Magnetic apogee detector troubleshooting

Paxton -

It sounds like your MAD is working fine if it will ignite a flashbulb.

The peak output current of a MAD should be something on the order of 5-6 amps through a flashbulb or Daveyfire (about 1.6 ohms). However, this current pulse will only last for a millisecond or so.

Are you measuring the current draw out of the battery? That will always be relatively low. The battery charges up the big capacitor through a 1 kohm resistor. The voltage drop is probably happening during the charging phase. 8.5 V sounds kind of low, I'd get a fresh battery

Do not use an incandescent bulb to test the output of the MAD. The MAD does not put out enough current to light this kind of bulb (you may see the bulb flash briefly just as the MAD is tipped over). Use an LED with a current limiting resitor in series to prevent the MAD from burning out the LED.

- Robert Galejs

Paxt> Hey folks, got a problem with my MAD...I think.

Thanks Robert. The battery I was using was "good" but not "fresh." When I put it in a rocket it will be "fresh."

About the LED's. I guess I have fried a couple since I haven't been using a resistor with it, but the MAD doesn't actually pop them.

Did some playing, and the 9V will pop a LED, but in the circuit it just starts bright then dims to a constant level.

I am wondering if there is a way to make the MAD work without a CAP and have the battery dump as much juice over those 2Ohms as possible. I am in a EET program, but I am still in the first DC analysis course. Not good enough to figure all this out on my own yet.

Pax

Some LEDs will stand up to the capacitor discharge and some will not. I'm not sure what LED characteristics make the difference. Add in a current limiting resistor and you should be fine.

The initial bright flash is the capacitor discharging through the LED and the constant dim level is the 9 mA "recharge" current passing through the LED.

It's certainly possible, but I'm not sure why you would want to. The capacitor discharge is fine for flashbulbs and e-matches. You'll run into two problems if you try this. First of all, you would likely exceed the HEXFET operating limits, burning out that component. Second, you will probably draw down the voltage on the battery, causing odd circuit behavior.

- Robert

According to Paxton :

I fried the one that came with the kit because the instructions didn't suggest a limiting resister (as far as I determined at the time). Must be marginal, because other people have done it without a resister and not blown one.

I think that the circuit as designed with the current dump cap is the best way to go. First, a 9V battery may not be able to deliver enough current to pop some igniters without "help" - you'd certainly have to remove the "fill" resistor (or make it MUCH smaller, as well as make it larger to stand up to the heat dissipation) to get the steady state current higher.

So you may have to have a higher current capable battery.

Secondly, consider what happens if the lead wires short and the rocket is lying flat on the ground after flight. You'll fry the switching FET and/or drain the battery quickly. Under unusual circumstances, you might even start a fire.

Robert's circuit avoids all of these problems.

Other circuits that do without the dump capacitor have TO-220 form factor FETS. Which are big beasties. Even then it's not clear how well they cope with accidental shorts.

The best firing circuit will limit current to safe levels even under dead short conditions, and _should_ also preserve the battery, at the same time as providing as much current as possible to get the igniter fired. Several of the ones on the web don't do any of these well.

Dump capacitor arrangements have the advantage of determining a relatively fixed amount of energy to apply to the ignitor. Without dumping, you want to actively switch the current _off_ after the ignitor fires, as well as limiting the amount of energy fired into the igniter wires.

I was reading through some of your documentation, and you have a circuit diagram of the prototype, but I can't seem to find one of the kit. Is that the "simple" version of the diagram, or separate. If separate is it possible to get it from you?

Pax

Just played around with some more stuff.

I can't even fire a 1Ohm estes ignighter with the MAD. The battery alone will toast it, but the MAD doesn't do anything. Still verified that is is working with my meter and a LED, but it wont fire a match.

This is with the battery running about 8.5volts, but still, it should be fine for ground testing, right?

Pax

An estes ignitor is a relatively high current ignitor. It takes about 2 amps for a second or so to fire. The MAD is not designed or configured for that. It's really designed to reliably fire an ematch, and it does that very well.

Indeed, once you realize how fragile estes igniters are, you immediately realize that they're not a good idea for a recovery system anyway. By the time you have the thing securely connected to lead wires and bashed about a bit, you have a very significant chance of it not working. Which is okay for a first stage motor igniter, but lousy for recovery - it's a bit hard to replace it in time...

Yeah, I was a bit disappointed that my MAD wouldn't fire an estes, but, I'd NEVER use one even if it could.

snipped-for-privacy@nortelnetworks.com (Chris Lewis) wrote in news:bp3il8$1k3v41$ snipped-for-privacy@ID-118425.news.uni-berlin.de:

What sort of electrolytic (1000uf?) does the circuit use? A low-ESR type,or an ordinary cheapo cap? That could restrict the current discharged.

You could increase(double/triple) the capacitance to get more discharge current,although you still will not get enough to fire a low-ohm igniter. The newest low-ESR caps are pretty small. (ESR=equivalent series resistance, ----[C]---/\/\/\-----...)

Perhaps a 9V lithium battery would help,too,although more expensive than a regular alkaline 9V.

I am only interested in making sure my MAD is working correctly, not wanting to use estes for ejection. I am not stupid, just ignorant and desperate :-) I guess it makes sense that a microsecond of current over 1amp wouldn't fire an estes because it is based on a heated wire element. I am just getting frustrated because the MAD is only designed to work with certain devices and I don't have any of them to test with. I have a crapperhead, a first fire, and the estes igniters. I haven't had time to see if I could bum some daveyfires off somebody and test with what I will ideally be using. I have a flashbulb left, but I think the real test would be the ematch.

Thanks for all the input guys.

Pax

According to Jim Yanik :

Looks to me to be an ordinary 1000uf 16V capacitor. Two parallel leads off the end.

It strikes me that estes ignitors are probably quite variable in fire current. And not robust enough for safety.

Wouldn't make any difference. The max battery current is fairly low due to the series capacitor "fill" resistor. The only thing you can do to increase ignitor oomph is increase the charge voltage.

Well it's not intended pyro source you can use the sub miniature light bulbs from rat shack. I use them all the time for testing my Adept ALTS2 and Olsen FCP/M2. I'd use them to test my MAD unit if I wasn't such a putz with the surface mounted itsy bitsy components :(

Ted Novak TRA#5512

Paxt> I am only interested in making sure my MAD is working correctly, not wanting

snipped-for-privacy@nortelnetworks.com (Chris Lewis) wrote in news:bp4hkk$1kfhn0$ snipped-for-privacy@ID-118425.news.uni-berlin.de:

Well,changing to a low-ESR cap and/or substantially increasing the capacitance -will- increase the amount/duration of instantaneous current to the ignitor.(more average power) I don't think going to 2700uf would destroy your FET.

Digi-Key has low-ESR caps of small size.If you decide to change to a low- ESR cap,you could get another one of say 3X capacitance,too and see what happens. Too bad you don't have a current probe and a scope to make a decent measurement.

I never put together a circuit schematic for my kit, but its pretty easy to follow the traces on the PC board to see how it all works.

- Robert

I'm usually satisfied if the MAD lights up the LED at the correct angle.

I've never heard of a MAD that would light the LED but not fire an e-match. The firing circuit is quite simple.

A flashbulb makes a nice inexpensive test unit if you really want to see something ignite, but they are getting harder to find.

- Robert

the notorious t-e-d wrote:

Yes, nothing special about the capacitor...

The MAD was not designed to fire an Estes igniter.

The peak current won't change. That would take an increase in voltage or a decrease in resistance. An e-match typically has a resistance on the order of 1.5 ohms or so, the resistance of the capacitor should be well below that. Increasing the capacitance will increase the duration of the current pulse which will deposit more energy into the e-match. Increasing the capacitance may or may not cause a FET problem, the existing set up does not.

- Robert

reposted from aRocket as a courtesy to the author

- iz

---- repost follows:

Date: Mon, 17 Nov 2003 10:49:01 -0500 From: Robert Galejs Subject: Six magnetic apogee detector kits up for auction

I just placed six old style MAD kits up for auction on Rocketry Online.

My new kits have a built-in on/continuity LED. The kits for auction do not.

Bidding starts at $10 per kit.

- Robert Galejs

Robert Galejs wrote:

I learned something useful in class today, woohoo.

So, with the equation T(time) = R*C(E/(Ic*R)) I come up with T =

1.6*.001(9/(.370*1.6)) which gives me: time above 370mA = 4.35ms. So I guess my MAD is working fine, my testing methods are all broken.

Pax