Raven 2 battery

[Feckless Counsel]

I plan to power a Raven 2 system from two lithium oxide power cells.

Peak voltage will be 8.2V. Minimum battery impedance is 16mR. I'll be using igniters at 800mR minimum resistance. Wiring and contact system measures approximately 15mR. Raven's FET is 20mR.

Total system impedance equals 851mR. Divided into 8.2V gives 9.63 ampere.

I'll be applying that current for 1 second. Fusing power here is 92.74 amp-squared seconds. Raven's datasheet suggests fusing tolerance of about 9A over 4 seconds or 324 amp-squared seconds.

My application is only 29% of the maximum, correct?

[Adrian A]

Wow, 16 mOhms for the battery is really low. This system will probably work, but it's risky. Consider what happens if you get a short in your ignitor as a result of the firing (not uncommon). The current would be 8.2/(51 mOhms) = 160 Amps! That would toast the output for sure. Worse, when a FET fails from overcurrent it typically fails closed, so that could short out your your altimeter for the rest of the flight, preventing further deployments.

A 9V battery has about 2 Ohms of internal resistance, so it only puts out 4-5 Amps, even in a short. The small 130mAhr batteries I recommend have about 0.5 Ohms internal resistance, so they put out about 8-9 Amps in a short circuit.

By the way, the Raven's 3rd and 4th channels have lower-resistance (larger and more expensive) FETs which can handle more than 9 Amps. So those are the best channels to use if you are going to be igniting a motor, since motor ignitors often require more current and provide more opportunities for shorting.

[Feckless Counsel]

Adrian,

Thanks for your immediate response.

My employer manufactures cordless power tools. We have lots of interesting cell samples here. Many afford high gravimetric energy density and super low impedance. Great for tool's breakthrough torque. Maybe not so great for electronics in sport rocketry.

May I ask you share the manufacturer's datasheet for Raven's output devices?

Feckless Counsel

[Adrian A]

Channels 3-4
http://www.toshiba.com/taec/componen...//71/12354.pdf

Apo and main:
http://www.fairchildsemi.com/ds/FD%2FFDMA410NZ.pdf

[Feckless Counsel]

Adrian,

Thanks. I'll reconsider my choice of battery.

Feckless Counsel

[goffa]

By the way, the Raven's 3rd and 4th channels have lower-resistance (larger and more expensive) FETs which can handle more than 9 Amps.

Adrian, the link to the data sheets for outputs 3,4 is broken.
do you know how many amps they can handle?
as per my post on your featherweight forum, wondering if they could handle the 16A shorting current of my lipo7.4 for 1sec.

[Adrian A]

I don't have the spec sheet handy, but I recall the FETs used in channels 3 and 4 have a 1-second rating around 10-12 Amps. However, I have used them in an air start application where the in-flight measurement was 16 Amps on accident, and they were ok. I think running them that hard would be an unnecessary risk that I don't recommend. It might work anyway.