-=> George White said to Greg Mayman
 -=> about "VEHICLE LED'S" on 09-21-03  12:12.....

 GM> First you sniff a bit of the AC input, amplify it and clip it and
 GM> feed it back to the transistor bases so they turn on/off at zero
 GM> crossing.

 GW> You also, for a self contained unit, need to take some of the AC and
 GW> create a sufficiently high rail relative to the emitter to enable you
 GW> to drive the base hard on when there is very low AC from the source.
 GW> The most difficult part!

Yes, amplify it up to a couple of volts. while the AC is still
just past zero crossing.

I have included the quote from my own message to show that we are
apparently saying the same thing 

 GW> Correct. The dedicated isolator units are just two isolation diodes.
 GW> Leaving the alternator sensor connected to the primary (it should be
 GW> the vehicle or existing) battery means that continues to be charged
 GW> correctly and the slave (usually a caraven, trailer or separate RV)
 GW> battery will get charged up as well. For systems without a separate
 GW> alternator sense wire (and they do exist) things are more difficult,
 GW> the batteries are never fully charged.

No, they _would_ reach near full charge eventually, just that
they'd take a bit longer.

There are two factors here:

One is that the alternator regulators that sense the output of
the alternator directly are almost always set about higher than
optimum to allow for voltage drop in the cables to the battery.
Some will charge the battery to 14.5v.

The second factor is the behaviour of the diode. The voltage drop
across the diode is not a constant, it is proportional to the log
of the current. Also the voltage drop DECREASES as the temp goes
up. So at elevated temperatures and reduced current, the diode
drop goes down considerably.

This means that the alternator can still be putting current into
the battery right up to the optimum voltage, but at a lower
charge rate than it would have with the battery connected
directly to the alternator.


 GW> Most modern alternators don't
 GW> have any facility for altering the terminal charge voltage.

Very true.

 GM> Theree is a lot of stuff on the 'net, but only very rarely do you
 GM> find data for outdated devices. I suspect the same would apply to
 GM> CD-ROMs :-(

 GW> But at least you can keep the _old_ CDROMS and still have the old
 GW> data.

The problem is that most of the old data never got onto CD-ROMs
in the first place.

 GW> They were normally just diodes to protect the gate against breakdown.
 GW> They clamped the gate voltage to 1 diode drop above or below the other
 GW> terminals.

Yes, they could be clamping against reversal of the control
voltage.

OTOH if they are zeners, they could also clamp the forward
voltage. Most insulated gate FETs are rated to a maximum
gate-source voltage of 15v.

From Greg Mayman, in beautiful Adelaide, South Australia
   "Queen City of The South"    34:55 S  138:36 E

... Any man who hates dogs and babies can't be all bad.
___ Blue Wave/386 v2.30

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