Limited current, current gain circuit for alternators

[PlaneCrazy]

Greetings, all. Let me tell you about my latest disaster.

My 30-yr old beaut was designed with rice grain bulbs for the instruments - mostly T3.5, but some are T10. The T3.5's installed were originally 0.5W, but the only ones I can get these days are 1.2W. Twice the brightness, twice the heat... so these end up melting the buttons.

So I decided to replace these with LEDs. And they look a-MAZE-ing!

Unfortunately, I picked up an unforeseen problem.

My alternator is not even two years old. I had it replaced shortly after installing the LEDs. But I discovered that, while previously the charging light would die as soon as the engine started, now I have to rev the poor engine to redline to get the charge going. I checked the voltage regulator, the slip rings, the ground cable, everything recommended by motor forums and auto electricians.

A study on how these self-excited alternators work has led me to conclude that the LED I use in the charging circuit is to blame: the alternator requires the indicator lamp to provide exciting current. But the LED draws much less current than the alternator requires. So I could put back the incandescent lamp and watch the plastic melt, or I can keep the LED and ruin my engine.

Or could I perhaps take Option 3?

If I could design a circuit, connected in series with the alternator, to provide the alternator with the required current, which relies on the indicator but boosts the current up to a limited point. In other words, if I could install an incandescent bulb, the current would be limited (as would the light - but really, it's a tiny indicator, nobody's going to miss half of twice the light) to what the alternator needs, but if I install a LED, the current would be boosted to what the alternator needs.

Transistors are used to provide current gains. But what would limit the current? What would this circuit look like?

FWIW, the alternator requires 10A of exciting current.

Thanks

P.S.

If the ON times were shorter, there'd probably be less of a problem. But this is not always possible. The diesel has to glow, sometimes for up to a minute, to get the engine started. And some of the electric features only work when the key is in the ON position (not the ACC position).

 

[MikeMl]

You are making it way too hard. Just parallel the new LED-resistor combo that replaced the old alternator lamp with a suitable externally-mounted 1W resistor.

The old lamp was 0.5W, so its resistance would be:

P = E^2/R

R = E^2/P

R = 12*12/0.5 = 288 Ohms

Because the LED resistor combo is already providing some current, try a 330 Ohm 1W resistor, or if you have a hard time procuring a 1W resistor, use two 150 Ohms 1/2W resistors in series...

 

[crutschow]

Mike, since the old incandescent lamp has a very non-linear resistance would that be a factor in starting the alternator excitation? In other words, would a fixed value resistance equal to the hot resistance of the bulb still start the alternator properly? Just wondering.

 

[MikeMl]

Mike, since the old incandescent lamp has a very non-linear resistance would that be a factor in starting the alternator excitation? In other words, would a fixed value resistance equal to the hot resistance of the bulb still start the alternator properly? Just wondering.

My Volvo I/O (Swedish) Chevy V8 powered boat (USA) came with a Paris-Rohne (French) marine alternator. That alternator is a copy of the original Motorola design which requires the back feed from the idiot lamp to bootstrap it, just like the OP's case. The boat has no idiot lamp; rather it has a voltmeter which shows if the alternator is charging or not. To provide the bootstraping, the boat wiring includes a 74 Ohm resistor that is fed from battery voltage via the ignition switch and goes to the terminal on the alternator that feeds the Voltage Regulator. If this resistor is disconnected, the alternator will not bootstrap. In a car, the idiot lamp serves the same function as the resistor does.

So, maybe the OP needs a resistor that is closer to the cold resistance of the original lamp (68 or 75 Ohms?) , rather than the 200 Ohm hot resistance I calculated?

 

[PlaneCrazy]

Interesting thing, the voltmeter...

I'm sure I could just add a shunt resistor. But I'd like to make the whole thing forwards and backwards compatible. For interest's sake, if nothing else. Is there really nowhere else that one might require a current limiting current gain?

 

[MikeMl]

Interesting thing, the voltmeter...

I'm sure I could just add a shunt resistor. But I'd like to make the whole thing forwards and backwards compatible. For interest's sake, if nothing else. Is there really nowhere else that one might require a current limiting current gain?

I'm not sure what you are asking. You have already modified the instrument panel. Y0u created the problem by removing the lamp that was there (which passes enough current to bootstrap the alternator) with a LED (that does not). To fix the problem, you need to parallel the Resistor-LED with a 75 Ohm power resistor. You can add the extra resistor either external to or internal to the instrument cluster.

Here is a pretty good explanation of what the lamp/resistor does. Search down the page for Alternator Warning Lamp.

 

[Diver300]

Many cars had exactly the same problem when the instrument panel got redesigned to one that took a lot less current for the generator warning. Also, on a car like yours, a blown generator warning light would stop the charging completely.

For those reasons, lots of cars have a resistor in parallel with the generator warning light. It is by far the easiest fix.