Firing circuit

Basil

Hi All,

This probably has a very simple answer but its avoiding me.

I am trying to design a simple circuit to provide 10A to an electric match in a rocket. I also want to be able to test the continuity of the circuit.

Is there a simple way to test the continuity with the prop? I was thinking a simple Mod on the attached circuit, running a low current through the ignitor by making the output pin on the prop an input and connecting it through a large resistor.

Please let me know what you all think, im not super strong on hardware design.

Thanks,

Alec

T Chap

An idea below. I would not let the mosfet float.

**Edit: fatally flawed design here**

Post Edited (originator) : 3/19/2007 5:37:07 AM GMT

Mike Green

Don't forget the fail-safe interlocks. You don't want the thing to go off if there's a program bug until you're ready.

Peter Jakacki

Basil said...
Hi All,

This probably has a very simple answer but its avoiding me.

I am trying to design a simple circuit to provide 10A to an electric match in a rocket. I also want to be able to test the continuity of the circuit.

Is there a simple way to test the continuity with the prop? I was thinking a simple Mod on the attached circuit, running a low current through the ignitor by making the output pin on the prop an input and connecting it through a large resistor.

Please let me know what you all think, im not super strong on hardware design.

Thanks,

Alec

Basically that idea is fine but if the Prop can detect a voltage then so can the mosfet, though it wouldn't quite work that way. Make the detection resistor a high value of around 100K or more and feed this into another input pin. The current is so low that the prop port can clamp it effectively from it's 10V but you can always add extra clamping.

I agree with using a resistor to the gate (for the purpose of protecting the port from catastrophic mosfet failure only) but make the value a little larger (10K) as you are not worried about nanosecond responses (RC delays). As was also pointed out you have nothing stopping the gate from floating high if the drive pin is not an output (during reset etc) so include a pulldown either on the port pin (this can be 100K or less) or from the gate. But watch out for the gate pulldown as you only have 3.3V drive and you would be forming a voltage divider.

Mike's suggestion is a good one as you would not want the rocket to be inadvertently fired. (I was just uploading new firmware and then it went KABOOM )
How you do this can also be tricky but usually it's good to have a circuit that needs refreshing or else it defaults into the disabled state.

The diode across coil is unnecessary as the mosfet has an intrinsic clamp diode and is well able to handle the coil.

*Peter*

Basil

Hi All,

Thanks for the quick replies!

Orig, thanks for the diagram, its a great help!

Mike, I have a number of failsafes planned for pre-launch saftey (Breakwire etc.). Or are you meaning something more specific I need to design in to my circuit?

Basil

Hi Peter,

Thanks for that. I can sort of draw that in my head, will have to wait till im not at work try it out [noparse]:)[/noparse]

Any more suggestions/diagrams would be great.

I have a quick question, will taking the 10v for the firing circuit (its actually 9v) direct from the battery, prior to the regulator which provides the 3.3v, cause any trouble with the prop when this fires? I.e. Brownout etc.

Thanks,

Alec

Mike Green

Alec,
1) I was just concerned since I didn't see any failsafes in your diagram.

2) You can run the 3.3V regulator off the 9V supply as long as your 9V supply can't drop below say 5V under the load of the ignitor.

Basil

Hi Mike,

Oh, that diagram was just a generic diagram of the net [noparse]:)[/noparse]
Its only a small part of the overall circuit.

It was a basic starting point to my requirments, the diagram which Orig posted seems to be alot better!

I have a basic understanding of electronic circuits but not with FET's, hence all my questions.

This forum would have to be one of the most helpfull I've seen!

Alec

T Chap

Alec, although the idea of the opto on the base of the ignitor could work (depending on the ignitors current requirement to fire), I would not go that route. The 100k resistor idea is better.

Basil

originator said...
An idea below. I would not let the mosfet float.

Hi Orig,

I just found out the TIL191 is obsolete, would the attached product (SFH6943) be a good replacment? (Its a quad coupler which is fine as im planning on 4 pyro charges.)

Forgive me is this is a simple question, im new to opto's also


EDIT: Posted this just afer your reply Re. the 100k Resistor.

Thanks,

Alec

Post Edited (Basil) : 3/19/2007 3:04:20 AM GMT

T Chap

I just grabbed that part out of a library for illustration purposes. Yes use the quad you listed, no problem.

Basil

originator said...
I just grabbed that part out of a library for illustration purposes. Yes use the quad you listed, no problem.

Cool thanks. This should give me something to chew on.

Alec.

Basil

originator said...
Alec, although the idea of the opto on the base of the ignitor could work (depending on the ignitors current requirement to fire), I would not go that route. The 100k resistor idea is better.

Hi Orig,

Forgive my ignorance, which opto are you refering to? OK1 or OK2

Also, with OK2 (The continuity Opto), is does not look like there is anything stopping the 12v passing through the ignitor and opto straight to ground.
Would this not be the same as shorting the ignitor and firing it? Or does the opto limit current somehow?

T Chap

OK2. If you notice, I accidentally omitted the resisitor to GND on the LED on OK2! This means, the ignitor will fire at once on power up, which is not a very good design [noparse]:)[/noparse]


However, depending on how much current is required to fire the ignitor, at some value for the resisitor coming off the OK2 LED to GND, it will work, but that value is unknown without tests, and still would not be an ideal design.

The opto is a good practice for outputs of processors going to larger voltage devices, so the first opto is ok, it provides a layer of protection against the mosfet shorting and sending 10v straight to the Prop pin. I short mosfets all the time doing experiments with motors. When you are experimenting and learning about mosfets, always meter from the drain to gate, source to gate, drain to source prior to testing, especially if you are breadboarding, hobby boarding etc, it is just a good practice at first.

Basil

Thanks for the tips [noparse]:)[/noparse]

Ok, so in summary OK1 is fine for the ignition part, but you'd do something else for the continuity test rather than using the opto OK2?

Im still stuggling a bit with what to do for the cont. test part though as I cant seem to find a way to do it without firing the ignitor

T Chap

For continuity, just do what was discussed earlier, use a 100k off the bottom side of the ignitor to one Prop pin(pin state is input).

The Prop reads if it is a 1, if so you have continuity, or you know the ignitor has "power". Can the ignitor fire off a 100k resistor to GND? I would say it is unlikely, just make sure the pin is always an input, not a low output just to play it safe.

Mike Green

The trick for a continuity test is to use a very small current. You could really use a 100K resistor between the Propeller continuity test pin and the junction of the ignitor and MOSFET. To avoid a floating Propeller pin, put another 100K resistor between the Propeller pin and ground. You're really looking for either a low resistance connection between the "low end" of the ignitor and +10V or an open or very high resistance connection. In the first case, the Propeller pin will be pulled to 1/2 of 10V or 5V but the protection diode will clamp that at about 4V with a maximum current of about 10ua. In the second case, the combination of the 100K resistor to ground and any leakage within the MOSFET (very small) will keep the I/O pin near ground. You really don't need the optoisolator since the 100K series resistor is a very good protection even from static discharges.

Basil

originator said...
For continuity, just do what was discussed earlier, use a 100k off the bottom side of the ignitor to one Prop pin(pin state is input).

The Prop reads if it is a 1, if so you have continuity, or you know the ignitor has "power". Can the ignitor fire off a 100k resistor to GND? I would say it is unlikely, just make sure the pin is always an input, not a low output just to play it safe.

Oh, takes me to miss that!

Thanks heaps, ill try those suggestions out and see how we go.
Ill try to think of a way to make that a bit safer too [noparse];)[/noparse]

Until tommorrow,

Alec

T Chap

Interesting, I never tried to turn on a mosfet with something like a 100K, if it worked then that is great protection and less parts. How much current is required to turn on the specific mosfet is key though I suppose.

Mike Green

originator,
The MOSFET gate is essentially a capacitor with the other side effectively grounded. The gate series resistor and this capacitor make an RC circuit with the usual equation defining the charging time. A 100K resistor is just fine as long as you're not in a hurry, like trying to amplify a 20MHz signal without significant distortion or switch a bus signal in under 50ns. Typical gate capacitance is around 250pF.

T Chap

While on mosfet theory, except in cases of logic level mosfets, is there a general rule of thumb for Vgate versus Vdrain for the fet to turn on fully(NPN)? Or is that specific to the fet.

Mike Green

Here's a link to the Wikipedia: en.wikipedia.org/wiki/MOSFET

Basil

Mike Green said...
The trick for a continuity test is to use a very small current. You could really use a 100K resistor between the Propeller continuity test pin and the junction of the ignitor and MOSFET. To avoid a floating Propeller pin, put another 100K resistor between the Propeller pin and ground. You're really looking for either a low resistance connection between the "low end" of the ignitor and +10V or an open or very high resistance connection. In the first case, the Propeller pin will be pulled to 1/2 of 10V or 5V but the protection diode will clamp that at about 4V with a maximum current of about 10ua. In the second case, the combination of the 100K resistor to ground and any leakage within the MOSFET (very small) will keep the I/O pin near ground. You really don't need the optoisolator since the 100K series resistor is a very good protection even from static discharges.

Hi Mike,

I think that make sense.
Would if be something like the attached picture?

Oh, the electric matches typically·have test voltages of around 15ma -> 40ma and firing currents of 400ma -> 1A @ 9v

This has been a great learning experience!

OzStamp

Hi

Attached a circuit that I have personally used
for years with a BS2 STAMP (and other micro's) to drive
a very high speed solenoid valve.

The zener and diode across the SOLENOID (load) are there
for very rapid trun off.

The zener down the bottom is probably overkill but is it to
protect the GATE on the Mosfet from max high Volts (20VDC)
The resistor values will probably be close what you would want
in the 12-24VDC range...

The opto isolation on the left would get its signal from the PROP
Maybe insert a led in there for indication as well..
Remember you only 3.3VDC drive...

Cheers

Ronald Nollet Australia

T Chap

Yoiu still needd the gate series 100k to the Prop trigger pin.

Mike Green

The MOSFET needs to be one with a lower gate threshold (IRL instead of IRF) so the Propeller's high output can saturate it.

Basil

originator said...
Yoiu still needd the gate series 100k to the Prop trigger pin.

Good morning

Thanks Ronald for that diagram.

Orig, that part got left behind on the rest of the circuit diagram, attached if how it should be

Thats a whole bunch of 100k resistors!

Thanks all, This looks like it should work.

T Chap

Post the first lauch progress report!

Don't forget to put a physical switch in the ciruit somewhere just to be safe, maybe on the 9V input, enable the switch when everything is ready for continuity check + launch .

Basil

Mike Green said...
The MOSFET needs to be one with a lower gate threshold (IRL instead of IRF) so the Propeller's high output can saturate it.

Hi Mike,

Okie tokie
Thanks for that.

originator said...
Post the first lauch progress report!

Don't forget to put a physical switch in the ciruit somewhere just to be safe, maybe on the 9V input, enable the switch when everything is ready for continuity check + launch .

Ill be standing well back when this is tested

I have a power switch, an 'Arm' switch and a failsafe, should be enough

rjo_

Basil,

Please make a video!!!!

Rich

Phil Pilgrim (PhiPi)

Basil,

Your circuit shows a voltage divider in the MOSFET gate circuit. With a high of 3.3V from the Propeller, the gate will see only 1.67V — not enough to turn it fully on. If the purpose of R1 is to limit inrush current to charge the gate capacitance, 100 ohms will suffice.

But even 3.3V won't be high enough for some "logic level" MOSFETs, since many were designed for 5V logic. Be sure to check the specs to make sure it will saturate with 3.3V on the gate.

-Phil