[AR] Re: NO-fire, SURE-fire ignition standards ?

  • From: Peter Fairbrother <peter@xxxxxxxxxx>
  • To: arocket@xxxxxxxxxxxxx
  • Date: Wed, 6 Jan 2021 19:41:25 +0000

On 06/01/2021 12:17, JAMES ROSSON wrote:

Decades ago smallest on board electronics used A23 (12v 55mAh) battery with 
storage capacitor to fire upper stage igniters.

I haven't tried, but I'm fairly sure any of the >100mAh very small motor lipos will directly fire a modern 200mA no-fire ematch - they can be <6g. At say 200maH, they can be rated to 70C (14A!) discharge, though even the more normal 10C-20C models should do.

Something like the 502030 might be OK, though it isn't made for that job. You may have to remove the protective circuit board.

They only worked with low current (<50ma safe) e-match type, or photo flash 
bulb/thermite.  Internal resistance on stack of button cells inside A23 was too 
high to fire ~1.5-2 ohm low current initiator directly, but with capacitor storage 
they worked well with fresh battery.

Sadly low current e-matches types became hard to obtain by unlicensed rocket hobbyist in US after 9/11, due new DHS sales restrictions.

I had wondered why no-fire went from 50mA up to 200mA.

The small A23 battery became less popular in on-board electronic designs about same time.

Also perhaps because microprocessors started to need lower voltages. I'd probably go for a single lipo battery. I run arduino minis on them quite a lot, if you get the 3.3V versions and remove/bridge the voltage regulators they work fine.

If I were going ultra small on board electronics today, might use CR11108 (1/3 N) lithium cells with 3v and 160mAh? Three fit inside same "N" sized alkaline cell battery holder for 9v
equivalent, but less space. Key to success with any small batteries is capacitor storage to boost short circuit current the bridge wire creates. A fresh single 1/3N battery would probably fire low current e-match without capacitor if extra volume was crisis?

Max pulse current of Energiser 2l76 CR11108 is 80mA, so I doubt it. Small lipos have a much larger current delivery capacity than Li/Mn cells.

PS - if using any relays inside rocket, make sure they are vibration safe.  The 
cheap PC board 1A relays are not vibration safe and should not be used for 
firing on-board igniters.  Use solid state 1A switch, as it is cheaper/smaller 
option compared to never close due vibration relay. DAMHIK

Indeed. I am slowly building a new fireworks box and it will not have relays, the concussion is said to trip them.

Peter Fairbrother

Cheers!

On January 5, 2021 at 8:16 PM Peter Fairbrother <peter@xxxxxxxxxx> wrote:


On 05/01/2021 21:00, Dr Edward Jones wrote:
[...]
What's common practice with small battery ignition, even with a relay?

4 x AA, fresh Energiser or Duracell preferred.

Maybe 6 x AA if you are igniting up to four ematches at once over a
distance with diodes in circuit. Chap I know recommends 3+n AA, where n
is the number of ematches.

2x18650 is overkill if you want to go rechargeable.

This is mostly for firework displays where I'll fire maybe 30-100
ematches in 20 minutes.

Smaller cells will work too, in fact a single AA will fire an ematch. I
just tried. :)



Peter Fairbrother

My main box has 4xAA to run the arduino and 6xAA for the matches. 36
relays, the unpowered breakout boxes have 3A diodes for the matrix.
Maximum firing wires would be 50m long, though 20m is more usual max. I
seldom fire more than 2 ematches at once but it is built for firing up
to 4 (and I could easily add more AAs if needed).




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