Rocket launcher MOSFET

[Lee697]

I'm adapting a circuit that fires a model rocket engine after a time delay. It is basically two 555's in monostable configuration, the first trigger the 2nd after a time period, the second then fires the igniter through, in the original plans, a darlington power transistor. I have built one launcher this way, but would like to use a power MOSFET for my MkII...

This is the basic schematic I have. This has been tested, and seems to function....
(I've obviously not included the timing/trigger parts of the 555)

Is this the correct way to wire a MOSFET - does the MOSFET, or the 555 need a resistor on the MOSFET gate?
Is 10k OK for a pull-down resistor?

Cheers
Lee

[tom66]

Yes, it's correct.

Your pull down can be essentially any value lower than about 1Mohm. The purpose is only to keep the FET gate off if the 555 timer's output is in a high impedance state. (I'm not sure if this is even possible - maybe on the CMOS version if RESET is asserted?)  The lower the value, the more power wasted although 10k will waste so little power (on the order of microwatts) it's not worth thinking about.

[Richard Head]

It may be worthwhile to include a 1N4148 in series with the power supply to the 555 and a 100uF cap across the 555 power pins. Then if the 1.5 Ohm load pulls the supply down momentarily the 555 output won't collapse for a while.

[Lee697]

It may be worthwhile to include a 1N4148 in series with the power supply to the 555 and a 100uF cap across the 555 power pins. Then if the 1.5 Ohm load pulls the supply down momentarily the 555 output won't collapse for a while.

Thanks...
I have a 47uF cap across the power supply, I don't understand what the diode is for though??

[Lee697]

Yes, it's correct.

Your pull down can be essentially any value lower than about 1Mohm. The purpose is only to keep the FET gate off if the 555 timer's output is in a high impedance state. (I'm not sure if this is even possible - maybe on the CMOS version if RESET is asserted?)  The lower the value, the more power wasted although 10k will waste so little power (on the order of microwatts) it's not worth thinking about.

Thanks - I'll keep the 10k, I run it off a fresh 9Ah SLA battery in the field, not too concerned about small power leaks.....

[Richard Head]

The diode is so that the cap doesn't discharge into the 1.5 Ohm load. The diode becomes reverse biased and the cap supplies the chip for a few hundred milliseconds while the ignitor is fired.

[max_torque]

With something like this, that has some possible "safety" consequences, i'd want to also design in some simple fail safe stuff too!

(for example, a manual switch that shorts the output of the fet to gound when on, which will prevent any firing voltage being applies in case of "accidental" triggering by the 555 etc.  It's also a good idea to include a very high impedance current source to act as a "wire test" for the firing circuit, before you actually go ahead and fire it!

[Lee697]

With something like this, that has some possible "safety" consequences, i'd want to also design in some simple fail safe stuff too!

(for example, a manual switch that shorts the output of the fet to gound when on, which will prevent any firing voltage being applies in case of "accidental" triggering by the 555 etc.  It's also a good idea to include a very high impedance current source to act as a "wire test" for the firing circuit, before you actually go ahead and fire it!

Definitely.
What I've shown is simply the firing part. I have a working version with darlington transistors doing the firing.

There is a continuity tester, which acts as a safety, puts 10mA or so through the ignitor to test before ignition - this disconnects the transistor when switched to test. There is a key switch also, which isolates the transistor gate (or base, previously). I had noted previously that the firing 555 occasionally falsely triggered on power-up - so there is a third monostable 555 which must be triggered before ignition countdown is started - it further isolates the transistor through a relay, which only stays closed for 30s or so.
The power-up false triggering was eventually fixed with tweaking of an RC delay on pin 4, but can still happen if you rapidly cycle the power switch - this is certainly not normal behaviour though.... In any case now the 'disarm chip' that controls the relay never false triggers. I don't think I could make it fire unintended if I tried now.....

The original schematic is here.... he only has the one safety interlock, and has an annoying buzzer I left out....
http://www.sentex.ca/~mec1995/gadgets/rocket.html

Lee

[Lee697]

The diode is so that the cap doesn't discharge into the 1.5 Ohm load. The diode becomes reverse biased and the cap supplies the chip for a few hundred milliseconds while the ignitor is fired.

Thanks - might put the diode in. In testing I've had a previous model discharge into a 1 Ohm resistor for 3-4 seconds, nothing unusual ever seen, I've not long had my scope though, might watch what happens with the scope this time....

[Richard Head]

If your source impedance is low enough (such as is the case with a car battery) then the voltage drop will be negligible so the diode won't be required.