Edward Scissorhands under the dash.

[Tom H]

Todd, Thanks. I see and understand. This is what we would call "Remote Sense" capability in the instrumentation world. I shoulda known! By making this connection separate we can connect it electrically "closer" to the battery thereby assuring that the 14.1 V control voltage is maintained at (or near) the battery and not just at the Alternator output. This "compensates" for any voltage drop in the main B+ charging cable.

Thanks for the links too.

Tom

 

[Nickodell]

Thanks, Rich,

I am sure it will charge properly if you wire the S directly to the B+. Electrically it is the same as wired to the switch with the switch ON. What I am not sure about is what happens when the car is turned OFF but the S is still connected to the B. It means the Alt is still ON. But I'm not sure what the net effect is. My guess is that it will put a slight drain on the battery while stored. Tom

Tom: That is exactly what happened to Matilda when I replaced the Lucas Smoke alt. with a 60a AC/Delco. In fact, it gave the interesting effect of the engine continuing to run with the ign sw off and the key out. I fixed that by splicing a diode into the Delco equivalent of the S wire, preventing feedback. Before doing this, not only would the engine continue to run, if you stopped it by putting the car in top gear and letting the clutch out gently, the battery drained if you left it for a couple of days.

Of course, once I fitted the batt master switch and religiously turned this off every time I left the car, the batt drain became a non-problem, diode or notproblem.

 

[sunbby]

More alternators

Tom: That is exactly what happened to Matilda when I replaced the Lucas Smoke alt. with a 60a AC/Delco. In fact, it gave the interesting effect of the engine continuing to run with the ign sw off and the key out. I fixed that by splicing a diode into the Delco equivalent of the S wire, preventing feedback. Before doing this, not only would the engine continue to run, if you stopped it by putting the car in top gear and letting the clutch out gently, the battery drained if you left it for a couple of days.

Of course, once I fitted the batt master switch and religiously turned this off every time I left the car, the batt drain became a non-problem, diode or notproblem.

Nick,

That is curious. Did you have it wired as in the team.net example, with the "S" sense wire to the switched battery side or to the unswitched side? I think the example provided is wrong, the sense wire should be connected to the unswitched side.

If you wire the sense node to the unswitched battery then the engine should not continue to run, since it can't supply the ignition with voltage. Having the S wire connected to B+ can not cause the engine to run with the ignition switched off. It shouldn't drain the battery either; I think the input impedence is pretty high.

If the sense is connected to the switched side then it can feed the coil keeping the engine running when the ignition switch is off; and it provides a path from the battery through the alternator then to ground when off, draining the battery. I suspect the diode prevents these 2 symptoms but it is not needed if wired correctly. Note, in this case the battery drain is from the alternator through the sense wire to ground; a different direction than the postulated drain when the sense is connected to unswitched battery.

Whew. Did any of that make sense?

 

[V_Mad]

I think much of this has already been said, but here is my version FWIW:

In any voltage regulating circuit I have worked on, the sense terminal is just a passive input, and therefore in the case of a car alternator, connection to the switched ignition circuit should not maintain the voltage (unless there is another low impedance path); therefore the engine should not continue to run on.

Having said that, it is inadvisable to connect it there (or any other switched place) anyway as the alternator's output (B+) will go 'out of control' when the sense is diconnected from the alternator, possibly causing the alternator to burn out.

Sense terminals are designed to be connected to a remote point along the main supply wire where all the other circuits (fused and unfused) are 'star connected from it, eg the fuse box or starter solenoid live side. This remote point is then maintained close to the specified voltage (around 14V) regardless of load, and is ideal as a stable supply for critical loads such as headlight bulbs etc.

If the sense wire is connected directly to the B+, the voltage will be lower as you move along the battery/main supply wire, but this effect can be minimised by using a heavy duty (large cross section) cable conductor.