ProFET smart switch

Basil · 2008-08-25 01:56

Hi All,

Some of you may recall I have been developing a rocket altimeter (for along time now

)

I am working through a few changes to the hardware and this nifty little device was brought to my attention on another forum.

BTS5246-2L

Has anyone here used something like this? Im thinking of using it to replace the pyro circuitry (see attached schematic). Any comments?

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-Alec

My our page

QuattroRS4 · 2008-08-25 09:00

In BMW's (ca. e39 5 Series) containing an LCM (Light Control Module) there was a pretty common·tendancy (fault) for one headlight to stay on even with the lights turned off and key removed from the ignition. This was caused by a short circuit in one or more High Side Power Switch device in the LCM·- namely a·BTS426-L1 - an earlier version of what you have mentioned. An Easy Repair but ...in your scenario how would a short circuit on the output stage affect you ?

That is the down side. - On the Up side -·I have used High Side Switches in many applications without issue - these have survived short circuit and over current conditions without issue - but I felt it necessary to inform you of a scenario where I have seen these to short the output stage (as mentioned).

Hope this is of some help.

Regards,
John Twomey

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'Necessity is the mother of invention'

Post Edited (QuattroRS4) : 8/25/2008 9:07:29 AM GMT

Basil · 2008-08-25 10:03

A short circuit would fire the pyro charges and possibly eject a parachute while the rocket is still traveling at 800kmph+...not good [noparse]:D[/noparse]

I assume there's a good chance the L2 version has improved on this fault?
I work with industrial products like you (you do aye? I work for an imported/distributor in NZ) and know the requirment for relaible product. If you have had no troubles then thats enough for me [noparse]:)[/noparse]

QUESTION: Which methods did you use to help improve safety?

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-Alec

My our page

QuattroRS4 · 2008-08-25 10:53

These items are ideally suited to replace switches,relays and fuses ! - Due to the fact that the current limit can be set - If exceeded the monitoring feature shows a fault ... They have integrated reverse biased diodes - to protect against back emf while switching inductive loads .. all sounds great .. and it is. Damage usually occurs when Vmax has been exceeded in the output stage .. There is an inbuilt protection and is·probably not likely in your scenario but perhaps possible .. If it was me .. 1) I would still fuse the output stage 2)Ensure that the switching signal current cannot be exceeded via resistor 3) ensure that the Switching signal is normally pulled low - to ensure it is not nuisance triggered. 4)Loss of device ground can be an issue with these devices - most obvious in automotive applications even though the device is set to stay in the off state in such cases..

Specs state: 'Protection functions are neither designed for continuous nor repetitive operation.'

In general I have found these devices to be spot on .. but Like I outlined I have seen failures in automotive applications.
This particular component seems to have improved protection .. and·current limits can be set -·which is perhaps why the versions I mentioned are no longer available !

Which is worse it triggers during launch and mangles the 'chute then falls like a stone·or it fails to trigger and falls like a stone ? Obviously worst case scenarios ..I notice you chose a two O/P model - You could use second stage as an aux. parachute ? - Of course provided that a fault condition would not put the·whole device in thermal shutdown .... Ok so use two 'single output' devices - 1 for main second for Aux.

Finally - I don't believe you will have an issue but worth throwing out the Pros and cons ..

Let me know how tests go ..

Regards,
John Twomey

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'Necessity is the mother of invention'

Post Edited (QuattroRS4) : 8/25/2008 11:08:07 AM GMT

Basil · 2008-08-25 21:05

Hi [noparse]:)[/noparse]

Thanks for those comments, that helps alot!

Most of those scenarios are unlikely to happen on my device for a few reasons:
a) The whole thing is running on a 9v battery so Vmax is not likely to be exceeded :P
b) The switching current will be directly from the prop

Most high powered rockets (those this will most likely be used in) have dual chutes, a drouge at apogee and a larger one a bit closer to the earth.

I plan on using the 2 output version for this functionality, and if I can squeeze on 2x 2 output IC's I will for redundancy[noparse]:)[/noparse] (The 4 output runs on too higher voltage so I cant use it)

In addition to this, most people who are really keen on saftey will use 2 altimeters of different types so if there is a fault in one it wont be repeated in the other.

One last question sorry.
In my deployment circuit, youll notice I have some outputs to and ADC. This is so I can check volt drop across the pyro outputs to check continutity. I check volt drop rather than just straight continututy because carbon from the pyro's firing can foul up the terminal blocks connecting the pyro's and give false readings.

I haven't fully got my head around the diagnosis outputs, but it looks as though this test is achievable with this device also. Would that be correct?

Thanks,

Alec

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-Alec

My our page

Sniper King · 2008-08-25 23:11

Boy, i would be wary of making a change like that, Failure is going to catastrophic!

Tried and true may be the way you wan to go.· This switch i have used often to control up to 4 amps.

2n7000 and an IRF5305 work pretty good together.

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·- Ouch, thats not suppose to be hot!··

Michael King
Application Engineer
R&D
Digital Technology Group

Basil · 2008-08-25 23:28

Hi [noparse]:)[/noparse]

my device has not yet been tested even with the current system of FET's.
The whole concept of using FET's though is well accepted in other altimeters, hence the reason for all my questions.

I guess there is always a chance of a mis-fire, even with FET's. Its just a matter of weighing up pro's and con's [noparse]:)[/noparse]

Thanks for your comments,

Alec

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-Alec

My our page

sylvie369 · 2008-08-26 02:13

Basil said...
Hi [noparse]:)[/noparse]

my device has not yet been tested even with the current system of FET's.
The whole concept of using FET's though is well accepted in other altimeters, hence the reason for all my questions.

I guess there is always a chance of a mis-fire, even with FET's. Its just a matter of weighing up pro's and con's [noparse]:)[/noparse]

Thanks for your comments,

Alec

I expected to see the words "test, test, and test again" in here somewhere. Any high power deployment system should be ground tested until it's clearly reliable, and then flown with a backup (a commercially-available altimeter) for its first flights.

Basil · 2008-08-26 02:21

Hi [noparse]:)[/noparse]

Thats DEFINATLY going to happen [noparse]:)[/noparse]

At first I wasn't going to go commercial with this, but have had alot of interest. I wouldn't feel comfirtable selling a product like this which wasn't tested extremley well!

Thankfully, there are some people at the local club who have offered to fly my altimeter in an upcoming launch, with mine piggybacking a proven commercial product.

Mine will be functioning fully just not connected to the pyro's themselves [noparse]:)[/noparse]

The prototype I have is using FET's and will be tested soon. I am thinking for the future with this post as I would like to reduce board complexity and size.

Thanks,

Alec

EDIT: Oh, ground tests are working swell [noparse];)[/noparse]

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-Alec

My our page

sylvie369 · 2008-08-26 03:15

Basil said...

Mine will be functioning fully just not connected to the pyro's themselves [noparse]:)[/noparse]

You can't use redundant charges? That's the best way to test, as long as there's a way to keep them from firing at the same time.

I flew a Missileworks Wireless Remote Control that I wasn't that sure of, so I used a PerfectFlite MAWD as backup. Both units had full charges attached.

Alternatively, at least hook up e-matches to the pyro output of your unit, so you can see that they fired during flight (even if you don't have charges on them).

Basil · 2008-08-26 03:27

Oh, thats what I meant. pyro's but no pyro charges. Sorry, quite an important word that...'charges' :P

I will need to test the power circuitry to be sure it doesn't brown out.
I have dual 9v battery inputs and a 1000uf cap so it should provide enough energy, but as you say, test test test [noparse]:)[/noparse]

Im also attempting to implement some sort of recovery algorithims in the code, so if for whatever reason there is a brownout (say someone flying with 1 battery and the cap not being able to provide the juice), the prop will recover and continue logging from where it left off, albeit with a short gap in the data.

Thats another reason im keen to know if there is a way to test for the reset method.
If it was USB, then run serial comm's 'mode' upon startup.
If it was brownout, then shortcut the prelaunch code and go straight to inflight
If it was a poweron, then boot as normal.

Any ideas?

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-Alec

My our page

ProFET smart switch

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