I know it has been discussed using a relay to cut power to the coil but this could cause a spark to fire at the wrong time and as such isn't considered safe.
What if a relay was used to connect a component (resistor, capacitor?) across the primary side of the coils? Would this stop it from generating a spark at all until the circuit was opened again? i.e. the EDIS would still be firing the coil but something would be stopping the field collapsing in the manor that generates a spark in the HT side.
I'm a bit out of my depth here but just a thought...
Hard rev limit
Moderators: JeffC, rdoherty, stieg, brentp
David's Thyristor Circuit
<a href="http://www.with.nildram.co.uk/MJLJ/Limiter.htm">David's Thyristor Circuit</a> does exactly this - it grounds the coils through the resistors and the thyristors when the Rev Limit output is active. The thyristors allow the 'in progress' spark to continue but then block any further sparks
I've two problems, however:
- I've built it and it doesn't work. It works fine on the bench with a light bulb instead of the coils and manually simulating the Rev Limit output. On the car I get no sparks at all - it must be earthing the coils all the time somehow
- The circuit is simple and I *think* I understand how it works, but don't know why is doesn't work in the car. I haven't had time to put a 'scope on it, but I will do...
David seems to have disappeared and I don't know how to contact him...
I've two problems, however:
- I've built it and it doesn't work. It works fine on the bench with a light bulb instead of the coils and manually simulating the Rev Limit output. On the car I get no sparks at all - it must be earthing the coils all the time somehow
- The circuit is simple and I *think* I understand how it works, but don't know why is doesn't work in the car. I haven't had time to put a 'scope on it, but I will do...
David seems to have disappeared and I don't know how to contact him...
Contact David
These forums used to have a feature before where you could look up user's e-mail address and the date they joined. It still has this feature but only when that user longs in. Ask Brent maybe he can help you with e-mail address.
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andyharding
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- Joined: Sat Jul 01, 2006 10:01 am
I've asked an electronics
I've asked an electronics engineer I know to look into this and see if he can come up with anything. I'll keep you posted...
http://picasso.org/mjlj/?q=node/469 ... where there are some interesting discussions that I've not read before
rev limiters
re: http://picasso.org/mjlj/?q=node/469
I made some comments including whether the thyristor properties
are needed. It looks to me that using a relay to connect the coil
to ground through a resistor of say 8 ohms (I use 12) to inhibit the
spark would work. I cannot see how switching the relay on or off at a random time would cause a spurious spark provided the resistor value
was a lot higher than the coil's resistance. That is the beauty of
the circuit in that it uses a low power circuit (the extra resistor)
to block the actions of a high power circuit.
John
I made some comments including whether the thyristor properties
are needed. It looks to me that using a relay to connect the coil
to ground through a resistor of say 8 ohms (I use 12) to inhibit the
spark would work. I cannot see how switching the relay on or off at a random time would cause a spurious spark provided the resistor value
was a lot higher than the coil's resistance. That is the beauty of
the circuit in that it uses a low power circuit (the extra resistor)
to block the actions of a high power circuit.
John
OK, back to basics then (I
OK, back to basics then (I like that!)...
Measured coil resistance = 0.8R
Hand connecting a 10R (measured as 10.4R) 5W resistor across the EDIS outputs and ground certainly kills the spark on the test bench. Gets a bit hot after 10 seconds though! Needs to be more like 12W capability according to the maths.
...goes off to try something like a pair of <a href="http://www.maplin.co.uk/Module.aspx?Tab ... leNo=37521"> these </a> directly connected to the Rev Limit output
Measured coil resistance = 0.8R
Hand connecting a 10R (measured as 10.4R) 5W resistor across the EDIS outputs and ground certainly kills the spark on the test bench. Gets a bit hot after 10 seconds though! Needs to be more like 12W capability according to the maths.
...goes off to try something like a pair of <a href="http://www.maplin.co.uk/Module.aspx?Tab ... leNo=37521"> these </a> directly connected to the Rev Limit output
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andyharding
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- Joined: Sat Jul 01, 2006 10:01 am
The only problem I can think
The only problem I can think of doing it this way is the increased load the resistors will place on the EDIS unit. Not sure if this would be a problem or not although a blown EDIS would be better than a rod through the block!
The EDIS is just a switch
The EDIS is just a switch the grounds the 'negative' side of the coil, just like the points in a standard Kettering ignition (it's a lot cleverer though!).
When it's open circuit there's no coil current and when its closed circuit there's maximum coil current - and when it switches from closed to open, there's a big negative dI/dt than causes a collapse in the magnetic field in the coil that induces a big voltage in the secondary that makes the spark etc etc
In normal operation (sub rev limit), the resistors aren't in the circuit, so there's no problem at all.
In rev limit operation with the EDIS 'closed' the resistors don't really do anything as the current is flowing through the coil into the EDIS and to ground - so normal EDIS operation
In rev limit operation with the EDIS 'open' the resistors are carrying all the coil current, but the coil current is less than normal since the (about 12R) resistors are limiting the current to around 1 amp or so - so no problems for the EDIS again
In rev limit operation with the EDIS transitting from 'closed' to 'open' there isn't such a big dI/dt in the coil, since the current continues to flow via the resistors instead of the EDIS. The EDIS doesn't know about this, so no problems for the EDIS I think. This reduction in dI/dt is what has been observed by John - if the resistors are too high (circa 25R) there is still a big enough dI/dt to cause a spark
In rev limit operation with the EDIS transitting from 'open' to 'closed' there is some current flowing via the resistors already, but it transfers to flowing via the EDIS. This isn't exactly as normal for the EDIS, but won't harm it so no problems for the EDIS
So that's all the scenarios catered for, there seems no harm to the EDIS in any of them and there doesn't seem to be any case where the resistors cause extra 'load' on the EDIS...I *think* !
The only thing that does worry me a little, without prototyping, is that the back emf induced in the primary coil by the dI/dt in normal operation (about 100-200 volts IIRC) will be too much for the relay contacts and they may start arcing/fusing together. Any thoughts anyone?
When it's open circuit there's no coil current and when its closed circuit there's maximum coil current - and when it switches from closed to open, there's a big negative dI/dt than causes a collapse in the magnetic field in the coil that induces a big voltage in the secondary that makes the spark etc etc
In normal operation (sub rev limit), the resistors aren't in the circuit, so there's no problem at all.
In rev limit operation with the EDIS 'closed' the resistors don't really do anything as the current is flowing through the coil into the EDIS and to ground - so normal EDIS operation
In rev limit operation with the EDIS 'open' the resistors are carrying all the coil current, but the coil current is less than normal since the (about 12R) resistors are limiting the current to around 1 amp or so - so no problems for the EDIS again
In rev limit operation with the EDIS transitting from 'closed' to 'open' there isn't such a big dI/dt in the coil, since the current continues to flow via the resistors instead of the EDIS. The EDIS doesn't know about this, so no problems for the EDIS I think. This reduction in dI/dt is what has been observed by John - if the resistors are too high (circa 25R) there is still a big enough dI/dt to cause a spark
In rev limit operation with the EDIS transitting from 'open' to 'closed' there is some current flowing via the resistors already, but it transfers to flowing via the EDIS. This isn't exactly as normal for the EDIS, but won't harm it so no problems for the EDIS
So that's all the scenarios catered for, there seems no harm to the EDIS in any of them and there doesn't seem to be any case where the resistors cause extra 'load' on the EDIS...I *think* !
The only thing that does worry me a little, without prototyping, is that the back emf induced in the primary coil by the dI/dt in normal operation (about 100-200 volts IIRC) will be too much for the relay contacts and they may start arcing/fusing together. Any thoughts anyone?
Another option, ask the prof.
You can ask the man about how Ford ECUs do hard-cut rev limiter. <a href="http://picasso.org/mjlj/?q=filestore2/d ... 820/EEC-IV Tech.pdf">Read the ECC-IV Tech</a>. The man usually don't accept questions, as there is a mailing list for questions (dead one). So, be warned, if your question is not interesting (or you are not interesting) your question will be ignored; ecc-iv is obsolite, so it's just the way things are now.
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andyharding
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- Joined: Sat Jul 01, 2006 10:01 am
An electronic circuit would
An electronic circuit would be better than a relay and then the contacts burning out wouldn't be a problem plus the reaction time would be faster.
Brent: the EDIS contains a small surface mount PCB in a clear jelly. Doesn't look mod'able. I'll take a pic and post it shortly. (p.s. I sent you an email...)
Brent: the EDIS contains a small surface mount PCB in a clear jelly. Doesn't look mod'able. I'll take a pic and post it shortly. (p.s. I sent you an email...)
Relay contact rating, resistor sizes
Relays can cope with 100-200 V. If you get one with contacts rated at 240 VAC it will withstand up to 240*1.414=340V. The current would be less than 2 amps. It would be best not to get a very small physical size relay - typical auto size should be OK. It would need a pole (separate pair of contacts) for each coil; a clever arrangement of diodes might let you use just one pole but I don't see much point since relays with up to three poles (for 3 phase switching) are easily available and for four poles (8 cylinders) you could use two relays or choose not to switch off the fourth coil. Even with 6 cylinders you might leave 2 of them running to get a smoother effect, unless you are worried that the firing cylinders might ignite the unburnt fuel from the others in the exhaust.
Regarding a comment about the resistors getting hot, presumably the limiter would not be on for long. Once the speed dropped or the Megajolt switched off the relay would open. However it would be wise to get resistors capable of handling at least 10 watts, e.g. a metal cased on bolted down, and not put where fuel might drip onto it and catch fire in a fault situation!
I too don't see any risk to the EDIS; it would be more risky to dig it open and try making modifications, especially as some aspect of the circuit might serve multiple non-obvious purposes.
Regarding a later comment, I would think that a relay would be plenty fast enough for most applications. The slow response might induce a hunting if you sit on the limit using the limiter to control the speed, but you risk an exhaust explosion from the unburnt fuel so should not be doing that anyway. I am happy with my current electronic arrangement, but relays work pretty well and are simpler to understand for most people, and I would be content to use one. They are certainly easier to fix in the field, and explain to other people.
John
Regarding a comment about the resistors getting hot, presumably the limiter would not be on for long. Once the speed dropped or the Megajolt switched off the relay would open. However it would be wise to get resistors capable of handling at least 10 watts, e.g. a metal cased on bolted down, and not put where fuel might drip onto it and catch fire in a fault situation!
I too don't see any risk to the EDIS; it would be more risky to dig it open and try making modifications, especially as some aspect of the circuit might serve multiple non-obvious purposes.
Regarding a later comment, I would think that a relay would be plenty fast enough for most applications. The slow response might induce a hunting if you sit on the limit using the limiter to control the speed, but you risk an exhaust explosion from the unburnt fuel so should not be doing that anyway. I am happy with my current electronic arrangement, but relays work pretty well and are simpler to understand for most people, and I would be content to use one. They are certainly easier to fix in the field, and explain to other people.
John
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andyharding
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- Joined: Sat Jul 01, 2006 10:01 am
I just tried connecting a 21
I just tried connecting a 21 Watt 12 Volt indicator bulb across the coil and it kills the spark. It also makes the other coils spark weaker??? I've measured the bulbs resistance at 1 Ohm. It doesn't light up.
So far it looks like the best solution will be connecting two 10 Watt 10 Ohm resistors across the coils with some kind of solid state circuit. I'll see what I can come up with.
So far it looks like the best solution will be connecting two 10 Watt 10 Ohm resistors across the coils with some kind of solid state circuit. I'll see what I can come up with.