Qualifying 1/2 inch Press In Diodes 3

[JSUN]

Our company has a test spec for qualifying diodes (in alternators) in the aircraft industry. This spec incorporates the more common tests i.e forward voltage test, current leak test, rockwell hardness test for the case, break test for the leads. But we have one test that I do not understand why it is in the spec and I wanted to see if anyone else would know.

The spec requires a sample of diodes to be cooled to 30-40 F for 20 minutes. Once cooled, they are to be completely submerged into water at 180 F. As the diodes are placed into the hot water, there should not be any bubbles/leaks noticed coming between the seal and case.

This is testing the seal properties, but is this test really necessary. Many of the diodes are failing because of this. Please let me know if there are other test that could qualify the seal or if it is even necessary.

Thanks,
Jason

 

[edison123]

May be it is required for the air craft industry where safety & reliability come first ?

 

[itsmoked]

You need to continue with that test. It is an important one. Aircraft can cruise at high altitudes where things are very cold. Then they rapidly descend into warm air. Same as your test but while in operation.

If you have problems with this diode you should get a field engineer in from the diode maker and discuss it with them.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[roydm]

If many diode fail the test it seems important. If they bubble it means they suck in moisture as well. Sounds bad!
Roy

 

[edison123]

itsmoked has nailed it.

 

[waross]

I'm with itsmoked. I would see the issue as not a problem with the test but a problem with the diodes. This seems to be a pretty easy test.
In service, any air or gas in the internal cavities of a diode case on the ground will be at ambient temperature and pressure. As the plane climbs, the pressure difference will rise faster than the temperature difference of the trapped gas. If there is a leak, some of the internal gas will be forced out. As the diode case cools, some air will be drawn in to equalize the pressure. When the plane descends the pressure difference will force more air into the case. On the ground, the case will warm up and expel some of the trapped air. If the case is leaking, it will pump air at every change in altitude. I am surprised that the cases are not completely filled with some potting compound for aircraft service.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[JSUN]

Thank you guys for your helpful information. The diodes are completely filled with potting compound.

Does anyone know of a standard Mil/SAE spec for testing the potting/sealing of a diode?

The test that we are performing is straightforward, but I just wanted to know if there may be other standard tests that could be done as well.

Thanks,
Jason

 

[roydm]

This seems like a reasonable test, why would you want to try and by-pass it.
I have seen a similar problem with pH probes the potting compound looks good but is rigid allowing moisture to leak in and short the probe out.
Roy

 

[JSUN]

roydm,
I'm not bypassing the test. I just wanted to know if there was a standard (MIL) for testing the seal of the potting compund. I have looked in a few specs but cannot find anything. If there is no standard, we will continue with our test criteria.

-Jason