The International Cessna 170 Association

Karl,

I don't really know how to answer that, as in 30+ years in the field I've never seen one fail - and I've used a bunch of them in just this application of spike suppression in relay circuits. They are pretty darn reliable if properly specified in the first place. Any switching diode with a Peak Inverse Voltage rating of 600V or more, and a power rating of 1 watt or more, should be fairly bulletproof in this application. I'd worry more about the reliability of the connections than the diode itself.

Just guessing, but I suspect that if a diode does fail it would go to a resistance somewhere between open and closed circuit. In this application, if the resistance is high enough that the wattage rating is not exceeded (about 200 ohms in a 14V system for a 1 watt rating), you'd probably never notice unless you checked it. If the resistance is low enough to draw enough current to significantly exceed the wattage rating, the diode would eventually melt or burn up and cause an open circuit.

Miles

I've seen the results of many failed diodes, from small reverse-current devices to larger rectifier diodes and when they've failed..... they've ALL failed to an open circuit.

I love it when someone understands the importance of Peak Inverse voltage, especially if you live in a dry climate.

For those who questioned the legality of using this new "can" style of relay...Cessna SB65-89 approved the use of the new style relay to replace the older "box" style, despite the fact they failed to revise the parts catalogs to reflect that change. The current PN is 111-140D and will cost you $48 from Cessna.

Here's a Cessna illustration of the diode and relay which should clear up a lot of these questions:
I am informed that it's important to use a SILICON diode....not a germanium diode...and that one rated at 1/2 amp and 100 volts are larger is fine. (Why silicon and not germanium? Probably for the same reason that Ol'Gar likes single-malt but not the single-malt Islay Scotch that I tried to give him....something about having his taste-buds castrated while he was in Saudi.)

The difference between a batt master relay and a starter relay is significant and one should not be substituted for the other. The Batt relay is continuous duty, and while it can support all the aircraft electrical loads (including that imposed by the starter) it is not designed to open/close WHILE the large demand is commanded. It is designed to open/close with small or no electrical demands being present....yet then allow higher demands to pass-thru. The starter contactor (relay) is NOT continuous and is designed to be able to withstand opening/closing with high-current demands already present and yet avoid excessive erosion/arcing of the contacts. However a starter relay will overheat in continuous service due to it's higher-capacity activation-coil.

gahorn wrote:I am informed that it's important to use a SILICON diode....not a germanium diode...and that one rated at 1/2 amp and 100 volts are larger is fine.

In doing some reading online it seems that the current (no pun intended) method of handling these voltage spikes is with a transient voltage suppressor rather than a regular silicon diode because they handle the problem of contact arcing/welding better. Here's some light bedtime reading for you electrical engineers: Tyco 1and Tyco 2. Miles, if you were to use one of these which one would you use? If it's the appropriate size, the price is right on this one from Jameco.
Edit: Aeroelectric article on the subject.

n2582d wrote:

gahorn wrote:I am informed that it's important to use a SILICON diode....not a germanium diode...and that one rated at 1/2 amp and 100 volts are larger is fine.

In doing some reading online it seems that the current (no pun intended) method of handling these voltage spikes is with a transient voltage suppressor rather than a regular silicon diode because they handle the problem of contact arcing/welding better. Here's some light bedtime reading for you electrical engineers: Tyco 1and Tyco 2. Miles, if you were to use one of these which one would you use? If it's the appropriate size, the price is right on this one from Jameco.

Miles do you have an opinion on the merits of a "Transient Voltage Suppressor" compared to a "Silicon Diode"?

I've not really studied up on transient voltage supressors very much, but it seems to me that a good silicon diode (very low $$$) can handle just about anything a 14vdc electrical system could throw at it. My airplane (55 years and counting) and avioncs (18 years and counting) have been doing fine without either. I've been meaning to install diodes on the master and starter contactors (yes, I gave up that leaky old pull-start rig for lent ) "just in case" but haven't gotten around to it.

Miles

Miles,
Thanks for the reply. My concern is two fold first I am reading and hearing about cases where Solenoid contacts are welding themselves shut (I have added a warning light near my starter switch to alert me if my starter stays engaged), second after reading the "tyco1 and tyco2" links in Gary's post I am concerned that by adding Diodes we may actually be the complicit in our solenoids welding their contact shut due to the mentioned bounce. Thanks for giving your insights into this issue.

John and Gary,

I apologize for not taking the time to read and comment on the Tyco 1 and Tyco 2 links earlier. I had never considered the effect of transient suppression on the dynamics of the relay armature. I suspect that this would be more of a problem with starter contactors than master contactors, as master contactor loads are significantly lower. I agree with the following quote from Tyco 2 as a "best of both worlds" solution: The use of a reversed-biased rectifier diode in series with a zener diode will provide the best solution when the relay can be polarized. This suppression is often recommended by Siemens Electromechanical Components (SEC) for use in automotive circuits. The impact on release dynamics is minimal and poses no loss of reliability. For the purpose being discussed, our aircraft starter circuits are essentially the same as automotive, and should benefit similarly.

Although the data sheet for the Vishay suppressors Gary referenced don't specifically say so, I suspect they function similarly to the switching diode+zener combination in that they allow some reverse voltage to be generated across the coil to aid in contact opening speed. On the data sheet, I'm assuming that the "Breakdown Voltage" specification is the reverse voltage when the device just starts to conduct, and that the "Maximum Clamping Voltage" specification is the maximum reverse voltage the device will allow on de-energizing the coil, I would choose one with a Maximum Clamping Voltage in the same range (or somewhat lower) as the "AC" voltage rating on the switch you're using to activate the starter, probably somewhere around 115 volts. Although not critical, I'd probably choose a bi-directional model to preclude installation errors.

This one Gary found from Jameco has a Maximum Clamping Voltage of 548 volts, and wouldn't be much better than nothing in our application. For a first try, I'd go with 1.5KE75C or 1.5KE75CA (maximum clamping voltage just over 100V).

Hope this helps.

Miles

I ended up buying these from Mouser Electronics.

Gary,

You should be good to go with that one. From what I read in Tyco's application notes, you should get good dropout times on your contactors, much better than clamping the reverse voltage at 1V. And, you certainly can't complain about the price!

Miles

cessna170bdriver wrote:... For the purpose being discussed, our aircraft starter circuits are essentially the same as automotive, and should benefit similarly.....

I'll just add the (probably obvious) that the original, pull-starter does not use a relay, so this discussion would not apply to that starter circuit.

Also, the circuits utilized in our airplanes do not incorporate the fragile "solid state" switching that is being discussed in the Tyco document.


Yes, Gary, those should work just fine, as would any other source of silicon rectifier diode.

Something worth keeping in-mind about this subject:
The Tyco documents are basically concerned with protecting solid-state components which switch the relay on/off. Unless modified, our airplanes have simple toggle or slide-switches which are not susceptible to damage from un-suppressed relays/switches. But keep reading!

Also, the size of these relays (battery/starter relays and the circuits such as auxilliary land/taxi light system which use those same relays) are sufficiently robust as to not need suppression at all. (So I do not personally have any qualms about whether or not simple diodes might increase the failure-rate of relays which already have an incredibly low rate of failure.)

The concern which provoked Cessna to issue SNL 62-8 incorporating diodes was not switch or relay longevity or damage..... it was possible damage to modern avionics which might have been installed and which might be connected to the electrical buss when the relays were deactivated.

Oh, you have a radio-master switch that does not use a relay and all your radios are always disconnected from the buss by that? You are still not immune from the problem.

Perfect Example: You now have a solid-state generator regulator? Every time you switch your cockpit master switch to the off position you are probably slapping your regulator across the face! Same thing can occur with solid state alternators and their regulators and overvoltage devices.

When these airplanes were designed and built they had vacuum-tube radios and did not have electrical accy's such as flap motors, etc., which create the large transient voltages observed when the relays open. Since delicate, solid-state avionics have replaced those older radios, and since virtually all modern Cessnas have electric flap motors, elect-motor-driven hydraulics, etc. etc., it became necessary to protect those delicate avionics (and other control circuits such as solid-state panel lighting, etc.) from the transient voltages.

What I am suggesting is that the diodes recommended by Cessna, silicon, reverse-current/rectifier diodes are fully adequate to accomplish the task of protecting those items. The use of "transient voltage suppressor" diodes, Metal-Oxide-Resistors, and other such devices are fine for the purpose also and they may offer the additional benefit of being kind to the relays themselves.

But be aware that those other devices do not provide the ulitmate protection for the avionics/devices which Cessna intended to protect in the SNL. The second Tyco document seems to agree with that conclusion in the table at the lower-left, which demonstrates a common silicon diode keeps the transient voltages at the lowest level (.7), and which is the same conclusion reached by Mike Mladejovsky, PhD EE Mechanical Engineering Dept., Univ of Utah in his work on this same issue and reported on by the Cessna 120/140 association.

gahorn wrote: The Tyco documents are basically concerned with protecting solid-state components which switch the relay on/off. Unless modified, our airplanes have simple toggle or slide-switches which are not susceptible to damage from un-suppressed relays/switches.

The way I read the documents, at least Tyco2, they do recognize the need to protect solid state switches moreso than mechanical switches, but the main concern of the article was that a plain switching diode used as a suppressor alters the release dynamics of the contactor (slows it down), contributing to the welding of the contactor contacts (see the section entitled Effects of Coil Suppression on Relay Dynamics and Life). The devices shown in the datasheet Gary referenced, actually allow some reverse voltage (the clamping voltage specification) to allow the contactor to behave closer to it's non-suppressed release speed, reducing the duration of the arc, lessening damage to the contacts. I did mention that starter contactors (for those of us that have them) are more susceptible to this sort of damage than master contactors due to the higer currents being switched. Those with pull starters don't need to be concerned.

While most mechanical switches are much more robust that most solid state switches, even the mechanical types have limits. For example, if releasing a starter contactor coil generates 400V unsupressed, and the control switch is only rated for 115V, that control switch life will be shortened (a typical solid state switch might not last through the first cycle). One of these days , I'd like to borrow a good oscilliscope from work and take some real data.

Miles

Boy can you learn alot just lurking here.

Or get even further confused!!
Scott --- 53B