how big of a capacitor to ignite a small igniter

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kencraw

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I'm making my own little accelerometer based delay timer board ("little" being the operative word) to make a 24mm 2-stage composite motor rocket. Weight is at a premium in my design, so I'm trying to use as small of a battery as possible. If necessary, I can probably get away with a very small ( 2-cell?) lipo battery. However, the idea I'm considering right now is a coin battery connected to a pump-up DC-DC converter connected to a capacitor. Thus I could take a 1V low-current battery and turn it into a 12V high-current discharge. The question is, how big of a capacitor would I need?

Anyone have any guidance on this? (Or do I need to go buy a bunch of igniters and do my own tests?)

As an fallback, anyone know if a single cell (3.7V) lipo battery will ignite an igniter, or do I need to go to a 2-cell 7.4V lipo?
 
As an fallback, anyone know if a single cell (3.7V) lipo battery will ignite an igniter
Certainly, the Raven+Power Perch does this with a very small lipo (its large capacitor is only used to hold the electronics voltage up, not to supply energy to the igniter.)

This is a much easier solution than using a cap IMHO, as it doesn't require you to characterize the igniter behavior and that can be very complex.
 
Or do I need to go buy a bunch of igniters and do my own tests?

As an fallback, anyone know if a single cell (3.7V) lipo battery will ignite an igniter

I have fired my igniters on a single cell 3.7V LiPO without any problems, but that doesn't mean all types would do so (BTW I have used a new AA alkaline battery to fire my igniters as well). I would definitely get a bunch of the type you plan on using and do ground testing until you are comfortable with the success rate before flying with it. Do new testing with any change of components as well. It is difficult to anticipate all of the unintended consequences when a seemingly small or simple change is made to a design.
 
That's correct Rich, this is to ignite the sustainer motor of a 2-stage rocket.

Go with a Cesaroni upper stage motor, and you can use their supplied low-current e-match. The CTI motor light instantly almost every time, which will ensure the your upper stage lights exactly when you want it and with very little current/voltage needs. Little 1s Lipo's work great, and I bet you could even go smaller than that.
 
That's correct Rich, this is to ignite the sustainer motor of a 2-stage rocket.

My apologies, I read that in your original post, my brain is a little fried by a full day of class on Hazardous Material Awareness Training for Transportation, a dull class when you have spent the last 15 years moving and removing hazardous and radioactive waste, along with hazardous materials.
 
I'm making my own little accelerometer based delay timer board ("little" being the operative word) to make a 24mm 2-stage composite motor rocket. Weight is at a premium in my design, so I'm trying to use as small of a battery as possible. If necessary, I can probably get away with a very small ( 2-cell?) lipo battery. However, the idea I'm considering right now is a coin battery connected to a pump-up DC-DC converter connected to a capacitor. Thus I could take a 1V low-current battery and turn it into a 12V high-current discharge. The question is, how big of a capacitor would I need?

Anyone have any guidance on this? (Or do I need to go buy a bunch of igniters and do my own tests?)

As an fallback, anyone know if a single cell (3.7V) lipo battery will ignite an igniter, or do I need to go to a 2-cell 7.4V lipo?

For the capacitor idea it depends on the igniter. MJG JTeks need 1.25A for 0.5s to be safe. Nominal res 1.2ohm Energy needed = 1.25*1.2/.5 =3 Joules

Capacitor stored energy = 1/2CV^2 C = 6/144 =0.0416F or about 41000uf. (That's a big cap....) Defeats the small battery concept....
 
You will find a LiPo battery has lower weight, higher capacity and smaller size than a capacitor.......

A single LiPo should be sufficient if you use a FET....

Bob
 
This little battery (9V A10) and capacitor (on the Perfectflite MAWD altimeter) will fire any low current ematch.

image02.small.jpg
 
For the capacitor idea it depends on the igniter. MJG JTeks need 1.25A for 0.5s to be safe. Nominal res 1.2ohm Energy needed = 1.25*1.2/.5 =3 Joules

Capacitor stored energy = 1/2CV^2 C = 6/144 =0.0416F or about 41000uf. (That's a big cap....) Defeats the small battery concept....

Thanks for the example. You bring up a good point: I should probably inquire with the igniter manufacturer to see what they recommend current-wise.

You're also probably right that the cap may end up being bigger than the battery, particularly since your equation assume perfect energy transfer. The smallest LiPo I've found is a 110 mAH 2.65g one. That's small enough I should probably just leave well enough alone.

Thanks everyone for their feedback/input!
 
I made an error in that joule calculation (it was late)

E = W x S = 1.25^2*1.2*0.5 = 1.16J, so the cap required would be around 16000uf. Still a big cap.
 
This little battery (9V A10) and capacitor (on the Perfectflite MAWD altimeter) will fire any low current ematch.

View attachment 278659
Take a look at your sled. Do you see the size of your capacitor and your battery? A 1S LiPo is the size of the white switch on your altimeter can replace both of them.

A 1S LiPo has significantly higher capacity and current delivery capability than any alkaline coin cell battery, and delivers more energy and power than the capacitor on the MAWD. This is the reason why the current generation of hobby rocket altimeters do not use capacitor discharge pyro circuits.

The StratoLogger CF is the current replacement for this unit. The SLCF weighs 0.38 oz versus the MAWD weight of 0.7 oz. and the list price is just 55% the MAWD list price with significantly enhanced capability. The GP23AE 12 volt dog collar batteries sold by PF (but not recommended for the MAWD) have a capacity of ~50 mah and will source less than 50 ma. A 200 mah 1S LiPo in the current SL CF altimeter will weigh less and source more than 5 amps for about the cost of the 12 V alkaline .

The MAWD is still a good reliable altimeter (but I wouldn't use it with a coin cell battery), but todays PF units are smaller, lighter, and far more capable for half the price.

Bob
 
Take a look at your sled. Do you see the size of your capacitor and your battery? A 1S LiPo is the size of the white switch on your altimeter can replace both of them.

A 1S LiPo has significantly higher capacity and current delivery capability than any alkaline coin cell battery, and delivers more energy and power than the capacitor on the MAWD. This is the reason why the current generation of hobby rocket altimeters do not use capacitor discharge pyro circuits.

The StratoLogger CF is the current replacement for this unit. The SLCF weighs 0.38 oz versus the MAWD weight of 0.7 oz. and the list price is just 55% the MAWD list price with significantly enhanced capability. The GP23AE 12 volt dog collar batteries sold by PF (but not recommended for the MAWD) have a capacity of ~50 mah and will source less than 50 ma. A 200 mah 1S LiPo in the current SL CF altimeter will weigh less and source more than 5 amps for about the cost of the 12 V alkaline .

The MAWD is still a good reliable altimeter (but I wouldn't use it with a coin cell battery), but todays PF units are smaller, lighter, and far more capable for half the price.

Bob

For sure. I was merely answering the OP's question about capacitors.

I have 3 MAWDs, several av-bays to accommodate them, and a bunch of the A10 batteries. This setup is over 10 years old, but still works great for me. I haven't had the need to upgrade my electronics. Yet.
 
I'm making my own little accelerometer based delay timer board ("little" being the operative word) to make a 24mm 2-stage composite motor rocket. Weight is at a premium in my design, so I'm trying to use as small of a battery as possible. If necessary, I can probably get away with a very small ( 2-cell?) lipo battery. However, the idea I'm considering right now is a coin battery connected to a pump-up DC-DC converter connected to a capacitor. Thus I could take a 1V low-current battery and turn it into a 12V high-current discharge. The question is, how big of a capacitor would I need?

Anyone have any guidance on this? (Or do I need to go buy a bunch of igniters and do my own tests?)

As an fallback, anyone know if a single cell (3.7V) lipo battery will ignite an igniter, or do I need to go to a 2-cell 7.4V lipo?
Here's the MAWD manual. https://www.perfectflite.com/Downloads/miniAltWD manual.pdf Goto page 22 for the specs.

The MAWD is designed to operate at a nominal 9 volts. The capacitor stores 190 mJ and can deliver 27 amps peak current into a short.

E = 0.5CV^2 so the capacitor is C = 2E/V^2 = 2x0.19/9^2=0.38/81= 0.0047 F = 4700 uF. If you go to a lower voltage you need a bigger (larger capacitor) which is heavier so this is the reason why all CD systems use higher voltage. The advantage of voltage is high current. I=V/R. For any resistance, the current drive is proportional to voltage. Since a CD ignition system has a high voltage, small changes in the igniter resistance make little difference.

The big negative of a CD system is the limited energy stored in the system. If the 190 mJ stored in the capacitor can not fire the pyro, it will take several seconds to recharge the capacitor. That's too late in a rocket.

A LiPo is a lightweight battery with a very high capacitance and low internal resistance so it can serve as both a high capacity battery and a high current source for short periods of time. The best of both worlds.

Bob
 
A 200 mah 1S LiPo in the current SL CF altimeter will weigh less and source more than 5 amps for about the cost of the 12 V alkaline .
Of course, the SL CF isn't speced to operate below 4V, and the voltage of a 1S lipo goes below that after about 20% of total capacity is used, so you may want to be careful about your state of charge.

I'm not sure what voltage regulator PF is using, you can certainly get regulators that have smaller dropout voltages, but they may not allow for the max voltage PF does.
 
The Stratologger and the Pnut are essentially the same altimeter with the addition of 2 pyrocircuits in the SL. The Pnut is delivered with a 1S LiPo as the power source and others have used a 1S LiPo with the SL. I don't see anything in the manual that says it won't run on a 1S LiPo.

Bob
 
I wouldn't trust a capacitor to fire an igniter (something with a heavier bridgewire requiring more current than a standard ematch). Go straight to the igniter with the battery, just about any 1S LiPo is going to be able to supply enough current to light it for the few seconds that you need, and make sure your FETs (or BJTs if you go that way) are up to the task.
 
I don't see anything in the manual that says it won't run on a 1S LiPo.
Page 10: "The altimeter’s electronics can be powered by any source of 4 volts to 16 volts." If it said 3.5V then a 1S lipo would be OK at any reasonable state of charge. I looked at an SL CF under a microscope but couldn't identify the voltage regulator being used. Perfectflite is on record as saying they don't recommend a 1S lipo, by implication (see
https://www.rocketryforum.com/showthread.php?127978-StratologgerCF-Battery-Options post #3) All that said, it'll probably work, but as a professional EE I am paranoid about operating outside datasheet specs.
 
Page 10: "The altimeter’s electronics can be powered by any source of 4 volts to 16 volts." If it said 3.5V then a 1S lipo would be OK at any reasonable state of charge. I looked at an SL CF under a microscope but couldn't identify the voltage regulator being used. Perfectflite is on record as saying they don't recommend a 1S lipo, by implication (see
https://www.rocketryforum.com/showthread.php?127978-StratologgerCF-Battery-Options post #3) All that said, it'll probably work, but as a professional EE I am paranoid about operating outside datasheet specs.
From the same thread please read https://www.rocketryforum.com/showt...oggerCF-Battery-Options&p=1488032#post1488032 that shows from multiple flights a 1SLiPo works fine.

The above post used 2 e-matches in parallel and the OP here wants to use a small igniter so his setup should be ground tested several times to make sure it works. A low current igniter is in order and not all igniters are low current. Ohms law always applies so R = V/I = 4/5 = 0.8 ohms so any igniter with a resistance not less than 0.8 ohms should work just fine.

Bob
 
Take a look at your sled. Do you see the size of your capacitor and your battery? A 1S LiPo is the size of the white switch on your altimeter can replace both of them.
Bob

I get the point, but you exaggerate a little bit. For sake of accuracy, the switch bank on the MAWD measures 17x7x4 mm. The 1S LiPo is a fair bit larger.
 
E = W x S = 1.25^2*1.2*0.5 = 1.16J, so the cap required would be around 16000uf. Still a big cap.

As a counter example, the igniters that I use, standard pyrotechnics igniters according to German Type-A specs (the most sensitive ones), can be fired reliably with 470µF @ 5V. A tiny 100µF/16V capacitor can store enough energy to fire two of them.

In other words: Choose wisely and test, test, test. ;)

Reinhard
 
As a counter example, the igniters that I use, standard pyrotechnics igniters according to German Type-A specs (the most sensitive ones), can be fired reliably with 470µF @ 5V. A tiny 100µF/16V capacitor can store enough energy to fire two of them.

In other words: Choose wisely and test, test, test. ;)



Reinhard


Wow those are really sensitive. I'd be scared of those...... That's flash bulb area.

My calculation is conservative, I assumed 0.5sec to fire, if the matches heat up sooner then the capacitor can be smaller. A test I did along time ago showed that 5ms at 8 amps will fire a JTek. So thats about 0.25 joules
 
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Wow those are really sensitive. I'd be scared of those...... That's flash bulb area.

The Daveyfire N28B was the standard for a while and it was sensitive. I believe its all fire energy was around 4mJ. The 2200uF capacitor on an RDAS Compact charged to 9V has about 90mJ so they work well together.

If you want really safe, the NASA standard igniter is required to do nothing when exposed to 1 Watt of power. For a long time.
 
The Daveyfire N28B was the standard for a while and it was sensitive. I believe its all fire energy was around 4mJ. The 2200uF capacitor on an RDAS Compact charged to 9V has about 90mJ so they work well together.

If you want really safe, the NASA standard igniter is required to do nothing when exposed to 1 Watt of power. For a long time.

Hi David, the NASA standard igniter reference is interesting - thanks! Is that for the Shuttle SRBs or something from Apollo?
 
Hi David, the NASA standard igniter reference is interesting - thanks! Is that for the Shuttle SRBs or something from Apollo?

A NASA standard igniter (or perhaps it was initiator, ahh, yes it was) is just the first step in an SRB igniter. They get used everywhere.
 
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