1 [[!meta title="SeparatePyroBattery"]]
3 # Using a Separate Pyro Battery with Altus Metrum Flight Computers #
6 [elsewhere](http://gag.com/bdale/blog/posts/Batteries_and_Pyro_Circuits.html),
7 the most reliable solution for pyro control circuits in large and complex
8 rocket projects is to use a separate battery to provide the current
9 used for actually igniting the charges from the one used to power the
10 electronics. This document explains how to do this with various
11 [Altus Metrum](http://altusmetrum.org) flight computer models.
13 ## Pyro Circuit Background ##
15 All Altus Metrum flight computers use low-side FET switches to control pyro
16 events. What this means is that the pyro circuit provides the equivalent of
17 a switch to ground on one pin of each pyro circuit, and a source of power on
18 the other. The continuity sense circuitry is on the switch side of the pyro
19 terminals, and was designed to cope with any pyro voltage through nominal
20 12 volts (15 volts DC maximum). Using a separate pyro battery is therefore
21 as simple as just ignoring the terminal that provides a source of voltage and
22 only connecting the side that provides the switch to ground.
24 The negative lead of your pyro battery needs to be connected to ground on
25 the flight computer. The plating around the mounting screw holes is one
26 good place to find ground. The negative terminal of the LiPo battery
27 connector is also grounded. And on TeleMetrum boards there's an explicit
28 ground connection hole you can solder a wire to.
30 The positive lead of your pyro battery needs to connect to one side of each
31 e-match, with the other side of the match connecting to the appropriate
32 pyro terminal on the flight computer. This obviously means you need to make
33 connections off-board. Whether you choose to do this with a screw terminal
34 strip, crimp connectors, or by soldering wires together is of course entirely
37 ## TeleMetrum, TeleMini, and EasyMini ##
39 Very early [TeleMetrum](http://altusmetrum.org/TeleMetrum) boards had an
40 unpopulated connector footprint on the board
41 for a separate pyro battery, but due to a wiring error in the design of the
42 board, that connector was never useful. Just ignore it.
44 Each of these products can be used with a separate pyro battery, but unlike
45 TeleMega and EasyMega, there are no extra screw terminals to make it easy.
47 On TeleMetrum, if you look at the bottom of the board where all the
48 surface-mounted components are, on the end where the pyro screw terminals
49 are located, you can see a
50 fat trace connecting three of the screw terminals together. This is the
51 source of pyro power, which comes from the LiPo battery through the power
52 switch and then to one side of each pyro channel. The pins that trace
53 connects are therefore the ones you want to ignore. The others are the
56 Here's a photo of the pyro end of a TeleMetrum board augmented to show what
59 <img src="TeleMetrumSepPyro.jpg"/>
61 [TeleMini](http://altusmetrum.org/TeleMini)
63 [EasyMini](http://altusmetrum.org/EasyMini)
64 work the same way electrically, but the screw terminal physical
65 arrangements are different. The two pyro channels have a pin each that
66 are connected together and to the pyro voltage. Ignore those, the other
67 two screws are the switches to ground.
69 Here's a photo of the pyro end of a TeleMini board augmented to show what
72 <img src="TeleMiniSepPyro.jpg"/>
74 And here's a photo of an EasyMini board augmented to show what
77 <img src="EasyMiniSepPyro.jpg"/>
79 ## TeleMega and EasyMega ##
81 Because [TeleMega](http://altusmetrum.org/TeleMega) and
82 [EasyMega](http://altusmetrum.org/EasyMega) were designed to
83 be especially well suited for use in large
84 and complex projects, they include extra screw terminals on board to make use
85 of a separate pyro battery easy. In fact, use of a TeleMega or EasyMega
87 battery requires putting a jumper between two adjacent screw terminals to
88 connect the main LiPo battery to the pyro power rail. All such boards
89 ship with this jumper installed from the factory.
91 To use a separate pyro battery, just remove the jumper and connect the
92 pyro battery to the two adjacent screw terminals marked 'gnd' and 'pyro',
93 using the 'gnd' terminal for the negative battery lead and the 'pyro'
94 terminal for the positive side. Connect the e-match leads to the pyro screw
95 terminals as usual... no off-board connections are required.
97 Note that when using a separate pyro battery in this way, only the "main"
98 battery will be charged by the on-bard LiPo charger. You will need to make
99 arrangements to charge the pyro battery if it's a rechargeable variety.
101 Here's a photo showing a TeleMega board wired this way:
103 <img src="TeleMegaSepPyro.jpg"/>