I'm sure Eric makes a fine product that will work as he advertises, but I have to take a very slight exception to some of his statements. True, diodes can fail open or shorted. Generally in this application, failing open is relatively benign since the system will still function as intended - there will be no system fault and the only problem will be a loss of protection of the switch against arcing. Failing shorted would seem to be a serious fault, but most(see exception below) diodes, zener or not, won't survive the resulting high current and will consequently destroy themselves and go open. The result is an open circuit whether the diode junction itself fails open or shorted. A standard diode (we would call them "free-wheeling diodes") used in this application only sees current during the suppression mode and absorbs about 0.7 volts
for maybe 0.1 sec at 0.5 amps peak, decaying to zero. The total energy is very small (about 0.02 watt-seconds), as most of the inductive energy is dissipated in the resistance of the coil. A zener, on the other hand, is there to absorb the inductive energy itself. It might dissipate 50 volts for 0.01 seconds at the same average current for about 1watt-second of energy. the instantaneous power is much, much higher - .35 watts for the diode and 25 watts for the zener. Point is that the zener is "working" much harder, although for a much shorter period of time, and it is designed to be able to do that. Point of all this is that I don't buy the statement that "their electrical failures are virtually impossible...". I deliberately left off the qualifier "when used as specified" because that could be said about any of the other suppression methods as well.
Either form of diode suppression
results in a diode continuously "bucking" battery voltage when the switch is turned on. A standard free-wheeling diode (something like a 1N4007) has a voltage rating of maybe 1000 volts, so it will not conduct under most any voltage transient seen in an aircraft, which could be up to 150 volts. On the other hand, a zener is designed to conduct in the reverse direction at a given voltage. These are like most back-to-back zeners in that they are bidirectional, having the same voltage rating in each direction. The "Snap Jack" supplied, I presume, for a 14-volt system is the P6KE18CA which, if I read the data sheet correctly, will start to conduct at 18 volts (1mA) and will be conducting 24 Amps at 25 Volts. Sorry, but I would not use a device with that little overhead. Instead I would pick one with a rating of at least 35 and maybe more like 60 volts. Remember, this device is connected directly to the battery when
the switch is turned on, so it will try to absorb any voltage transient in the system. Essentially it is protecting the whole electrical system. I'm sure that's why Eric picks a device with such a high power rating, far and away higher than is required to absorb the transient from a single solenoid coil(the rated peak power dissipation is 600 watts for 1msec, 100 watts for 10msec).
Certainly overrating the device will reduce the probability of failure, but it could make the failure more serious. It could, if shorted, conduct enough current to to either melt the switch wire or open any protection device upstream (CB). In that case the system would be rendered inoperative. The tradeoff then is an unlikely serious failure mode vs a more likely benign failure mode. In the pass car business we strive for the unlikely failure mode, but the typical aircraft design practice is to accept a higher
potential failure RATE in favor of the benign failure MODE (system keeps working). A complete electrical failure in a car, even though it stops the engine dead, is far less serious than a complete electrical failure in an aircraft even though the engine keeps running.
Sorry about the long post and, again, I'm sure that Eric sells a reliable, high quality product. For me, I'll use no suppression at all, thank you. What would be my failure mode? The master might not shut off if the switch contacts weld. Then I would have to disconnect the battery or let the battery go dead.
Gary
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.