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High altitude - near space use of mosfets and IGBTs.

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Carbon_Rod

Mon Sept 19 2016, 02:01AM

Registered Member #65 Joined: Thu Feb 09 2006, 06:43AM
Location:
Posts: 1155

Patrick wrote ...

Elaborate please i dont understand.

Foil Heating Films come with adhesive backings to glue down to cases, batteries, and PCBs.
Note 24v 30W heaters are around $10 on ebay...

These heaters are absolutely required around batteries, but only require a sealed mechanical temperature switch to operate.

1. the mass of the equipment itself will minimize temperature fluctuation if you drive above ambient.

2. a 42'C atmosphere will have better battery performance, optics will not fog, and water condensation is mitigated.
Note, lithium batteries rapidly fail outside SOA temperatures, and I suspect this is a problem with your set-up.

3. Under severe conditions an over-temperature 60'C cut-out should be placed in series with a mechanical thermostat. A small ceramic capacitor should also be placed across the switches to reduce possible EMI. This set-up also allows recovery of dormant electronics from low temperature standby where semiconductors don't work, solar flare latch-ups, or acts of clod.

Most importantly, copper foil in plastic doesnâ€™t weigh very much...
Hydrogen has better lift, and is cheaper...

Patrick

Mon Sept 19 2016, 02:56AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Carbon_Rod wrote ...

Hydrogen has better lift, and is cheaper...

I keep advocating for hydrogen, but they just wont do it.

Cold batteries. . .

I suspect the reason they cant get the ignitors to send the rockoons up is precisely this this reason. I think their batteries are cold, and only half heatedly go off. which isnt enough heat to get hobby motors to set off.

They do use BIG ones though.

two huge race car batteries, they're on top of each other in parallel.

Carbon_Rod

Mon Sept 19 2016, 03:35AM

Registered Member #65 Joined: Thu Feb 09 2006, 06:43AM
Location:
Posts: 1155

Patrick wrote ...

suspect the reason they cant get the ignitors to send the rockoons up is precisely this this reason.

I gather you meant to launch raccoons into LEO, but a bit of potassium chlorate with sugar may be needed as a starter if thereâ€™s low O2.

Helium is getting recycled on some campuses because of its increasing supply instability.

Patrick

Mon Sept 19 2016, 06:54AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Carbon_Rod wrote ...

Patrick wrote ...

suspect the reason they cant get the ignitors to send the rockoons up is precisely this this reason.

I gather you meant to launch raccoons into LEO, but a bit of potassium chlorate with sugar may be needed as a starter if thereâ€™s low O2.

Helium is getting recycled on some campuses because of its increasing supply instability.

Well right now we'd like to reach 100,000 ft with the balloon then launch to 300,000 or so, then come back by some means, probably streamer.

and yes helium is becoming a huge cost concern. i metered out $3000 myself before they found a piece of tape missing. but it wasnt on the check list and they didnt tell us about it. About $1000 was lost in 20 minutes.

Hydron

Mon Sept 19 2016, 10:17AM

Registered Member #30656 Joined: Tue Jul 30 2013, 02:40AM
Location: UK
Posts: 208

Helium should cost a lot! - it's utterly un-renewable (is a by-product of gas production) and we'll be in big trouble when we run out (e.g. MRI machines use a lot of it).
We can probably find or make alternatives for fossil fuels, but creating an element in large quantities is still science fiction.

The US has done the world a huge disservice by selling down their strategic reserve at a fixed rate, pushing the price down and encouraging people to waste it (e.g. party balloons, not bothering re-capturing it when practical due to lack of cost-effectiveness).

Use of helium should really be limited to productive purposes only (sounds like this project is one!), and re-captured when at all possible. Think twice before buying your next party balloon!

/rant

Blackcurrant

Mon Sept 19 2016, 03:22PM

Registered Member #2989 Joined: Sun Jul 11 2010, 12:01AM
Location: UK
Posts: 94

Here's some mad ramblings may give you an idea
It seems odd that 1 Bar difference would cause reliability issues.It'll be interesting if you manage to find the reason behind this.
Maybe if you could find some of these failing igbt/mosfet parts you could maybe test them in a vacuum chamber find out how they fail. Say build the same circuit x times and run them until something stops working.
Rad-Hard components? maybe worth looking in to. A build up of rad damage over time?
Transistor DC gain can change quite a bit with temperature so if a design doesn't take this in to account it could be something as simple as not turning on, if you check over an old design this may have gone unnoticed.
Cold lithium batteries not giving out current leading to brown outs, PC +usb current monitor + fridge
Some electrolyte in capacitors can stop working at lower temperatures also could a vacuum maybe boil it off?
thermal expansion making your PCB go like a non eu banana?
I guess they use a lot of relays for power saving? or does someone have a large box of them that need using :)

Patrick

Mon Sept 19 2016, 07:42PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Blackcurrant wrote ...

I guess they use a lot of relays for power saving? or does someone have a large box of them that need using :)

Mostly they get them as old auto parts from the junk yard. Which i dont like the idea of.

i want to conduct at least three tests, each with 5 transistors blinking 5 lights.
1) control, just normal like here on the forum.
2) same as above but in vacuum vessel.
3) Vacuum vessel, dry ice and forceful gate drive.

Then ill try to reproduce their 75% failure rate, by duplicating their previous work.

BigBad

Thu Oct 06 2016, 01:40AM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
Location:
Posts: 600

One thought, you have to watch out for arcing at high altitude.

At 100,000 feet the air is no longer such a good insulator.

If it is that, potting everything is a pain, and adds weight, but should fix it.

johnf

Thu Oct 06 2016, 06:09AM

Registered Member #230 Joined: Tue Feb 21 2006, 08:01PM
Location: Gracefield lower Hutt
Posts: 284

What a lot of drivel. We use lots of electronics in vacuum at work. a few provisos
no electrolytics use tantalum or solid aluminium
Minimise plastic components ie use surface mount no plastic caps ie ceramic npo etc
We are using this stuff inside a linear accelerator where the ionising radiation flux is extreme CCd cameras turn into a technicolour snow storm in short order through Neutron damage. But a few micros and logic chips have survived for the 12 years I've been there and these are ordinary PICs or 8051 4000 series and 74LS series type beasts.
The dry ice will only get you to -70 degrees celsius I would use liquid nitrogen which is nearer the temp at max altitude you are / want to use

Sulaiman

Thu Oct 06 2016, 11:51AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

I suspect that temperature is not the problem due to the short flight time and thermal inertia,
easy to check ... record temperature in electronics bays vs. time or altitude
have the temp. sensor(s) in good thermal contact with hardware to prevent self-heating.

I also doubt that near vacuum would be a problem causing mechanical failure of plastic encapsulated silicon ... silicon works just fine in a vacuum
but ... there will be negligible convection cooling.
Build a small vacuum chamber to run your electronics in and use an ir thermometer or thermal camera to check this possibility, whilst monitoring telemetry.

I also think that since the electronics are much warmer than the air, condensation of atmospheic water is unlikely, so maybe a flush of the electronics bays with dry gas prior to launch will help ?

electrical breakdown of the air is a suspect, if >= 300 V is involved
see

especially from Conclusions / Validity onwards

ionising radiation would, I guess, turn devices on unexpectedly, not off,
have a look at circuitry that could cause failure if a device is wrongly turned on.

(all guesswork, no experience)

D'oh ... I think that an ir thermometer or camera will not work through glass .....

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