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MG Midget and Sprite Technical - alternator/ignition lamp question

If one wanted to put LED indicator lights in a customized dash, is it possible to use an LED in the alternator circuit? I understand that there must be an incandescent bulb in that circuit for the alternator to work properly, but, could the bulb remain in that circuit (maybe hidden behind the dash) with an LED wired in the same circuit mounted in the dash? The existing lamp has a white wire going to the ignition and a brown/yellow going to the alternator. The LED needs a pos. and a neg. so I'm not sure how that would be wired in, if it could be. How does the existing lamp glow when it has no apparent (to me) ground? Could a resistor be used in place of the incandescent bulb? I know just enough about electricity to get me in big trouble!
Jack Orkin

The warning light earths through the alternator when it's not charging. When it's working there's +ve on the brown/yellow wire which puts the light out as it has +ve on both sides. You should be able to replace the globe with a resistor and LED in parralel with it for the indication. Diagram below.

Here's how I came up with the values. If factory globe is 2.2 watts that's 0.2amp flow at 12 volts. A 60ohm resistor will achieve the same thing. A 5watt resistor is needed as there's a bit of current going through it. If you use a smaller wattage it'll either get hot or burn out. Not what you want under the dash. A LED is like a diode and needs a resistor in series to limit the current flow to about 20ma. At 12volts a 600 ohm resistor is needed to do that. If you want a duller LED increase the resistance.

If you're not too sure about how to connect it up get someone to give you a hand.

Does it look reasonable to others?

Greg H

LED is a one way device, requiring a voltage that is more + one one side than the other, and it matters which. When the engine is not running, this is from W (+) to Alt NY(-). So far so good, it will act as a "not charging" light.

However, part of the function of the indicator is to tell you that there is something wrong that gives more + at the Alt NY (++) than at Ign W (+), the equivalent of Alt + and Ign -. That is, power is flowing in the contrary direction, so polarity at the LED is reversed. This would be either an Alt/reg fault, or a wiring fault, as described in numerous recent threads. So the LE
D indicator will not work in these situations.

A resistor alone will let the system charge, but will not give you any info. Further, under the condition described by several, where the light gets brighter as rpm increases, the alt voltage rises very high, causing lots of heat damage to the alt. When the voltage gets high enough, the bulb blows and the alt quits charging, effectively acting as a fuse, and (maybe) saving some parts.

FR Millmore


How about a bi-colour LED to give one colour for OK and another for a fault. Excess voltage would still be a problem that would have to be dealt with.

Off to bed now in the UK so not thinking about it too much at the moment.
David Billington

Thanks for the responses. I did not realize that LEDs were like diodes. The LEDs that I got were from an auto parts store for use as replacements in cars, so the needed resistor for LED operation must already be in place in the housing, if I understand the concept. Interestingly, there is no mention on the package of polarity and only by checking each contact with a battery, did I notice that there is a small label around the housing with a part no., 12VDC and a symbol that must be for neg., but its sort of odd looking and not always centered over the neg. contact. But, the contact that it is closest to is the one it works with when connected to neg. I don't understand the potential overvoltage problem. Are there electronics in the alternator that regulate the voltage output, and these can go bad, resulting in overvoltage? Although this may be uncommon, I wouldn't want to not be warned about it if it did occur. Smoke and flames under the dash is not the kind of excitement I want in my midget! Maybe I'll reconsider my idea and leave a perfectly functional system alone...
Jack Orkin

LED = Light Emitting Diode

"Are there electronics in the alternator that regulate the voltage output, and these can go bad, resulting in overvoltage?"
Yep. Some of them are the diodes that rectify AC to DC, and some of them are in the voltage regulator. Doesn't much matter which, since it usually cooks all of the electronics, but the ones that cause high voltage cook parts that can otherwise be saved.

The high voltage is caused by a bad (open)"live side output diode" which confuses the regulator and cooks everything. "Light goes out at 850 rpm then glows brighter as rpm increases; may blow bulb"
The bulb blows around 18-20V, or usually 5-6000 rpm. The overvoltage is proportional to the brightness of the light, so if you keep the engine speed down to where the light is dimmer than when the engine is OFF, you can drive forever. Get annoyed by the yurk in front of you and do a high speed downshift/pass and you have to stop and replace the bulb. And he passes you while you are doing it/repeat!
When the bulb blows the system quits working and things quickly start to get dim, until then your headlights are REALLY GOOD!

You could devise any number of complex circuits to tell you stuff, which we might call a "Check Engine" light.
Or, it could poke a stick in your eye.
But why bother?
Light bulb is cheap and tells you much.
I've never seen a bad bulb that was not caused by a fault that it had been announcing for some time.
"OFF" is good, "ON' is bad, and modes/degrees of "ON" might tell you what is bad.

"leave a perfectly functional system alone... "
The perfect solution!

FR Millmore

For the vast majority of alternator faults it will work just fine but yes for a couple of faults where the IND voltage is higher than the battery it won't. e.g. open circuit on one of the positive main diodes or break in the main output wire back to the battery. Both these will result in less volts back at the battery than at the IND terminal. So in these you might get a dull globe from a reverse flow.

I'd say power flowing in the reverse direction is not that common though. A charging fault is more likley not to be one of these so with only a couple of faults that could cause this to occur perhaps overstatement to state "This would be either an Alt/reg fault, or a wiring fault, as described in numerous recent threads" because unless you've actually measured the polarity across the light it's hard to know why it's glowing.

Normally when the alternator is overcharging you won't even know. Although one of the prementioned faults may result in the light glowing brighter as the revs pick up it just means there's more IND volts than battery volts, not neccesarily over charging. Normally if there's fault with the regualtor and it's over charging both the IND and the battery, although at a higher voltage will have exactly the same voltage so the light goes out as normal, it just has more voltage across it. You could have any higher than normal voltage across the light but as it only sees the difference it happily goes out and will never blow. Output above 15.5 volts will start to kill the battery, one of the indicators of over charging is acid around the battery vents. You may notice brighter lights if it gets up too high but the only real way to know is put a meter across the battery and measure it. So don't be fooled into thinking the warning light will protect you from over charging.

The two main bits that can go bad in the regualtor to give and overcharge is the voltage sensing bit (zener diode) or the transistor that does the switching can go short cicuit, but the most common faults are worn brushes, diode fault or open circuit in regulator. All these will give no charge or reduced charge. Over charging due to a fault is less common.

So this circuit will generally work fine but there are a couple of faults where it won't give any indication. The globe is a simpler way to go. If I really wanted a LED I'd have no issue with doing it but if not really needed then stick with the light. Always apply the KISS principle when you can.

FRM i was posting at the same time. Agreed unless you need to change it leave it alone.
Greg H

"you might get a dull globe from a reverse flow"
An open in the output lead to Batt would give the same glow as when the engine is not running - bright and steady, until the regulator died. Alternatively, the light might keep getting brighter until it all goes up in smoke. (Not sure since "don't run with cables disconnected" and "check cables" are the first two rules of alternators.)
Open output diode gives the increasing with rpm glow, which will get much brighter.
"dull globe" is other faults, including bad connections in the N/W circuit, a most useful thing to know.

"...not that common though"
I think at least three threads on this in the last month or so, Spridget boards only. I've seen plenty, and have been working on these since Lucas discovered alternators.
I think this is about to be much more common, as the "Output diodes" are in fact part of the rectifier stack. This is old tech selenium rectifier, and I just discovered that they die of old age, even if sitting unused on a shelf. 50 years being the stated approximate failure time per the radio resto guys.

You are correct in general, that the light will not show overcharging, however, I don't think I've ever seen a Lucas alt overcharge without the open output diode fault.

FR Millmore

In the fault condition that causes the lamp to glow ever brighter until the lamp fails, the concern is that this protection will be lost.

Isn't the protection is already there in Gregs circuit?

Greg already shows a fuse, presumably of a sufficiently low rating, designed to prevent the lamp glowing ever brighter, by failing as it attempts to.

When the fuse “blows”, whilst you might not notice the sudden extinguishing of the LED glow, the next time you switch the ignition on, you should notice that the LED is dead. That would give you cause to investigate.
Lawrence Slater

Yes FRM you're right if the battery isn't connected with nothing to hold the voltage down the voltage will keep rising. Globe would blow for this one, but I still don't think it's that common for the BAT wire to become disconnected. The early alts with the large spades perhaps, or if people have been fiddling when they shouldn't have been then quite possible.

Yes it didn't take much for those old rectifier packs to fall apart. The regs weren't much better. I think a blown +ve diode is more likley fault of these with a dull globe which the LED circuit won't pick up. I haven't seen a lot of bright warning lights from reverse flow or disconnected BAT witres but I supporse it depends on how many old Lucas cars you work on.

Lawrence the fuse has to be large enough to allow full alternator output so wouldn't blow under overcharging as it doesn't take a lot more field current for overcharging to occur. Actually if the battery is fully charged it won't take much at all. With a normal charge circuit the fuse wouldn't blow either. As has been stated it's not as good as a globe as there's a couple of faults it won't pick up. If you really wanted to cover these faults you could put another LED in reverse or replace the LED with a small globe in. You'll get a glow, but with the large resistor in it, it will still try and charge. So still not perfect.

but I think Jack is back with the original light....
Greg H

Thanks to all for the education. And, yes, I know that LED stands for light emitting diode but for some reason it didn't click. I guess when you use acronyms for so long, you tend to forget what they stand for! Anyway, I guess I'll use the old indicator bulbs. Now, if anybody has a trick for getting those spiked washers off that hold the plastic bulb holders, I would greatly appreciate it!
Cheers, Jack
Jack Orkin

Hi Greg,
You've got me there. How can the fuse in your circuit take the full output current of the Alternator? A 16ACR is rated at 34A. You've already agreed the max current flow in the lamp is 2A (rounded up), and substituted a 60Ω (rounded down) fuse for it.

The lamp delivers excitation current to the field coil. When the ignition is first turned on, that current must be max circa 2A, but possibly less, if you also count in the admittedly low resistance of the field windings. Without your suggested circuit modification, which I like btw, if the lamp fails or is taken out of circuit, no current can flow via that part of the circuit, and if there's no residual field magnetism, the alternator won't generate current output.

When the lamp is in circuit, and hasn't failed, barring other faults, the alternator produces an output current, but not back along the line fed by the so called ignition warning lamp.

If the kind of fault discussed previously developes, the voltage on the Alt side of the lamp rises, and current begins to flow back to the bat. Eventually reaching a level where the lamp element can't handle it, and ut melts. I've never placed a 12V 2.2W lamp in series with an ammeter to measure how much it can take, but it sure won't be the full output current of the alternator, 34A.

So I think I must be misunderstanding what you mean when you say, the fuse has to be large enough to allow full output current. So in your circuit the fuse has to be rated at 34A? The lamp will fail way before that is ever reached, and if it didn't the wiring would.

Lawrence Slater

"and substituted a 60Ω (rounded down) fuse for it."

Sorry, I meant resistor, not fuse. A 60ohm resistor.
Lawrence Slater

That sound all strange, maybe my system is defying the laws of physics "Scottie""


I have an led in place of the original ignition light on my homemade dash board.

Performs perfectly simply wired as the original lamp!

The alternator does require a few revs initially to "switch it on" but then it is straight forward.

Robert (Bob) Midget Turbo

Lawrence, the fuse is in the excitation circuit only has to handle full excitation circuit. The main output circuit i.e. fat wire isn't drawn. It's a different circuit. Ommitted for clarity, perhaps it isn't. Actually thinking about it the fuse doesn't have to be large enough to allow full excitation current either, it only needs about 0.2A to get it going and once it has the excitation circuit is self exciteed from the alternator. In most instances the voltage on both sides of light are about the same the only real time when the light will see a high voltage is if the alt bat terminal is disconnected from the battery so battery is at 12volts and alternator voltage keeps going up. Match the fuse so it would protect the circuit you say? Good idea but don't think doable. So battery remaining at 12volts due to break in fat main alt wire or some other problem. Voltage on alt and IND at 30 volts (only example) If you did have 30 volts on the IND wire on the alt and only 12volts at battery you'd still only have 18volts across the globe so the voltage and resultant current flow is not that far from normal. So under 28volts I can't see how the fuse could help and even if set up so it blew then you would have to have the fuse at and exact size with little tolerance. If you couldn't then a minor increase in current rating would mean notable extra voltage. With such low current it'd be hard to get exactly the right rating too and with a lot of variables, such as wire resistance and voltage drops that'd have to be consdered, not an easy task, although perhaps doable on a test bench just doesn't sound practical.

Bob good to see someone's using a LED without a problem. I put the 60 ohm resistor in the circuit to ensure there was enough current to allow it to self excite. Sounds like if you're lucky not even needed or could be a much higher value. It'll all work ok but when fiddling you need a bit of knowledge otherwise you could just create problems.
Greg H

Hi Greg.
Hmmmm, yes. Well, Mr. Aus Red is calling at this precise moment, so if you'll excuse me, I'll read that tomorrow, when I can absorb it more easily, ---- than the grape.
Lawrence Slater

Hi Greg. Red or no, I'm still not convinced.

The circuit you posted is only that section that supplies the excitation current via the warning lamp. Ignoring the ignition switch, it is no more than a wire from Batt +ve, through a 2.2W lamp (or equivalent circuit-- yours), on to the Alternator field coil.

FR said, "under the condition described by several, where the light gets brighter as rpm increases, the alt voltage rises very high, causing lots of heat damage to the alt. When the voltage gets high enough, the bulb blows and the alt quits charging, effectively acting as a fuse, and (maybe) saving some parts."

The concern he had with your circuit, was that the 60ohm resistor would allow that current flow to continue and increase as the voltage rose, instead of acting like a 2.2w lamp, and failing because it couldn't handle anymore current.

I said that there is already a fuse in your circuit. That fuse only needs to be able to take the same current as the 2.2W lamp, which feeds the Alt it's excitation current. If the fault FR describes arises, the same conditions would exist. The voltage will continue to rise, and in this case the fuse will reach a point, where it can no longer carry the current. The fuse will melt, and the current flow cease in that part of the circuit, saving in this fault condition, at least some parts as FR says.

The fuse needs to be one that fails at circa 2A, which is probably how much that 2.2W lamp can handle before it's element melts.

Your circuit is an equivalent circuit. It's really no different to the normal setup, except you have an LED indication to tell you when the ignition is on, the LED goes out to tell you the Alt is charging, and a fuse that blows if the condition FR describes arises.

As to it being good for the indication of other fault conditions, that's a slightly different case. I think.

Lawrence Slater

Lawrence I've had a bit more of a think about it you're right, with a fuse of the right value it would sort of work.

Although for full excitation you need about 2amps this is provided to the regulator from the excitation diodes within alternator when it's going. So for the external circuit on start up about 0.2amps (at 12.5 volts) through the globe is all that's required (possibly even less). So then use 0.2amp fuse? You're right here this fuse would protect you if the reverse flow exceeded 0.2 amps which equates to 12.5volts above battery voltage i.e. total voltage from alternator 25 volts or more. So now the circuit would be protected similar to the globe. One little problem which you now doubt realise. The fuse can't be the same as the normal current flow i.e. 0.2 amps as it would blow, actually it can't even be close because if it was it would run very hot and in theory probably be glowing red. Not ideal so it needs to be over rated. So say we go up just slightly to what is probably the next possible size 0.3amps (300ma). This I think would be the minimum over rating you could use possibly even too close. Now 0.3amps = 12.5volts + 50% = 18.75volts before it would blow. So battery at 12.5volts + 18.75 so that's now 31.25volts at the alternator output before the fuse would blow on reverse flow. So you're right a fuse could be used as a protection device. But to call it a protection device you'd have to you the term very loosley as most 12volt stuff would be cooked by then, but then probablky so to if you had a globe. Now we're even covered about as well as the globe. Not well I think but about the same.

Now this situation only exists when the main lead on the alt isn't connected to the battery and the regulator can't hold it down low enough. Most overcharging faults the alt is still connected to the battery which acts like a load and holds it down lower, usually under 20 volts. In this situation as has been stated before even with the globe you'd never know it was happening.

So here's MKII. This is becoming a real time waster, especially condering I don't think anyone has any inclination of putting a LED in the circuit (apart from Bob).

I should be in the garage putting my car together. When it is together I could put the circuit in a simulate a few faults on it as a little project, but I won't, I'll leave that to someone else....

Greg H

Brilliant thanks Greg. :)

Now get back to work. LOL.
Lawrence Slater

This thread was discussed between 24/02/2012 and 27/02/2012

MG Midget and Sprite Technical index

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