damon henry via EV wrote:
Hi, I am wondering if anyone has adapted Lee Hart's Zener regulators
for lithium cell use.
Cor has covered the issues very well.
I've tried them with lithiums; but they don't work well. True zener
diodes (voltages less than about 6v) are pretty soft. There is about a
2-3 volt-wide range over which they gradually change from non-conducting
to fully conducting.
This soft "knee" makes them unsuitable for a zener-based lithium
regulator. They still draw significant current even 1 volt below the
voltage where you want them off.
"Zener" diodes above 6-7v are in fact Avalanche diodes. As the name
implies, they conduct much less below their rated voltage, and a lot
more above it, with a very sharp transition. The 6.2v and 6.8v diodes
used in the 12v zener-lamp regulator are this type. They conduct far
less current with an open-circuit battery (13v or less), but fully
conduct when overcharging (15v or more).
There are zener-like replacements; essentially integrated circuits
designed to behave more like an "ideal" zener diode. The 25-cent TL431
is a common example. It looks like a transistor (3 leads instead of two)
but acts like a 2.5v zener (adjustable with the 3rd lead). It "leaks"
about 0.4ma below this (which is actually its supply current), and
conducts up to 150ma above this. That's pretty good; about a 600:1
change. There are lots of similar parts, with lower leakage but lower
conduction.
PS: This zener-lamp regulator is more sophisticated than it look.
Avalanche diodes (as used in the 6v, 8v, and 12v versions) have a
positive temperature coefficient. That means if they overheat, their
voltage rises, which tends to limit the power so they don't fail. True
zeners (under 6v) have a *negative* temperature coefficient. If they
overheat, they draw *more* current, and burn themselves up!
A lamp is used (instead of an LED) so it also acts as a fuse. Normal
regulators use lots of semiconductors which can fail "on". This loads
the cell continuously, runs it dead, and destroy it! But if a zener
shorts here, the lamp sees too much voltage and burns out. Now you have
an open circuit, and saved your cell. :-)
The lamp also makes it "idiot proof". If you hook it up backwards, or to
the wrong place in the circuit, or there's a loose connection, or a cell
fails open, the lamp simply fails open -- no fireworks or fires.
--
The greatest pleasure in life is to create something that wasn't
there before. -- Roy Spence
--
Lee Hart, 814 8th Ave N, Sartell MN 56377, www.sunrise-ev.com
_______________________________________________
UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub
http://lists.evdl.org/listinfo.cgi/ev-evdl.org
For EV drag racing discussion, please use NEDRA
(http://groups.yahoo.com/group/NEDRA)
