Alternator kick-in
Discussion
I recently had the dash out on my S3, and while I was there I thought I would replace the warning lamps with LEDs so that I could actually see them on a sunny day. Since doing this, I've noticed that the alternator no longer kicks in at 2000RPM (now closer to 3000-3500). I've read some postings about the battery warning light supplying current to the alternator to help it start up - would changing the bulbs to LEDs have this effect? I would have thoght that the LEDs draw less current than the original bulbs. Perhaps I'm barking up the wrong tree!
The alternator always cuts in eventually, but I'm reluctant to rev to 3500 when its cold so usually have to drive with no alternator for a few miles first. Not a massive problem as I usually go on a fairly long drive :D so the battery gets topped up properly, but just irritating me!
The alternator always cuts in eventually, but I'm reluctant to rev to 3500 when its cold so usually have to drive with no alternator for a few miles first. Not a massive problem as I usually go on a fairly long drive :D so the battery gets topped up properly, but just irritating me!
Haven't looked at the circuit diagram (like they drew one anyway !) or anything useful like that. But LED's are quite different devices to bulbs.
A graph of voltage against current for a bulb will be linear.
But a graph of voltage against current for an LED will show nothing at all until the voltage across the LED itself rises above whatever the "switch on" voltage of a diode is (from school, 0.6V - I can't remember). In addition LED's are lower current devices than bulbs,
Obviosuly depends on the precise nature of the circuit - but you could be seeing a combination of these two effects.
WG
A graph of voltage against current for a bulb will be linear.
But a graph of voltage against current for an LED will show nothing at all until the voltage across the LED itself rises above whatever the "switch on" voltage of a diode is (from school, 0.6V - I can't remember). In addition LED's are lower current devices than bulbs,
Obviosuly depends on the precise nature of the circuit - but you could be seeing a combination of these two effects.
WG
Also - a bulb allows current to flow in both directions, and an LED allows current only in one direction.
On most cars, the warning bulf illuminates as a result of a "backfeed" when the alternator charge voltage drops below a certain level. I don't think it's normally a simple "supply and earth" circuit.
I don't have a wiring diagram at the mo for the S, so it might be different. But I think that, if the LED isn't allowing the necessary "backfeed" and also taking account of the "kick-in" voltage mentioned by WG, then this may explain it.
It may not though, I'm pretty vague on these modern LED thingys...
On most cars, the warning bulf illuminates as a result of a "backfeed" when the alternator charge voltage drops below a certain level. I don't think it's normally a simple "supply and earth" circuit.
I don't have a wiring diagram at the mo for the S, so it might be different. But I think that, if the LED isn't allowing the necessary "backfeed" and also taking account of the "kick-in" voltage mentioned by WG, then this may explain it.
It may not though, I'm pretty vague on these modern LED thingys...
although fortunately I decided not to refit the two nuts that take hours to remove so it shouldn't take too long!The no-charge warning light has to pass enough current to kick-start the alternator. The less current it passes, the more revs you will need to get the alternator going. If you disconnect it completely it will take loads of revs but should eventually kick in.
Strictly speaking the no-charge warning light passes current both ways - current flows backwards while the engine spins down after you switch off. I don't think that losing this function is particularly harmfull, but you may find that the LED doesn't like the reverse voltage. I think it would be sensible to swap this one LED back to an ordinary bulb just in case; if the LED does fail, it will be a right pita.
Strictly speaking the no-charge warning light passes current both ways - current flows backwards while the engine spins down after you switch off. I don't think that losing this function is particularly harmfull, but you may find that the LED doesn't like the reverse voltage. I think it would be sensible to swap this one LED back to an ordinary bulb just in case; if the LED does fail, it will be a right pita.
this is exactly the same problem i had on my cobra replica when i fitted led's instead of warning lights
i'm sure someone with more electrical knowledge than me can comment but can't you just fit a bulb in parallel with the led ?
this gives the alternator a kick start at a lower voltage but you still keep the nice bright light
reb
i'm sure someone with more electrical knowledge than me can comment but can't you just fit a bulb in parallel with the led ?
this gives the alternator a kick start at a lower voltage but you still keep the nice bright light
reb
Hi guys,
always thought the alternator-kick-in feature was because of old (DC) dynamo type wiring.
As on my boat by the way. The alternator there is DC wired, and therefore _not_ helped (excited) by current via the light, hence needs to start up on the rotor's (weak) remanent magnetism. This involves two (sets of 3) diodes in series, hence 1.4 V before the alternator kicks in (the exciter current starts, and the regulator comes in control). It has three wires on the regulator, like I saw on my S3C wiring diagram. 2000 rpm kick in on my 3000 rpm max 9 HP Stuart Turner diesel.
An (AC) alternator has one (set of 3) separate exciter diodes, who need 0.7 V before a current starts to run, exciting the rotor coil. The light should be wired in AC type wiring between the rotor coil feed and the ignition switch.
I used to see four terminals on these regulators, one extra providing 12 V to the regulator circuit, not to the rotor). I now realise the fourth terminal in the S3C might be the mass contact regulator --> alternator --> engine --> chassis --> earth. My boat needs 4 because the regulator is mounted on the "plastic" boat, excuse me, eh GRP.
Now, if I hear a smaller current (led) increases the kick in rpm's, I would say the light is wired the AC way, and just not big enough.
So I think somebody who is annoyed with the feature should try a bigger bulb, or a resistor around the bulb. Or led actually! Total current about twice or three times the current would lower the kick-in rpm's hopefully below 1000 rpm! And do no harm to the rotor coil, which needs several amps I think.
Personally I like these "features", scares people away from wanting to drive my boat or car... Ain't I bad.
mvg Boudewijn
always thought the alternator-kick-in feature was because of old (DC) dynamo type wiring.
As on my boat by the way. The alternator there is DC wired, and therefore _not_ helped (excited) by current via the light, hence needs to start up on the rotor's (weak) remanent magnetism. This involves two (sets of 3) diodes in series, hence 1.4 V before the alternator kicks in (the exciter current starts, and the regulator comes in control). It has three wires on the regulator, like I saw on my S3C wiring diagram. 2000 rpm kick in on my 3000 rpm max 9 HP Stuart Turner diesel.
An (AC) alternator has one (set of 3) separate exciter diodes, who need 0.7 V before a current starts to run, exciting the rotor coil. The light should be wired in AC type wiring between the rotor coil feed and the ignition switch.
I used to see four terminals on these regulators, one extra providing 12 V to the regulator circuit, not to the rotor). I now realise the fourth terminal in the S3C might be the mass contact regulator --> alternator --> engine --> chassis --> earth. My boat needs 4 because the regulator is mounted on the "plastic" boat, excuse me, eh GRP.
Now, if I hear a smaller current (led) increases the kick in rpm's, I would say the light is wired the AC way, and just not big enough.
So I think somebody who is annoyed with the feature should try a bigger bulb, or a resistor around the bulb. Or led actually! Total current about twice or three times the current would lower the kick-in rpm's hopefully below 1000 rpm! And do no harm to the rotor coil, which needs several amps I think.
Personally I like these "features", scares people away from wanting to drive my boat or car... Ain't I bad.
mvg Boudewijn
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