Alternators, LEDs and Resistors

[AdamQ]

I've fallen foul of the problem of the alternator not being activated having replaced the ignition/alternator light with an LED version.

I've found the various useful threads in the archives about diagnosis and solution and just wanted to check that the 40 Ohm version of one of these is what I need (I've linked to the 50 Ohm version because there's not a nice picture of the 40 Ohm version):

https://uk.rs-online.com/web/p/panel-mount-fixed-resistors/8916417/

Many thanks,

Adam

[Jonathan Kay]

Which alternator? And what exciting current does it want?

Jonathan

[AdamQ]

It's on a Duratec conversion and was supplied ten years ago by Raceline - I think at that time they were using 80A Nippon Denso alternators from Daihatsu Charades or something, but I'm not sure and I don't have specific details.

What I have found is that connecting the original (non-LED) ignition bulb between the NY wire on the alternator and +ve on the battery solves the problem.  Assuming that bulb is 4W, by my calculations a resistance of ~36 Ohms is required ...

[revilla]

Hi Adam, hope you're well?

Yes that should do fine.

Just wire it in parallel with the LED (and any built-in resistor that uses internally to run the LED off 12V), i.e. between the NY wire and any convenient ignition switched supply (preferably off the same fuse as the LED indicator). It will get warm when in use (i.e. when the warning LED is on) so one of those that will screw to the bulkhead as a heatsink is a good idea.

Just one thought - I'm not sure what voltage this will pull the alternator terminal up to, it depends on the design of the alternator. It might make the LED rather dimmer than before, as the voltage across the LED will fall as the voltage at the alternator terminal is pulled upwards by the resistor. Ideally you would want to drop the value of the series resistor used with the LED to match, but that's probably internal and inaccessible. I've had no problems on my alternators with a 2W bulb, so maybe a 75Ω resistor would be better?

Let us know how you get on.

Cheers,

Andrew

[Wrightpayne]

Andrew,

is the resistor in series or parallel as both terms used?

Ian

[revilla]

Excuse the scribbled diagram but I think it makes it clearer:

A naked LED will normally require about 2.2V across it. Connect it to a 12V source directly and it will go pop very quickly. So there will be a resistor in series with the LED (probably inside the tacho somewhere where you can't see it) to limit the current.

For example imagine that the alternator terminal pulls right down to 0V so you have battery voltage of approximately 12.6V to drive the LED and you want 20mA of current. 2.2V across the LED leaves 12.6-2.2=10.4V across the resistor. So a resistance of around 10.4/0.02=520Ω would be used.

You can get "12V LEDs" which appear not to need a series resistor - they actually just have the resistor built right into the LED.

You will be adding a relatively low valued resistor as shown, in parallel with the combines series circuit of the LED and its current limiting resistor.

One consequence of this is that the voltage at the alternator terminal will be pulled upwards. How much exactly depends on the circuitry inside the alternator. If we assume for example that it pulls it up to 4V, then we have only 12.6-4=8.6V to drive the original LED circuit. The series resistor will then have 8.6-2.2=6.4V across it and will pass 6.4/520=0.012A, or about 12mA. The LED will therefore only light to about 12/20=60% of its original brightness.

If the LED is unacceptably dim with the parallel resistor, let me know and I'll try to think of something better!

[Wrightpayne]

I thought it would be my lack of understanding of the circuitry rather than a typo but always willing to seek clarification / ask the stupid questions.

Cheers

Ian

[AdamQ]

Hi Andrew,

All good here thank you - I hope the same is true of you?

Many thanks indeed for the confirmation (I was rather hoping you'd be a long at some point ...!).  I took the plunge yesterday and ordered the 40Ω version and it arrived today.  I'll wire it in on Friday and report back ...

Thanks again,

Adam

[AdamQ]

Erm ...  Not quite sure how to put this ...  Perhaps best summed up by:

HELLLLLLP!!!!

I have wired the resistor in parallel with the LED by connecting it to the same NY wire as the LED is connected to and to one of the G wires that feeds the switches.  I've double-checked that I've used the correct wires using a multimeter.

All seems good - ignition light shines brightly with ignition on and there's a healthy 14.4V across the battery with the engine running (at which time the ignition light is out).

The problem comes when I try to turn the engine off.  Usually I press the button that turns the ignition on and off and all is as it should be.  Now, when I turn the ignition off the instruments all die but the engine carries on running.  Hmmmmmm I thought, well I've got a BMS so I might as well use it.  No effect - engine continues to purr merrily away.  I eventually killed it by disconnecting the green wire from the resistor.

I dare say that with a cold towel around my head and a bit of ruthlessly applied logic, I could eventually work out what's going on, but it seems so much easier to ask the clever men of Blatchat who know all there is to be knowed ...  I presume the resistor is preventing a circuit from being broken when the ignition is turned off and that the engine is simply running from the output from the alternator when the battery is disconnected.

I guess I ought to add that it's a 1999 car with the ignition light external to the tachometer, not integrated, and that it has a latching ignition button and a momentary starter button.  The BMS is one of the cheap and cheerful ones with solely two big connections for the battery cables.

TIA as they say ...

Adam

[Jonathan Kay]

Have you changed anything else since it was operating correctly with the original incandescant warning light?

... and to one of the G wires that feeds the switches.

Which wire and is it ignition-switched?

Jonathan

[revilla]

Hi Adam,

Short term fix, put your headlights on, the engine should stop!

You need a higher value resistor, or probably a better solution would be a diode in line with the resistor you've added.

What is happening is, with the ignition switched off, the alternator is actually feeding enough current back through that resistor to power your ECU and keep the engine running. I'm surprised it happens in the car with a resistor of that value, but that's what's happening! I saw a similar issue on a K Series I have on a test rig. On that I had a couple of 2W indicator bulbs, one for the alternator warning and one just to show me I'd got the ignition switched on. I increased the "on" light to 4W and it was enough to pull the voltage down and kill it.

Give me a few minutes and I'll show you what you need ...

[revilla]

OK, you need something like this, a 1N4001 rectifier diode:

https://www.ebay.co.uk/itm/1A-Silicon-Rectifier-Diode-1N4001-to-1N4007-Pack-of-10-25-or-50/111777815515?hash=item1a067a1bdb:m:mrn6u_XT7jetZ63QEHmYYiA

​

It's a completely standard part, you can get them anywhere, only pennies.

You need to pit it in series with the resistor you've just added. The easiest thing to do is just to crop the leads short, cut one of the wires and solder it in line, then use a bit of heat-shrink tubing around it to protect and insulate it. It doesn't matter which side of the resistor you put it, the important thing is that the end with the silver band around it should be pointing towards the alternator (if you follow the wiring). It will only allow current to pass one way, towards the end with the silver band.

This will allow the battery to feed the alternator but prevent the alternator feeding back when you switch off.

There are two reasons why the original warning light doesn't cause this. 1) It's got a much higher resistor in series. 2) It's an LED. The D stands for ... diode.

That'll fix it!

[revilla]

PS: There's a similar issue that affects the K Series, and possibly others. If the electric fan is running when you switch off, sometimes the engine runs on in the same way. The inertia in the fan keeps it spinning and the motor becomes a dynamo, powering the ECU up as in this case. Doesn't last long though before the fan runs out of energy.

[revilla]

PPS: It might seem implausible that the engine electrical systems could be "powered" through a 40 ohm resistor. On the K Series, the ECU has a permanent live feed which is not switched. The fuel injectors, ignition coil packs etc. are also either permanently live, or switched by a Master Relay under the control of the ECU, depending on the loom design. There is an ECU "Ignition Sense" input pin which is connected to the ignition switch 12V circuit and tells the ECU when the engine is switched on. The alternator only needs to feed enough current to pull that input up to some threshold voltage and the ECU thinks you're still switched on so keeps running the engine. With very little other electrical load on that side of the ignition switch, it doesn't take much. So being strictly correct, the alternator or fan isn't actually powering the ECU, more signaling to it that it is still powered. That's why I said the lights would kill it; anything that would load it up and pull the voltage back down beyond the capability of the alternator fed through that resistor would shut it down.

[AdamQ]

Andrew, that's superb - many thanks indeed - all installed and normal service has been resumed ...  

There were four diodes in the box with a second-hand ECU I bought recently (I wondered what they were ...!) and one of those has done the job.

And boy am I glad I took the trouble to put rivnuts rather than rivets around the scuttle - I am no longer the contortionist I never was!

And thanks for the detailed explanations of what's going on - like so many things, it all makes perfect sense when somebody who fully understands these things explains them - all interesting and informative stuff ...

Adam

[revilla]

Glad it's all sorted. You still get decent brightness from the original LED? There have been a few cases recently but I've never had feedback on the final solution and how it worked out. Nice to have a confirmed fix documented.

[AdamQ]

Yes, the original LED appears unchanged in brightness - if it is dimmer, it's only marginal and still perfectly functional, but I think it's unaffected.  Agreed about documenting a fix - and a fix it certainly is ...

I should be able to work it out (or I could just experiment), but, out of interest, will the diode stop the radiator fan keeping the engine running for those extra few seconds?  I found it slightly disconcerting the first time it happened, but it's never bothered me since.

[revilla]

Adam,

No the diode you have just fitted will only affect the alternator issue. You could put a diode in the fan circuit but it would need to be something rather more substantial than the 1N4001 I linked to above, which is only rated at 1 amp.

What still puzzles me is why there seem to be quite a few cases of this particular problem cropping up lately, when you would think if it had always been a problem it would have been discussed extensively and a fix found long ago?

One possibility springs to mind, but I've no idea if there's any truth in it. It seems odd that Caterham would have only fed the alternator from an LED. I just wonder if there is actually a resistor some other circuitry hidden inside the tachometer in parallel with the LED, but this is underrated and over time burns out. Just a thought. If anyone would like to measure the current fed to the alternator on their K Series car and report back it would be interesting. I measured mine a while ago, I can't remember exactly what it was now but it was something silly small like 4 milliamps, so if this is a fault my car clearly has it - I seem to be lucky enough have an alternator that will reliably start on that!

@Jonathan Kay, do we know anyone who has a K Series car, likes a bit of research and data and is handy with a multimeter? All we need is an ammeter between the brown/yellow wire and the alternator terminal with the ignition on but the engine not running. No problem if you need to remove a 2-pin plug to measure it, the DFM on the other pin won't affect things.

[John Vine]

...do we know anyone who has a K Series car, likes a bit of research and data and is handy with a multimeter?

Hi Andrew,

My S-in-L might be such a person.  I'll ask him.

ETA:  I assume the subject car will need to be one with an LED warning lamp built into the tacho?  If so, my S-in-L won't be able to help as his car (1999) has the old-style tacho and a separate alternator warning light (presumably an incandescent bulb).

JV

 

[Jonathan Kay]

That would get me off the hook, John. :-)

Andrew: Great idea, but today I am mostly sealing the garage floor. And after that comes the giant buzz wire game for this summer's wedding!

Jonathan

[revilla]

@John,

Yes it's really the current that the standard tachometer warning LED and any associated resistors normally feed the alternator with that I'm after. But thanks anyway.

@Jonathan,

Nice try but don't worry, if you haven't actually got a multimeter you can admit it... we're here to help :)

[revilla]

Quite why I didn't just check it on my own car at the time I can't remember now, must have been in pieces or something. Anyway for completeness I just measured the alternator excitation current fed through the LED in the tachometer on my car at 3.8mA! No wonder the alternators on these cars can be reluctant to start.

[EEK]

Is that the actual excitation current? I think the real excitation current is fed via the regulator pack and the current fed is dependent on the voltage read from the warning light circuit. At least that is my understanding.

[revilla]

@EEK - You may well be right.

But either way I've now seen 4 or 5 EU3 K Series cars where the current fed by the LED wasn't enough the get the alternator to start up and fitting a parallel resistor to more closely mimic the "load" of an incandescent bulb cured the issue. So one way or another they do seem to expect a certain level of current flow here to act as the signal to turn on and current passed by the LED with series resistor is marginal.

But as you say, it may not be that the actual current flow into this terminal provides the excitation directly.

[revilla]

Actually, after looking at the schematics for Bosch alternator regulators which all seem to be of this format:

In the case where the alternator is not charging, all of the diodes I have highlighted below will be reverse-biased and effectively out of the circuit, taking all of the stator wiring with them. The ones in green rectify the three-phase AC output from the stator to feed the battery but will be non-conducting when the stator is not generating a voltage. The ones in yellow (coupled with the three lower green ones) do the same to provide an excitation feed for the stator when running but again will be non-conducting when not generating. The ones in red look to be to suppress back-emf transients when switching the stator and again will not be conduction statically.

So the circuit effectively reduces to this in the non-charging state:

And from this you can see that the current through the warning lamp not only provides the actual excitation supply for the stator but also powers the regulator IC itself. The regulator IC switches the earth side of the rotor to regulate the excitation current DOWNWARDS but doesn't actually switch in any current from the battery side of things. The regulator IC has a supply connection form the warning lamp circuit, a ground connection, a sense input from the rotor and a switching output to control the switching transistor.

So unless I'm reading this wrong, yes the warning lamp feed IS the actual excitation current, AND the regulator supply.

Later regulators may add some extra functionality; I know these have a DFM output which is a PWM square wave which allows the ECU to monitor the alternator load, but to be honest this may well simply be taken from the transistor base signal in the above diagram as this would give a duty cycle proportional to the load.