Do note that the maker of the radiomatch system intended it for use with firing
relatively low current e-matches and squibs. It comes with a 9 volt battery
type connector that clips to a holder for eight AA alkaline batteries. That's
nice, but most of the motors I want to use with this system can't even use an
e-match. It does work with large batteries (like Gel cells and car batteries),
but they do recommend adding an inline fuse holder and 5 amp fuse. Makes sense.
I'm going to be adding a fuse to the battery leads on my system.
I won't go into the full instructions. But basically you press one button to
arm the system, then you have about 5 seconds to press one of the other two
buttons to fire pad 1 or pad 2. I don't think I'll be using the 2-pad
capability much but it's there if the opportunities arise.
As I do with ANY relay type system (even if this is radio-activated), I added a
little "black box" to the ignition outputs. The black box has a buzzer that
sounds whenever firing current is sent thru the ignition leads (have to be sure
the polarity is right, or use two buzzers wired parallel in opposite polarity
to assure one sounds). So if the relay system ever failed-ON, I'd be able to
hear it (I've seen two HPR models accidentally launched due to relay systems
Also the black box has a piezo beeper, SPDT switch, and 9 volt battery. The
SPDT switch is wired to disarm one ignition lead and check continuity to the
ignitor clips. Or when the switch is flipped the other way, to turn off the
continuity check and arm the ignition lead.
People might not want to bother with the whole little black box arm/continuity
thing for this or any relay type system. But I can't suggest strongly enough to
add a buzzer in parallel to the ignition leads so if it is firing accidentally
you can know it before hooking up clips. It's also very useful for ground
testing (without a live motor hooked up!) to get accustomed to using it and to
troubleshoot a misfire.
- George Gassaway
You could also use an auto-reset thermal breaker, the kind used in automotive
applications. They can be found at auto supply stores for a couple of bucks.
No need to replace the fuse or press a button when it trips. They have a
bi-metal strip internally that heats up at the rated current and separates the
contacts. It usually takes about 20 seconds to cool down to make contact again.
There are 20A thermal breakers in all the pad boxes in the MARS (NY) launch
system (as seen at the NYPOWER launches, NSL2000 and NARAM 43). 5 Amps should
be plenty for non-cluster protection.
I'm re-doing the design for the pad boxes and will have a website up with
the PC board layout, parts list, etc. The old version is documented in my
NARAM R&D report at http://web.syr.edu/~smdemar/rktreports.htm .
It sounds like the Radio-Match product could be used as is to drive the
coil of one of my relayer pad boxes without any additional circuitry.
The padbox design has a continuity tone that is unique for TEST, ARMED,
and FAILED RELAY. Plus the tone's pitch is higher for a good igniter, and
lower for a crappy igniter.
Of course I'm not talking about in-line fuses... these are thermal auto-reset
breakers. They take several seconds at the rated current to kick out. A five
amp rating is fine for this type of device with single igniters of all sorts.
However, I recommend at least 10A, preferable 20A if clusters will be used.
Limiting the battery is a very BAD IDEA. Smaller capacity batteries also have
larger internal resistance. They cannot deliver the instantaneous peak current
required to light many igniters. So, a hefty battery with a time-dependent
limiting device is much better.
The reasons for having the thermal breaker are:
1) They protect other components from heating up, fusing wires, welding relay
contacts, etc., when there is a dead short with some monkey's paw on the launch
2) They save battery life on the range when clips are shorted while the above
mentioned launch ape is waiting for a miracle after 30 seconds of button stomping.
3) It beats other types of over-current protection because it is slow and
resets itself (in about 20-30 seconds).
4) And, most importantly, this idea has been used now for 5 years and
thousands of flights on the MARS system.
5) The device is cheap and readily available.
Oh, and (6): because Bob Kaplow says you shouldn't!