# What's the maximum current a pyro channel can handle?

Older Altus Metrum products used Vishay Si7232DN FETs as switches.  They are 
rated for 25A continuous, 40A pulsed.  Newer products use Taiwan Semiconductor
TSM200N03D FETs which are rated for 20A continuous, 80A pulsed.  In pyro use, 
we're closer to the pulsed rating than continuous.  However...

Please note that Ohms law applies.  Whatever your battery voltage is and
whatever your e-match or other device's resistance are will limit the
actual deliverable current.  With a single-cell LiPo battery, the fully
charged voltage is 4.2 volts, and a feature of LiPo batteries is that
they have very low source resistance, so we can mostly ignore it.  A
typical e-match has 1.0-1.7 ohms depending on the wire length.  The FET
on resistance is less than 20 milliOhms, so we can basically ignore
that, too.  Only the resistance through wiring and the match matters.
So, by Ohms law, E = IR, I = E/R or 4.2/1, meaning the most current you
can deliver to the pyro element is 4.2 A.

A typical commercial e-match fires in about 13 microseconds under these
conditions, which is why we think 50 milliseconds is .. a very long time!

You'd think moving to a 9V alkaline would allow you to deliver 9A, but
it doesn't really work that way.  Unlike LiPo batteries, Alkaline cells
have a very high source resistance and their current producing ability
is nowhere near linear. 

If you're using a programmable pyro channel on Altus Metrum products that 
include that capability (TeleMega, EasyMega, EasyTimer) to do something like 
turn on other electronics, there is an additional consideration.  The screw 
terminal strips we use can handle large peak currents, but are only rated 
for a maximum of 6A continuous current per screw.  This means the best plan 
if you need to switch something power hungry is to include a high-current 
FET switch in your electronics, and use our FET to switch just enough current 
to control it.