit, not 75 ohms.
I know you must be right, but that's unintuitive. What's the mechanism by which resistance varies with frequency?
Standard ignition systems work for millions of cars, and in many cases are extremely powerful as is.
I can only think that if you have some sort of "fluffiness" issue, then the problem is likely to be elsewhere.
Ive used all sorts of plugs and leads over the years, from cheap shite, to pretty damn expensive, and in all but extreme cases ( engines running say close to or over 2 bar boost ) a well maintained factory ignition system has coped very well, albeit with perhaps a different plug, resistor type of course.
I can only think that if you have some sort of "fluffiness" issue, then the problem is likely to be elsewhere.
Ive used all sorts of plugs and leads over the years, from cheap shite, to pretty damn expensive, and in all but extreme cases ( engines running say close to or over 2 bar boost ) a well maintained factory ignition system has coped very well, albeit with perhaps a different plug, resistor type of course.
AFAIK most ignition systems will generate significantly more energy than is required so that they can accept all the losses from the resistive RF suppression devices. You can remove all the resistive suppression but that could damage the coil by extracting too much energy.
And to make it even weirder, the "resistance" isn't apparent until you get to sufficiently high frequencies, where the wavelength is sufficiently small in relation to the cable. If you measure a piece of antenna coax on a multimeter you'll read a short circuit, not 75 ohms.
I know you must be right, but that's unintuitive. What's the mechanism by which resistance varies with frequency?
Impedance of a coaxial cable is a defined by the geometry (id : od) and is constant with frequency (until you get to serious GHz...)
However the impedance 'seen' at the input to a coax depends on the load at the end of it and the frequency. A good example would be a quarter wavelength of a coax with a shortcircuit at the end would look like an open circuit to RF at the input.
GreenV8S said:
Pigeon said:
And to make it even weirder, the "resistance" isn't apparent until you get to sufficiently high frequencies, where the wavelength is sufficiently small in relation to the cable. If you measure a piece of antenna coax on a multimeter you'll read a short circuit, not 75 ohms.
I know you must be right, but that's unintuitive. What's the mechanism by which resistance varies with frequency?
Impedance of a coaxial cable is a defined by the geometry (id : od) and is constant with frequency (until you get to serious GHz...)
However the impedance 'seen' at the input to a coax depends on the load at the end of it and the frequency. A good example would be a quarter wavelength of a coax with a shortcircuit at the end would look like an open circuit to RF at the input.
Le TVR said:
GreenV8S said:
Pigeon said:
And to make it even weirder, the "resistance" isn't apparent until you get to sufficiently high frequencies, where the wavelength is sufficiently small in relation to the cable. If you measure a piece of antenna coax on a multimeter you'll read a short circuit, not 75 ohms.
I know you must be right, but that's unintuitive. What's the mechanism by which resistance varies with frequency?
Impedance of a coaxial cable is a defined by the geometry (id : od) and is constant with frequency (until you get to serious GHz...)
Yes, but you don't "notice" it until the wavelength of the frequency you're using is sufficiently small compared to the length of the cable. The RF output from a satellite LNB "sees" the characteristic impedance of the cable, but the DC feed up the cable to power the LNB just sees the resistance you calculate from the thickness and resistivity of the copper; sparkies don't need to worry about transmission line effects when wiring up a building, but power engineers transmitting 50/60Hz across hundreds of miles do.
Le TVR said:
However the impedance 'seen' at the input to a coax depends on the load at the end of it and the frequency. A good example would be a quarter wavelength of a coax with a shortcircuit at the end would look like an open circuit to RF at the input.
...which is very useful for making VHF/UHF tuned circuits... rather than fiddling with LC resonant circuits with values of L and C which are very small and vary when you put your hand near them, just cut the required length of coax and short the far end of it.
There's a reasonably good page on it here.
An HT system without suppression is transmitting all over radio spectrum from a few kHz, up to several GHz. That means that even assuming the antenna feed were perfect and the TV is shielded, the antenna itself will pick up RF spikes on the same frequency the TV transmitter is broadcasting on. As has been pointed out, any simple wire will act as a receiving antenna too, and may carry the energy back into whatever equipment it is connected to.
Despite 20 years amateur radio experience and an MSc in High Frequency Engineering, I'm still convinced that RF is more of a dark art than a modern science. But if you do want to take it further then there are some really good books by the RSGB and ARRL on radio theory and application.
Despite 20 years amateur radio experience and an MSc in High Frequency Engineering, I'm still convinced that RF is more of a dark art than a modern science. But if you do want to take it further then there are some really good books by the RSGB and ARRL on radio theory and application.
The problem with an ignition pulse system is that it is generated by a high voltage coil.
Suppression requires that the dV/dt function is reduced to the absolute minimum needed such that the Fourier spectrum is as small as possible. Normal EMC techniques would be capacitive decoupling and screening. But added capacitance will cause the HT coil to oscillate or 'ring' on each pulse which is not good for the plugs or the resultant spectrum.
The only real option to reduce the dV/dt function is series resistance, hence resistive leads, caps or plugs.
Similarly the total power dissipation of the HT coil will be derived from the working load dV/dt and dI/dt. Remove too much resistive suppression and the coil could exceed its maximum dissipation and fail.
You clearly dont have the right version of EEsof
Suppression requires that the dV/dt function is reduced to the absolute minimum needed such that the Fourier spectrum is as small as possible. Normal EMC techniques would be capacitive decoupling and screening. But added capacitance will cause the HT coil to oscillate or 'ring' on each pulse which is not good for the plugs or the resultant spectrum.
The only real option to reduce the dV/dt function is series resistance, hence resistive leads, caps or plugs.
Similarly the total power dissipation of the HT coil will be derived from the working load dV/dt and dI/dt. Remove too much resistive suppression and the coil could exceed its maximum dissipation and fail.
Zad said:
Despite 20 years amateur radio experience and an MSc in High Frequency Engineering, I'm still convinced that RF is more of a dark art than a modern science.
You clearly dont have the right version of EEsof
Zad said:
Despite 20 years amateur radio experience and an MSc in High Frequency Engineering, I'm still convinced that RF is more of a dark art than a modern science.
Like wot I said earlier 
All this RF discussion really is just a distraction from the original question!
The HT components will be designed for a certain resistance for some very good reasons. Let's leave it at that. If you are experiencing problems that are improved by removing the designed resistance this is not the solution but should be regarded as a clue to finding where the real problem is. High resistance in the HT coil secondary or maybe low voltage/high resistance on the primary. Insulation breakdown..... and so on
Best of luck and post up your findings when you get it sorted please.
Leo
leorest said:
Zad said:
Despite 20 years amateur radio experience and an MSc in High Frequency Engineering, I'm still convinced that RF is more of a dark art than a modern science.
Like wot I said earlier All this RF discussion really is just a distraction from the original question!
The HT components will be designed for a certain resistance for some very good reasons. Let's leave it at that. If you are experiencing problems that are improved by removing the designed resistance this is not the solution but should be regarded as a clue to finding where the real problem is. High resistance in the HT coil secondary or maybe low voltage/high resistance on the primary. Insulation breakdown..... and so on
Best of luck and post up your findings when you get it sorted please.
Leo
The car isn't running poorly,and by a 'fluffy' sound to the exhaust note,i am just trying to describe the tone as best i can.It performed reallt well on the rolling road with both resistive plugs and extenders,but i noticed when i changed to non resistive plugs that it sounds different,and less of the 'fluffy sound'.I then removed the resistive extenders also and the sound changed again,even less of the 'fluffy' tone.
The reason i'm confused as to how to run my ignition HT,is because of two things:-
A)it sounds best of all to my untrained ear,when using no reistance.
B)I'm not sure how much resistance,if any,i should have.The car originally ran resistive extenders but not resistive plugs,but this was with a distributor system.I now have wasted spark and a different ecu altogether,and the engine is modified a bit!
In theory, having HT resistance will produce a 'softer' spark i.e. from the point that the current starts to flow across the gap, you'll get less current over a longer time. It's possible that this could affect the way the ignition occurs within the cylinder. Having a multi-coil setup ought to produce a much stronger spark than a single coil/dizzy set up though, and I would have expected this to have a much greater effect than subtle changes from adding/removing HT resistance.
The changes that HT resistance cause on the HT side also apply to the inductive 'kick' back on the LT side. On my OMEX system, this was enough to make the difference between the ECU resetting when the first spark occured while cranking, and running properly. Strangely, the ECU seemed to be particularly susceptable to interference while cranking and more tolerant as the revs picked up i.e. from 200 rpm cranking to 1000 rpm as the engine fired up. I suspect that the current drawn by the starter motor was dropping off as the revs picked up, and this was allowing the battery voltage to rise and allowing the ECU voltage regulator to work properly.
The changes that HT resistance cause on the HT side also apply to the inductive 'kick' back on the LT side. On my OMEX system, this was enough to make the difference between the ECU resetting when the first spark occured while cranking, and running properly. Strangely, the ECU seemed to be particularly susceptable to interference while cranking and more tolerant as the revs picked up i.e. from 200 rpm cranking to 1000 rpm as the engine fired up. I suspect that the current drawn by the starter motor was dropping off as the revs picked up, and this was allowing the battery voltage to rise and allowing the ECU voltage regulator to work properly.
Gassing Station | Engines & Drivetrain | Top of Page | What's New | My Stuff