GlenL

Tass...


Looks like you're set. 68 Ohm resistor plus 1.2W bulb or just a 44 Ohm resistor. If it were my car (IIWMC?) I'd do just a resistor under the hood and watch the voltmeter to see if something has gone wrong.

And Alan...

This "crap" is that you did quite clearly suggest the bulb behavior might be important, and say it didn't have "thermal issues." Moreover, this "crap" is about my frustration with two kinds of posters: idiots and know-it-alls. I spent 20 minutes finding the info, doing the math and writing my response. Here you come, in know-it-all mode, confusing the issue with crap about how you might need a bulb after all. It has weird non-linear behavior! Well, duh! How smart of you to mention that. We were all impressed. But the buld is just an idiot light.

At least there were no "idiots" who'd post something like "I just wire it directly and use a battery maintainer."

I've taken to posting less because of this crap. In either case it takes additional time and effort to try to clarify the facts especially when the offender keeps repeating themselves. This leaves the original poster to decide what's valuable and ends up being a waste of time for me.

docmirror

How sad. This whole thread could have stopped at post #7 where Alan gave a very brief but ultimately perfect explanation of what needs to happen for the exciter circuit. He's not trying to make EEs out of people, just getting their car sorted out.

How much time and effort did he spend having to go back and say the same thing over, and over, and over again before people get it? And then to get piled on for being right? Yikes.

The resistor and incandescent lamp are wired to the exciter in parallel not series. It's a very old, and very well tried method of providing a variable resistance to a field.

dr bob

Ouch!

A lot of pissing for a resistor and a light bulb issue!

Dave--

In my last race car I used a beehive truck/trailer clearance light with a 57 bulb in it. No Resistor. Didn't do any fancy math or try to calculate the thermal issues with the bulb. It was big enough to see and catch the driver's attention. Big concern there was for belt failuire, since the belt drove both the alternator and the water pump. Driver wouldn't notice a problem until the car stopped running and the steam was pouring out. Use a 5A fuse in the circuit for exciter and connecting wire protection.


For those playing along at home--

The alternator self-excites once it actually starts making its own power. It just needs a kick-start and no issues after that. The internal excitation is through a silicon diode with about 0.7 volts drop. Otherwise the exciter sees battery terminal voltage. The lamp serves two purposes. One is to provide a path for initial excitation current to flow, as stated by others. Should the alternator stop prodicing power for any reason, the second feature comes into play. The lamp then sees the difference in potential between the ignition circuit on one sde, and the alternator on the other side, with the alternator statr windings isolated by diodes. Bulb illuminates. Purpose of the resistor and/or bulb is to make sure that the current in the circuit is high enough to build the initial magnetic field in the rotor to start charging when the alternator spins, yet that current needs to be low enough to protect the wiring in between when the alternator isn't making any juice but there is voltage at the exciter terminal.

IIRC. the #87 light bulb is the same oneused in the tail lights and marker lights on the 928. I buy them in boxes of ten, box lasts me a decade or more.


-----

More than a few great resources have abandoned the forums here because they were slighted somehow. Too much drama over a light bulb! There really is a golden rule when it comes to posting. It should NEVER deteriorate into an "I'm right and you are wrong!" situation. Post your contribution and let it go. Here are two EE's wrestling over how important the varying current flow in a light bulb works.

In this case it really makes no difference. There needs to be enough current flow through the bulb to get it to light up enough to gain the driver's attention if there's a problem. I'll speculate that if Porsche could eliminate the bulb they would, since it costs money and wire to install, protect, warranty, etc. A simple resistor circuit is cheap Note that many GM cars are produced with internal excitation, eliminating the bulb. A system display monitors voltage and warns the driver. Would a bulb be better? For 99% f the drivers the answer is yes, since we tend to lokk at gauges only to see what damage has already happened.


Dave--

I probably have a spare old-style clearance light and bulb in my trailer box in the garage. Yours if you need it.

Tass 928

Doc, thank you for catching that. I was going to wire them in series.

To everyone else who responded, thank you very much for your time and effort. Each response gave me a better understanding of the problem. It seems there are a couple of ways to achieve the desired result but I ultimately decided to go with the stock resistor/light bulb combo because that is what I have at hand.

Tass

John Veninger

Dave,

I have a large 5watt 50 ohm (need to check actual) resistor in series to the excite post on the alternator. I power it on when the ignition switch is turned on. Been working like that for many years on the track car.

QuattroRS4

I'm a tad late but I used to make units to cure this for marine, agricultural and custom cars without pulley changes and other such mods - its not a simple shunt mod .. details of it can be found here .. http://www.igniteautomation.com/docs/exciteme.pdf

Regards,
John