MOSFET Board

Hi everybody I just buy a MOSFET board the thing is to trig fireworks igniter with. they explain on their website how to plug things on. here is a picture = The igniter (+) plugged in the 24V (+) The igniter (-) plugged in the Mosfet output to get or not the ground (that's what i understood ...)

Here is my problem ... i don't know (... i mean .. i really don't know ) if its dangerous, or not, to have all the time my igniter (+) plugged in the (+) of the 24V power supplie. I know that the igniter's ground will not be connected until the mosfet open, but i don't feel confortable with this. What Do you think about it ?

The way i would like to connect the igniter looks like this = But i really don't know if it's possible ..

Hi, I have never used the MOSFET SHIELD before, but I found this site: http://emartee.com/Modules/bbs/redirect.php?tid=482&goto=lastpost&styleid=5

Their how-to-do means that your first sketch is ok. Have you tested it with a 24V load? E.g. two 12V bulbs in series.

Your second schematic isn't allowed. You have transposed + and [ch8211] at the board's pins and therefore at the Power MOSFET.

Don't feel uneasy if the power supply (+) is all the time connected to the igniter. It doesn't matter if the (+) or the ([ch8211]) wire is permanently connected and the other is switched by the board. Firework is dangerous at all. ;)

The way i would like to connect the igniter looks like this =

The way the loads have to be wired is dictated by what flavor mosfets are being used, N-channel or P-channel, and the first way that the manufacture shows implies N-channel devices are used. Safety considerations are equal either way and for your use should be reviewed closely. For instance what do the channel output do if the Arduino is in a reset condition?

Most firing circuits I have seen published use a manual switch contacts between the firing devices and any control electronics as a fail-safe backup arming function. Also I've seen some that have circuitry to detect that there is indeed a igniter wired up (broken wire detector). Are you planning such safety circuits.

Lefty

Good evening. Yes, fireworks are dangerous no matter what you use to set them off. I have been setting up and firing professional shows for about 5 years now. We primarily use electronic squibs(igniters) and occasionally hand light some smaller shells in a show.

Here is my problem ... i don't know (... i mean .. i really don't know ) if its dangerous, or not, to have all the time my igniter (+) plugged in the (+) of the 24V power supplie. I know that the igniter's ground will not be connected until the mosfet open, but i don't feel confortable with this. What Do you think about it ?

The way the manufacturer designed it is perfectly fine. You can not reverse the way you connect the power because the MOSFET and the opto-isolators (the little white chips) are designed to work with the negative side of the power. The board would need a total redesign to work with the positive side of the power.

Just make sure you are at least 100 feet from where the fireworks are when you ignite them. Even consumer fireworks can cause damage and death at close distances.

Hope this helps.

I am not the one that will plug FW, i am doing this for guys that do this job for a long time, but have a really “vintage” system, and would like to implement some “new technologie” stuff in it. … and for a low price…for sure…
I just have to found how to deal with the MOSFET board to sense the 24V too.
Because an important thing is to see if the circuit is closed and what is the loss (to estimate how many igniters are plugged and how much cable is here)
Maybe you have a tip for me ?

OK. HI .. I'm back !! :P The Mosfet Board works fine and after testing few igniters, i have no problem with ground and (+) connections ... I have one more question for you wise people ... Sometimes, an igniter don't "work" properly. It happens. not because of the mosfet board, but because of the igniter itself, sometime when the right voltage and amp is going through, it don't burst. When it happen, the circuit is closed, between the (+) and the (-) = mosfet, on the board. A 'short circuit'. Do you think it is dangerous for the mosfet board ? Do i have to add a component on each channel to prevent the board from this kind of short ? or maybe the board seems already protected for this ? Thank you for your help. .Y.

I think normally igniters fire in milliseconds. Means the MOSFet doesn't really [ch8222]notice[ch8220] its (short) load. If it is switched on over several minutes without firing of the igniter it could be heated up. This is extremely unlikely because of its low internal resistor between Drain and Source. The transistors are specified to 33Amp. This might be a value for optimal conditions (with a sufficient heatsink). Without any heatsink it should work for about 10Amp.

But make a test and put your fingers on the right MOSFet. If you can let them touching = no problem with the circuit.

Well you could always wire an inline fuse between each mosfet output to protect from 'shorted' loads.

Lefty

Hi,

The way the FETs are soldered on this board, I would definitely not recommend any current larger than 5Amps, otherwise you might toast the FETs or seriously burn yourself or something else. I checked the FETs' datasheet and their thermal resistance should be somewhere around 50 or 60 degress Celcius / Watt. The FETs ON-resistance is on the order of 70 milliOhm, so you should get a "junction temperature" of 105 degrees Celsius with only 5Amps... not good at all.

If you are running on 24V, you will get 5Amps of current with a load of 4.8 Ohms. This is easy to check with an Ohm meter. If you measure a resistance smaller than that on your faulty igniters then you should seriously consider limiting the current.

OK guys, thanks …
What if i put a resistor (lets say 100K) to limit the current, before the Mosfet.
Something like this =
24V (+) ----------------------------Igniter (+)
MOSFET out --------R(100K) ----- Igniter (-)

Do you think it will work ? 100K seems good ?

Do you think it will work ? 100K seems good ?

Sounds like spitting into the wind trying to fight Dr. Ohm's law. 100k would limit igniter current to .24 milliamps. Somehow I don't think that much current would fire the igniter. Just use fuses if you want to protect your mosfet outputs against external short circuits.

Lefty

As a current limitting resistor simply use a 24V bulb. I think trucks have a 24V mains. The advantage of a bulb is that its resistor is temperatur depending. In the very first moment when the filament is cool, its resistor is very low and the igniter can fire nearly with the full voltage.

If the igniter doesn't work, the bulb probably begins to light and its resistor increases. The power of the bulb is also important. I would use a headlight bulb. 24V/55W or similar.

YES, good idea the bulb !! And you're right "retrolefty", an igniter need 1A to burst. I forgot to tell you something, an igniter need only 3ms of current to burst. I programmed the arduino to close the circuit (mosfet board) only 10ms each time i want to trig something. Maybe 10ms in case of a short circuit is not enought time to burn the mosfet don't you think ? I use a 9,6 V battery ( 6 LR6 in serie) > 1800 mAH. I don't know how much current such a batterie can give in case of a short. And in a time of 10ms ... What's your opinion ?

What's your opinion ?

I think with only 10msec, and that battery capacity, your mosfets will handle it OK. That's an opinion, not necessarily a fact. ;) Just make sure your code is very stable and tested such that it nevers goes off into the tall weeds while leaving a output pin on. ;)

Lefty