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Ignition circuit, advise

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klugesmith

Sun May 06 2012, 05:36AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

ryanshow wrote ...
Harry: Yes, I require electronic ignition because I need to control exactly when the secondary stages ignite. I'm planning on having an accelerometer (and maybe a barometric altimeter) on-board which I'll use to tell when the flight has reached it's peak altitude so the second stage can ignite. I'm also going to record ignition times and such so I can create some fancy flight graphs.

I hope you realize that for maximum final altitude, you want to fire each stage as soon as the previous one has stopped thrusting. Adding delta-v's is much more efficient than addng delta-h's.

There is also a safety issue if you delay ignition of upper stages. You said your goal is to wait for the rocket to reach peak altitude from the previous stage's burn. Wait a second too long, and the rocket will have nosed over and be pointed toward the ground. If that is your nefarious intent, be sure to wear a hard hat. And consider simply using an ordinary delay-type rocket engine for the booster, perhaps experimenting with pyrotechnics to turn the ejection puff into a reliable ignition flame.

mister_rf

Sun May 06 2012, 11:20AM

Registered Member #4465 Joined: Wed Apr 18 2012, 08:37AM
Location: Bucharest, Romania
Posts: 145

Minor corrections to the charger/ignition circuit

ryanshow

Mon May 07 2012, 10:52AM

Registered Member #4720 Joined: Thu May 03 2012, 11:19PM
Location: Fresno, CA
Posts: 5

Thanks again for all the input everyone,

radiotech:
Yeah, I'm still considering the best approach for a safety mechanism. I'm thinking about adding on another relay that completely cuts off the supercap unless it's being held closed by a cap that is only charged when a person does the key/pushbutton process. The trick is that the ignition circuit still needs to be active after launch for when the 2nd or 3rd stage ignites, so there needs to be some sort of mechanism that prevents the ignition circuit from working before launch of allows it to function for up to say.. 30 seconds or a minute after launch.

klugesmith:
Noted! Yes, igniting the stage immediately after the previous stage has burned out will be ideal. While I love pyrotechnics, I love the idea of this being completely electronic controlled even more. It might make things a little more complex but I think it's worth it for the amount of control I'll be able to achieve.

mister_rf:
Wow! Thanks. I've reviewed the changes you made and I have a few questions about your implementation:
1) D7 -- is that for positive feedback to reduce required current?
2) Why is C7 there? If I had to guess... it would be to reduce voltage ripple generated by voltage regulator?
3) What is the function of D8?
4) For IN-, it looks like we're dropping down the 8.4v supply and manually adjusting it to be just below maximum IN+ potential voltage. If the battery loses voltage, will it not have to be adjusted again? Why not draw after the 2.75v regulator where we can (hopefully) safely assume the voltage will be stable? Actually.. now that I think about it.. is this why you added the balance inputs?

mister_rf

Mon May 07 2012, 02:25PM

Registered Member #4465 Joined: Wed Apr 18 2012, 08:37AM
Location: Bucharest, Romania
Posts: 145

1) D7 = flyback diode (sometimes called a snubber diode) used to eliminate flyback, the sudden voltage spike seen across an inductive load when its supply voltage is suddenly reduced or removed. With the snubber diode D7, this reverse voltage on the relay coil is limited to the forward voltage drop of diode (~0.7V) to protect the microcontroller board.

2) R17,C7, R18 = filter to reduce voltage ripple generated by the step-down converter.

3) D8 = Zener diode, an electronic component which can be used to make a very simple voltage regulator circuit.( in this case 6V2)

4) Capacitor charging is exponential so it never quite charges fully, so rather than waiting for a 100% charge, the controller only waits until it reaches a particular percentage of full charge. The voltage comparator is used to determine when the capacitor reaches the appropriate level, LM311 voltage comparator works by comparing the input voltage to the voltage reference provided here by the zener diode. In this circuit the reference voltage is fixed by adjusting the pot to so the wiper is set at 2.75V. LM311 compares the voltages at the + and â€“ inputs. If the + input is at a higher voltage than the â€“ input the comparator output will be high. If the â€“ input is at a higher voltage than the + input the comparator output will be low. So the output is switched ON or OFF depending on the relative voltages at the + and - inputs of the comparator. When the voltage on the supercap rises past 2.75V the comparator output will go high so as to output an 'ON-time ending signal' to the microcontroller...

Steve Conner

Mon May 07 2012, 02:35PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Thanks for your input, mister_rf. I have just one thing to add: Why not leave the battery and charger on the ground to save weight, and have the whole system run off the supercap in flight? Or maybe even replace the whole lot with a CR123 lithium battery. I believe these can source a few amps for short periods, as they're used in high-powered flashlights.

Using a MOSFET switch instead of a relay would save weight too.

I believe klugesmith's comments are correct, multi-stage model rockets are made by simply having the parachute ejector charge of the previous stage ignite the next one. I used to knock out the little clay cap to expose the ejector charge so it would flame more.

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