This page explains what CubeSat burn wires are, some common methods and materials to implement them, and then how to use the PyCubed burn wire circuits and helper function.

What is A Burn Wire?

The concept of a burn wire (as a deployment mechanism) is simple:

PyCubed Burn Wires

PyCubed mainboard has two independent burn wire circuits:

Nichrome wire is commonly used as a heating element (in many applications) since it has a high melting point and low oxidation rate. The PyCubed burn wire scheme is designed to use a piece of nichrome wire (or other resistive heating element) to complete the circuit between the burn wire output (BURN1 or BURN2) and ground.

Common Pitfalls of Nichrome Wire Burn Wires

1. Inconsistent burn times between boards
   • Cause & Solution

     Nichrome wire does NOT solder well. This results in inconsistent implementations of the nichrome wire loop for the burn wire which will have different resistances and therefore heating rates.

     Use the most aggressive flux you have when soldering the nichrome wire to a PCB. Also make sure to weave the wire through a few holes on the PCB prior to soldering to achieve better mechanical support.

2. Nichrome wire breaking
   • Cause & Solution

     The nichrome wire gets very hot during a burn. Any oils from your hands or other contaminates can react with the nichrome causing pitting, cracks, and eventually failure. Also take care not to "kink" the wire or make a sharp bend in the exposed loop.

3. Power system fails when attempting to burn
   • Cause & Solution

     This nichrome burn wire scheme is essentially just short-circuiting the battery.

     Most battery packs will engage their short-circuit protection if current is drawn this quickly. In the case of the PyCubed software, we turn the burn wire circuit on and off very quickly (PWM) as a way of regulating the current below the short-circuit threshold of common battery packs (PyCubed Battery included).

Material Choices

Nichrome Wire

We've successfully used TEMCo's RW0341 for a number of missions. This particular product is 30 AWG "Nichrome 80."

Choosing the appropriate nichrome wire diameter is a balance between:

1. being thick enough to mechanically serve as a tie down for the deployer

2. but thicker wire will have less resistance and therefore require a longer burn time. You want to minimize burn time since other aspects of the burn circuit (cables, transistors, etc...) are heating up during the burn as well.

Fishing Line

Lots of different kinds of finishing line work for burn wire tie downs. One brand we've used before is "FireLine."

• 20lb (9kg) worked well for our antenna deployment

• this particular product is stranded and made with "Dyneema"

• it's important to avoid a fishing line that melts and oozes around the nichrome wire.

Best thing to do is buy an assortment of finish line and try them.

Using the pycubed.py burn wire methods

The pycubed helper library (⭐pycubed.py) has a reliable and fault-tolerant method for operating the burn wire circuits. Note that enabling the burn wire circuits wont do anything unless a nichrome wire has been installed into the circuit as discussed above.

cubesat.burn() theory of operation

As we learned from the overview diagram above, PyCubed has two burn wire circuits that are downstream from a protection relay. At a high level, cubesat.burn() needs to do the following:

Now we will discuss how each of those steps are performed in more detail

1. open the relay

We use a general purpose input/output (aka GPIO) from the SAMD microcontroller to control the relay enable pin. As an extra layer of fault protection, the default state of this GPIO pin uses a "Drive Mode" called OPEN_DRAIN meaning even if a radiation-induced bit flip were to set the GPIO value to "high" it would not be able to open the relay.

self._relayA.drive_mode=digitalio.DriveMode.PUSH_PULL
self._relayA.value = 1

2. enable the burn wire circuit for a certain amount of time

Enabling the burn wire circuit isn't as straight forward as the relay because we need to pulse-width module (PWM) the transistor that controls the burn wire circuit in order to regulate the amount of current allowed through the circuit.

We configure the appropriate burn wire control pin for PWM with the following:

if '1' in burn_num:
    burnwire = pwmio.PWMOut(board.BURN1, frequency=freq, duty_cycle=0)
elif '2' in burn_num:
    burnwire = pwmio.PWMOut(board.BURN2, frequency=freq, duty_cycle=0)

We pause for a brief moment to ensure the relay is open before setting the duty cycle above 0% with the following:

burnwire.duty_cycle=dtycycl

With the duty cycle now set above 0%, all we have to do is wait for the desired duration:

time.sleep(duration)

3. disable the burn wire circuit

Now all we have to do is clean up.

Start by setting the PWM duty cycle back to 0%

burnwire.duty_cycle=0

Next, we no longer need to keep the PWM object we created, so we de-initialize it:

burnwire.deinit()

Finally, we configure the GPIO pin controlling the relay back to it's OPEN_DRAIN configuration:

self._relayA.drive_mode=digitalio.DriveMode.OPEN_DRAIN

Using cubesat.burn()

Now we understand what the burn() method is doing, let's review how to use it.

cubesat.burn(burn_num, dutycycle, freq) method takes the following arguments:

Initial burn testing

As the many caution statement above have said, we need to be careful when using the burn wire circuit because INHERENTLY it is capable of permanent damage to the burn wire circuits.

Begin with the following values and test using a complete burn wire + fishing line setup in order to dial in your burn wire configuration. SLOWLY increase duty cycle by 0.05 increments and retest.

burn_num  = '1'  # or '2', doesn't matter
dutycycle = 0.05 # START WITH THIS 
freq      = 1000 # don't change this unless you become really familiar
duration  = 1    # avg optimal time to burn your fishing line and double it