Rookie here. I’m installing a IXYS DSS2x61-0045A schottky diode in my experimental airplane project electrical system. I would like to use both diodes in parallel (same power source to same buss). Looking at the tech data sheet, the numbering of the screw attach tabs don’t match what I show on the diode I have. I want to make sure I will run wires to the correct tabs so that both diodes will be used.
Thank you.
Picture attached
Hello shawntedwards,
Welcome to the DigiKey TechForum.
If your diode is the exact same part number as DSS2X61-0045A, I can’t explain why your diode is numbered differently. According to the datasheet, the anodes are on the same side, and numbered 2 and 3, so that’s how it should be wired. You should do some testing, or run this in a test circuit to verify the correct polarity of each schottky diode, before wiring it in your final application.
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Thanks David. I have verified the part number and they match the tech data sheet. I purchased two of these diodes at the same time. The numbering is the same on both. I will need to determine how to test the diodes I have. Any chance to check the diodes in stock to verify the numbering while I figure out how to test the ones I have? Attached pic of diode part#.
Thanks
Hello shawntedwards,
Welcome to the forum.
Not recommended.
Diodes have a temperature coefficient that will cause increased conduction with increased temperature. With parallel diodes, you run the risk that one diode will hog the current causing a runaway spiral where more current leads to greater heating which leads to more current and so on.
Alternatives from a strictly electric perspective:
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Locate a larger single diode than can handle the current.
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Place low value resistors in series with each diode. This will tend to equalize the current it will also provide a level of shock resistance.
Recommend you consult FAA rules and inspectors to determine applicability as I am not qualified to make recommendations for product flight suitability.
Sorry I couldn’t provide additional assistance.
Best Wishes,
APDahlen
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Thanks for your help David. I’m guessing the 1 and 2 positions are mis-marked on the two diodes I have. This is for a small plane electrical system so I would prefer to have the parts properly marked for possible future maintenance. If there are properly labeled parts in stock, I’d prefer to return the ones I have for those.
Shawn
Thanks for info AP.
I wasn’t sure if using the diodes in parallel was accepted practice or not. My installation should see around 20amps max, probably around 10amps continuous. I will also have the diode mounted on a 15w heatsink.
I will look into a larger, single diode if that would be better for my application. I will also look at the resisters in series.
Thanks again,
Shawn
Shawn,
It’s difficult to say.
Personally, I don’t look at typical situations. Instead, I always attempt to envision the worst case. For example, consider the craft operating is in the heat of an Arizona summer after getting delayed on the taxiway for half an hour. We could fry an egg on the avionics. Things get even more problematic when we consider some future modification pulls more buss power than expected.
As for aviation, the closest I’ve gotten is the wire diagram for a Piper Aztec.
Perhaps someone with more expertise will add additional comments.
Sincerely,
Aaron
It’s not obvious where the alternative numbering comes from, but the datasheet representation is generally correct. In any case, the diode test function available on most multimeters should let a person remove doubt in a few seconds.
That would seem reasonable, considering that the diode could be expected to burn on the order of 12W under the stated maximum conditions. It should be understood however, that heat sink ratings are (necessarily) based on a lot of stated or implied assumptions that may or may not align with one’s particular circumstances. This post may offer insights on the topic.
That’d be light work for one of the diodes in the package individually, so the question of whether or not two in parallel would share equitably seems irrelevant in this case.
It may be of benefit to zoom out a bit and consider the context of how & why you’re thinking of using the device in the first place. After all, it doesn’t really matter how well one carries out the details if the overall plan is ill-fated…
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Thanks for all the info Rick. All good information for me to consider.
I did end up using my meter to check the diode and it was mis-marked. Looking back, as a few posters pointed out, I
shouldn’t rely on the markings anyway. I posted some pics of my test just to verify my process.
As for the context of how the diode will be used. It will between the main power bus and avionics/ essential bus of the primary power system of my experimental plane project. The relay in the picture is the avionics relay. During engine start most avionics will be kept offline via the avionics switch/relay.
The schottky diode comes into play when the avionics/essential bus is used as an essential bus when load shed may be required…e.g.Pri Alt failure. It will prevent back-feeding of items on the main power bus.
I will attach a PDF of my primary power system plan for reference. Please feel free to comment/critique. Like I said, I’m a rookie at this stuff and enjoy learning from the comments.
Thanks again. Shawn
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Here is PDF of my primary power system showing how Schottky Diode will “tentatively” be used. Red color shown on diagram is for shop use, to indicate wires have been run.
Primary Power.pdf (204.3 KB)
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Mmmkay… Subject to the terms and conditions (particularly the part that says free advice is sometimes worth what one pays for it) I’d offer the following thoughts/observations/suggestions.
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I wouldn’t take issue with using the diode block in the described manner if the current is maintained at the <20A levels described. Do the math and read the fine print on the thermal management situation however; performance under natural convection can vary significantly depending on mounting orientation, for example.
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Looks like that distribution network might involve a fair length of wire, potentially subject to abrasion and faults to ground. One may wish to consider a bus master fuse to offer protection against such.
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Is the diode really necessary? Since the avionics bus relay is powered from the “normal” bus that’s disconnected when the bat/esntl switch is in the latter position, it would appear not to be, at least under static conditions. Electrons are faster than relay armatures however, so there may be something of a race condition present between opening of the battery contactor and closure of essential bus relay if the bat/esntl switch is thrown rapidly. And if the “standby bat” represents energy storage on that bus, it would tend to increase the chance of the race ending unfavorably.
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Assuming the bat/esn switch is a center-off type, that’d maybe help the situation above somewhat. If it’s not a center-off type, at least one relay will be powered at all times and the battery will be stone dead after a week of disuse. Probably not a situation one would care for.
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The avncs master switch seems to be functionally bypassed by the bat/esntl switch. Is it really a “master” switch in that case, and could the fact perhaps be exploited to aid in dispensing with the diode?
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Maybe silly to ask, but how “non-essential” is the “fuel pump” thingy?
Thanks Rick, for the thorough review of my diagram with some interesting points. Especially regarding removing the diode completely. I have worked on and off this wire plan for over a year. Seems like at some point I considered removing the diode. For some reason I stuck with it but there may have been other considerations with the diagram revision at that time. The Batt/Off/Esntl switch is a three-position switch with off in the center position. The primary reason for the avionics/essential bus is to be able to secure certain avionics during engine start via the avionics switch, and “possible” load shedding if required when placing the Bat/Off/Esntl switch to Esntl. That situation could (not likely) occur during failure of the Pri Alt and operating on the Stby Alt. Basically, I need the avionics bus relay to remain open when avionics/essential bus is being powered from the battery only so the main power bus is not using power. I’m currently on the road for work and have limited time to review the various scenarios that need consideration before removing the diode.
I will definitely give it some more though.
Thanks again!
Shawn