RCBrust
Well-Known Member
I decided to take a look at firing currents for ejection charge initiators and thought you guys might be interested in the results. A lot of flyers, including myself, often use 2, 3, 4 amps or more to fire these things. If you look at the specs for MJG Firewires, they recommend a nominal firing current of 1A. The Chinese initiators available on eBay list a firing current of 0.4A. With that in mind, I ran some tests to see how the initiators perform at around 1 amp. I started with the eBay initiators but I have Firewires on order and will report my findings with those when they arrive. My 3 goals for the tests were:
1. Use the altimeter, battery and initiators within their recommended specs (or close to it)
2. Have a configuration where a single altimeter pyro output could drive multiple initiators
3. Be able to have one (or more) of the initiators fail open or fail short and not have any affect on the others
To that end I decided to use a 2s LiPo battery and a 5 ohm resistor in series with each initiator. A fully charged 2S LiPo has a voltage of about 8.4V at room temp and the initiators have a resistance of about 0.8 to 1.7 ohms, depending on the brand. Max current per initiator should be about 8.4 / (5 + 0.8) or 1.45A. To guarantee a minimum of 1A, the battery voltage would need to be at least 1 * (5 + 1.7) or 6.7V. Even at a temp of 0C, a decent LiPo battery should be able to deliver at least 75% of its capacity before dropping to that voltage.
The battery that I used was a Turnigy 300mA 2S LiPo. It provides a constant discharge of 10.5A and a burst discharge of 21A so current is not an issue.
To simulate non-ideal conditions, I fully charged the battery and then discharged 150mAh from it so it had about half of its capacity left. I then began firing off the eBay initiators in series with a 5 ohm 1% power resistor and monitoring the current with a digital scope. The first 10 were done at room temp (~17C or 63F) and the current pulses ranged from 10.5ms to 11.8ms with amplitudes of about 1.25A before the initiator fused open.
I then cooled the whole setup (including the battery) to about 0C (32F) and fired off 10 more. The current pulses ranged from 13.3ms to 16.8ms with amplitudes around 1.1A.
Considering that many of the popular altimeters fire their pyro outputs for 1 second and can source around 5A, I'd have no problem using up to 3 of these initiator/resistor combinations on a single pyro output while staying well within the capability of the battery and the altimeter, and still firing within the first 2% of the 1 second window.
Here's a sample of what the current looked like during a firing:
I was actually a little surprised at how clean the current trace was. I was expecting it to be noisier. It should be noted that even with a 16ms wide pulse of current, a single firing pulls about (1250mA x 0.016s) / (3600s/hr) or 0.006mAh out of the battery, i.e. the 300mAh battery can do this quite a few times.
Randy
1. Use the altimeter, battery and initiators within their recommended specs (or close to it)
2. Have a configuration where a single altimeter pyro output could drive multiple initiators
3. Be able to have one (or more) of the initiators fail open or fail short and not have any affect on the others
To that end I decided to use a 2s LiPo battery and a 5 ohm resistor in series with each initiator. A fully charged 2S LiPo has a voltage of about 8.4V at room temp and the initiators have a resistance of about 0.8 to 1.7 ohms, depending on the brand. Max current per initiator should be about 8.4 / (5 + 0.8) or 1.45A. To guarantee a minimum of 1A, the battery voltage would need to be at least 1 * (5 + 1.7) or 6.7V. Even at a temp of 0C, a decent LiPo battery should be able to deliver at least 75% of its capacity before dropping to that voltage.
The battery that I used was a Turnigy 300mA 2S LiPo. It provides a constant discharge of 10.5A and a burst discharge of 21A so current is not an issue.
To simulate non-ideal conditions, I fully charged the battery and then discharged 150mAh from it so it had about half of its capacity left. I then began firing off the eBay initiators in series with a 5 ohm 1% power resistor and monitoring the current with a digital scope. The first 10 were done at room temp (~17C or 63F) and the current pulses ranged from 10.5ms to 11.8ms with amplitudes of about 1.25A before the initiator fused open.
I then cooled the whole setup (including the battery) to about 0C (32F) and fired off 10 more. The current pulses ranged from 13.3ms to 16.8ms with amplitudes around 1.1A.
Considering that many of the popular altimeters fire their pyro outputs for 1 second and can source around 5A, I'd have no problem using up to 3 of these initiator/resistor combinations on a single pyro output while staying well within the capability of the battery and the altimeter, and still firing within the first 2% of the 1 second window.
Here's a sample of what the current looked like during a firing:
I was actually a little surprised at how clean the current trace was. I was expecting it to be noisier. It should be noted that even with a 16ms wide pulse of current, a single firing pulls about (1250mA x 0.016s) / (3600s/hr) or 0.006mAh out of the battery, i.e. the 300mAh battery can do this quite a few times.
Randy