Hi Greg,

On 19-Sep-03, Greg Mayman wrote to Roy J. Tellason:

 RJT>> Ok,  how would you do that?

 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.

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

 RJT>> It would seem to me that at the high current levels (tens of
 RJT>> amps) they're wasting a bunch of power as heat,  and the voltage
 RJT>> being dropped a bit is also going to cause the charging rate to
 RJT>> be just a little slower.

 GM> I have seen these things in the past, but I can't remember what
 GM> the heatsinks were like, probably they were pretty big. Yes, I
 GM> guess the charging rate would be slower.

You don't normally bother to do this for battery isolating units. Just
using a high power Shottky diode is enough, has no effect on the
overall charge rate (If you have, eg, a 50 Amp alternator you still
get 50 Amps out of it if the battery will take it).

 GM> And I've also wondered what the drop across the diode would do to
 GM> the regulator. Should it be set a bit higher than normal? No-one
 GM> I've spoken to about this seems to think it is necessary.

 GM> OTOH someone once said the regulator should be connected to one of
 GM> the batteries AFTER the isolator diode.

Correct. The dedicated isolator units are just two isolation diodes.
Leaving the alternator sensor connected to the primary (it should be
the vehicle or existing) battery means that continues to be charged
correctly and the slave (usually a caraven, trailer or separate RV)
battery will get charged up as well. For systems without a separate
alternator sense wire (and they do exist) things are more difficult,
the batteries are never fully charged. Most modern alternators don't
have any facility for altering the terminal charge voltage.

 RJT>> I used to be able to acquire the occasional data book here and
 RJT>> there, but haven't in years now.  It's probably a good thing,
 RJT>> as the ones I do have take up a bunch of space now!  :-)  This
 RJT>> stuff is also coming out on cdrom,  and may be downloadable as
 RJT>> pdf files,  I haven't looked yet.

 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 find data for outdated devices.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 :-(

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

 RJT>> I'm thinking those part numbers are actually "2SK707" and
 RJT>> "2SK799", that sound any better?

 GM> Could be. It sounds logical.

 GM> But I still don't have any data for those either.

 RJT>> There were some of those parts that also seemed to have diodes
 RJT>> across the gate-source connection,  though not all of them.
 RJT>> I've seen some of those parts that seem to have something of the
 RJT>> sort built in,  too, though I can't recall just why.  Maybe to
 RJT>> prevent breakdown at the gate?

 GM> Could be. Offhand I can't think of another reason. Were they
 GM> zeners or just ordinary diodes?

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

George

--- Terminate 5.00/Pro