I
use diodes around my relay coils, and was interested to learn that they are a
mixed blessing, saving the toggle switch, but slowing the coil action. I
wrote Eric at Perihelion Design to ask about his Snapjacks which use zener
diode technology. (Zener diodes act like regular diodes except that when
the voltage exceeds a certain level in the “wrong direction” they
become conductive and remain so until the transient voltage condition goes
away. Then they shut off. Thus they can trim transient voltage
spikes going both ways when put in “back to back” in a single
integrated device – Eric’s Snapjacks.).
Here
is what I wrote and Eric’s interesting response. These
new devices look to be a big improvement. I am going to order a
set. Fred
Dear
Eric:
There
is a lot of chatter in the Lancair world about transient suppression. I
read your stuff on Snapjacks – very interesting – and I am thinking
about ordering a set.
However:
there is always the question of failure modes and probabilities. What the
failure mode of the Snapjacks? Can they fail closed in such a way that
you can't operate the relay? Can they fail short and lead to smoking
wires or a fire around the relay?
The
reason I ask is the following list from a text discussing the relative failure
modes of various types of components:…
________________________________________________________________________
I
am also interested in transient suppression. I build it into most of the
devices I sell, because I can't depend on the customers' attention to this
detail. As a result, the more of my parts you have, the more transient
suppression you have as well.
My
comments on the methods of failure--
“The following list....:
- Capacitors (shorted), especially electrolytic
capacitors. The paste electrolyte tends to lose moisture with age, leading
to failure. Thin dielectric layers may be
punctured by overvoltage transients.
One comment--I have avoided the use of electrolytics as far
as possible in all my design work for the reasons stated. Often a
REALLY BIG electrolytic gets used where a smaller tantalum plus a series
inductor plus a small parallel ceramic and a zener should be used. This is
sloppy engineering.
- Diodes open (rectifying diodes) or shorted (Zener diodes).
True as a Rule of Thumb...but it depends on parts and
circumstances.
- Inductor and transformer windings open or shorted
to conductive core. Failures related to overheating (insulation breakdown)
are easily detected by smell.
True. Let's try to
minimize transformers and relay coils.
- Resistors open, almost never shorted. Usually
this is due to overcurrent heating, although it is less frequently caused
by overvoltage transient (arc-over) or physical damage (vibration or
impact). Resistors may also change resistance value if overheated!
True. Design note: The almost-obsolete carbon-comp resistors
perform well in voltage-transient and overvoltage conditions.
My SnapJacks are
Bidirectional Zeners and as such never see current except during transient
suppression. It is hard to judge their failure modes since their
electrical failures are virtually impossible when used as specified.
Mechanical failure from vibration would be possible, and then they would
fail open. I have never seen or heard of a failure short of a
lightning strike.
The issue of zeners,
bidirectional zeners --or even diodes in general-- failing short always brings
up the idea of adding a fuse. This fuse could be far less reliable than
the diode. A better solution would be to add a series resistor to limit the
fault current, but it would be problematic as to whether or not this improves
the function of the suppression diode.
By the way, the question of
lighning strike suppression needs addressing. Lightning is attracted
differently on composite or metal-skinned aircraft, but in general, adding arc
suppression to the "corners" and the main busses of the aircraft
seems to be a cost effective thing to do.
You're welcome to republish
this on any list.