Topics

Cutdown device?


Larry
 

I am making progress on the "Life in the Stratosphere" experiment.  I am not sure I am convinced but we have  decided "life" might be falling off the balloon. So we don't want to start collecting  until we are free from the balloon.  How do you make a cutdown device?  You wrap the line from the parachute to the balloon in nichrome wire?  Just a couple of wraps?  How much power should I have  in my power budget for this?  Any idea how long this is going to take to burn the payload free?

Larry
KJ6PBS


Chief Surfer
 

Hello Larry!

I invite you to review a cut down I made here.

Joe


Michael Hojnowski
 

Hey Joe,

I'm planning to make a quite similar setup myself.  Do you happen to know the current draw you get when that nichrome is engaged?   I'm trying to see if I can fire it with a FET and maybe run both the microcontroller I'll be using AND the nichrome from the same power source to save weight/volume on the cutdown device.  Knowing the current draw would help me understand whether I can ride out the "brown-out" when the voltage gets pulled down, or whether I might have to supplement with some caps/diodes or something to protect it.

Thanks for any info you might share,
Mike

On 11/21/2020 10:31 AM, Chief Surfer via groups.io wrote:
Hello Larry!

I invite you to review a cut down I made here.

Joe


Chief Surfer
 

Hello Mike!

Using a FET would be pretty cool!

I set up my cutdown for the current measurement.  I used my DMM on the 20 A scale.  When I made the connection, the DMM spiked around 5 A.  After a several seconds, it settled in around 4.25 A.

The cutdown power supply for the nichrome is three Energizer Ultimate Lithium AA batteries.  The energized length of nichrome is about 3/4 inch.  I used 26 AWG nichrome 80.

Joe


Bill Brown
 

Hi Mike,

  I designed a cutdown many years ago that had a CR123 battery.  I supported the flight train through some ring lugs to keep it from moving (knotted the line where it went through the ring lugs.  I used a MOSFET to actuate the nichrome wire...a few inches coiled tightly around the flight line.  It flew above the parachute and had a small 434 MHz RF receiver which was activated by turning on the 434 MHz transmitter in the main payoad with a coded serial string.  The cutdown box weighed about 3 ounces and just spun around the parachute after cutting down the balloon.

- Bill WB8ELK




-----Original Message-----
From: Michael Hojnowski <kd2eat@...>
To: GPSL@groups.io
Sent: Sat, Nov 21, 2020 7:52 pm
Subject: Re: [GPSL] Cutdown device?

Hey Joe,

I'm planning to make a quite similar setup myself.  Do you happen to know the current draw you get when that nichrome is engaged?   I'm trying to see if I can fire it with a FET and maybe run both the microcontroller I'll be using AND the nichrome from the same power source to save weight/volume on the cutdown device.  Knowing the current draw would help me understand whether I can ride out the "brown-out" when the voltage gets pulled down, or whether I might have to supplement with some caps/diodes or something to protect it.

Thanks for any info you might share,
Mike

On 11/21/2020 10:31 AM, Chief Surfer via groups.io wrote:
Hello Larry!

I invite you to review a cut down I made here.

Joe


Michael Hojnowski
 

Joe,

That's awesome!  Thanks so much for making those measurements!  That'll really help with my planning.

Thanks again!
Mike / KD2EAT

On 11/24/2020 11:49 AM, Chief Surfer via groups.io wrote:
Hello Mike!

Using a FET would be pretty cool!

I set up my cutdown for the current measurement.  I used my DMM on the 20 A scale.  When I made the connection, the DMM spiked around 5 A.  After a several seconds, it settled in around 4.25 A.

The cutdown power supply for the nichrome is three Energizer Ultimate Lithium AA batteries.  The energized length of nichrome is about 3/4 inch.  I used 26 AWG nichrome 80.

Joe


Michael Hojnowski
 

Bill,

Ya, I plan a similar setup.  I want to make a tiny 3D printed box that the balloon line passes through between parachute and balloon.  The box will route the string just like Joe's setup.  It will have a small ESP32 board with bluetooth which can fire the nichrome.  The main payload will have an ESP32 bluetooth/LoRa  flight computer that can decide when to initiate cutdown.  I can either stick an accelerometer in there or use altitude or whatever.  I may put a second cutdown box between the chute and payload box so that if I get stuck in a tree, I have the option of abandoning the chute and cutting away.

I've fallen in love with the ESP32 boards.  They're SUPER cheap ($5 for the cutdown board I plan to use) and have a ton of functionality, and they're just as easy to program as arduino.  I'll probably be switching over to them for most of my applications.

So many projects, so little time!
Mike / KD2EAT

On 11/24/2020 12:17 PM, Bill Brown via groups.io wrote:
Hi Mike,

  I designed a cutdown many years ago that had a CR123 battery.  I supported the flight train through some ring lugs to keep it from moving (knotted the line where it went through the ring lugs.  I used a MOSFET to actuate the nichrome wire...a few inches coiled tightly around the flight line.  It flew above the parachute and had a small 434 MHz RF receiver which was activated by turning on the 434 MHz transmitter in the main payoad with a coded serial string.  The cutdown box weighed about 3 ounces and just spun around the parachute after cutting down the balloon.

- Bill WB8ELK




-----Original Message-----
From: Michael Hojnowski <kd2eat@...>
To: GPSL@groups.io
Sent: Sat, Nov 21, 2020 7:52 pm
Subject: Re: [GPSL] Cutdown device?

Hey Joe,

I'm planning to make a quite similar setup myself.  Do you happen to know the current draw you get when that nichrome is engaged?   I'm trying to see if I can fire it with a FET and maybe run both the microcontroller I'll be using AND the nichrome from the same power source to save weight/volume on the cutdown device.  Knowing the current draw would help me understand whether I can ride out the "brown-out" when the voltage gets pulled down, or whether I might have to supplement with some caps/diodes or something to protect it.

Thanks for any info you might share,
Mike

On 11/21/2020 10:31 AM, Chief Surfer via groups.io wrote:
Hello Larry!

I invite you to review a cut down I made here.

Joe