Internally Regulated Alternator Trivia - Bootstrap Current?

[geezer]

Interesting question. I think the light is a visual aid as well as a failsafe in case the resistor suffers an open, ensuring the alternator field is energized. Likewise, if the light fails, the "mystery" resistor referred to would still energize the field winding.  

The diagram showing the resistor is probably not realistically illustrated but shown as an example of theory of operation. That leaves me thinking a fusible link is one means to provide the resistance value represented in the diagram. Just thinking out loud.

[Captain Obvious]

Zed Head, Yeah, I know I'm often long winded with my posts. I really try not to, but I have a hard time complete ideas out concisely. I like to think that if you can stand to make it to the end of my long explanations, it would be worth it? It's always the tradeoff being clear and being "that guy". 

 

So even if you don't check your charge lamp during the lamp test mode, a quick glance at the voltmeter will tell you if your alternator has bootstrapped correctly and snapped into regulation. And barring a spontaneous alternator failure, you only need the bulb at startup to get you over the hump. Once the alternator is generating it's own rotor winding current, you should be golden until you shut off the engine. The only other thing the charge lamp would do is light up while you're driving if your alternator or something else close to it went belly up and get your attention immediately.

 

I think adding another path for some excitation current would be a good idea, and I agree that a simple failure mode carbon comp resistor (as opposed to a bulb filament or relay coil winding) would be a reliable way to do it. I would use a value that's on the same order as the bulb filament and/or brake warning relay coil resistance. That brings me to Geezer's post...

 

Geezer, I'm not sure if that resistor on the diagram is a "representation" of other resistances in the system or if it's an actual component they put in there to increase system reliability. But I can tell you that putting a fusible link where that resistor is shown would not work in this situation. The resistance of the link would be too low and the charge lamp would never light because all the current would flow through the link instead of the bulb. You need something with more resistance than a piece of wire.

 

Theory says that if the charge lamp consumes 3.4 W at 12V, it should have a filament resistance of about 42 Ohms, and that's about what I would use in parallel with the bulb. Something like 40 to 50 Ohms. Maybe since my car seems like it wants a tiny bit more excitation current, I might push it a little lower than that.

 

Another question for discussion would be... Where to physically locate that resistance? It would work anywhere, but what are other's thoughts on the location? I've got my idea, but want to hear other suggestions first.

[siteunseen]

 

Zed Head, Yeah, I know I'm often long winded with my posts. I really try not to, but I have a hard time complete ideas out concisely. I like to think that if you can stand to make it to the end of my long explanations, it would be worth it? It's always the tradeoff being clear and being "that guy". 

I would rather read an explanation standing on my head than get that wise arse "use the search function".  You and Zed Head are the ones who take the time to explain, it's that "teach a man to fish" thing. 

[Zed Head]

Words can be fun.  If you're on a roll, in the flow, might as well keep going.

 

I realized in my sleep last night that I hadn't measured my gauge wires in the way relevant to the issue.  If Nissan was going to make a fail-safe gauge L circuit they would have added resistance across those two wires, in parallel to the bulb filament.

 

So I re-measured this morning, in case there was something hidden in the bulb socket, with no bulb in it, and...still nothing.  OL.  

[Captain Obvious]

Haha! Glad you did that. I was internally questioning the way you took your measurements, but I considered the chances that they included a resistor in the gauge assy so remote that I wasn't going to even bring it up. I figured that the chances that maybe I misunderstood what you did combined with the very remote possibility that they included one there didn't make it worth it to continue to chase it. So thanks for closing the loop!

[siteunseen]

So thanks for closing the loop!

...we're gonna rock down to electric avenue...

[Captain Obvious]

And then we'll take it higher? 

[siteunseen]

I had to throw something in even though I have know idea what y'all are talking about.     

 

I do know closing the loop makes thing work!  

[Captain Obvious]

Speaking of closing the loop... I'm sitting here right now wearing my new favorite shirt from The Back Forty Beer Company!! Thanks bud! 

[siteunseen]

I glued a magnet to the Bosco's free beer token, now it's in my refrigerator magnet collection. 

[Chickenman]

 

OK, nothing special about the 78 gauge. I would have been surprised if there was. So if that resistor DOES exist, it's not in the gauge. Thanks for checking. If it exists, it's probably a plug-in like the tach resistor, but I don't have any 78 harness stuff to to poke around with either.

 

About the brake warning lamp stuff... ( Snip )

Thanks for the concise explanation. My Charge light and Brake warning lamp came on when I blew my Alternator. Was driving me nuts at first. I thought I had some sort of strange wiring short that blew the Alternator circuit and grounded the Brake Warning circuit at the same time.

 

Once I replaced the Alternator the Brake Warning light went out. 

[Captain Obvious]

Glad to help.

 

I'm sure you've mentioned it in the past, but out of curiousity, what year Z are you working with? Because if it's not a 78, I think it's just a matter of time before your brake warning relay goes belly up. In the original configuration, they are energized any time the engine is running and the heat eventually takes it's toll.