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Is this mosfet good for my project?

Xenobius

May 15, 2012
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Hi Steve,

This is the finalised schematic which includes both reading logic and switching mosfet where

T1 is the Ignitor
O1 is a low logic from Arduino (to turn on)
and I1 is the low logic from cct to identify that the ignitor has blown.

What do you think?
Thanks :)
 

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Xenobius

May 15, 2012
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Hi all,

I have a small concern with the mosfet chosen. Firstly it is very small which concerns me about heat issues, secondly according to the datasheet, the absolute rating for the following is this:

Voltage 60V
Current 5.5A
Power 5W

What does this mean? Does it mean that for my application ie: 12V @ 1A for 100mS (12W) this mosfet is not good?

Here's the datasheet
 

BobK

Jan 5, 2010
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The 5W is the power dissapated in the MOSFET, not in whatever it is connected to. Assuming the MOSFET is either on or off all the time, the only power it will use is the drain current squared times Rdson, which is about 1/10 Watt for 1A. If it is switched at high frequency there can also be power dissapated during switching.

That said, the 5W spec that you see is not what you are looking for anyway. It is the max power at 2% duty cycle, you can get nowhere near that for continous operation. The actual dissapation allowed for the device is not really stated in the datasheet because it is too dependent on how it is mounted to a pcb. You are certainly safe at the 1/10th Watt level.

Bob
 

Xenobius

May 15, 2012
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Wow I'm really grateful :) What I find intriguing is how could such a small device handle 5.5A @ 60V. I mean this is a 2mm x 3mm device! But as you said, since RDSon is very small, its like passing 5.5A @ 60V over a 3mm wire which is nothing :)

Thanks Bob!
 

(*steve*)

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The important thing to note is that the mosfet is fine when switched on or off.

It's the transition between those states that needs to be done right :)
 

Xenobius

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Ic, but as you told me in another post, it shouldn't be a problem for me since I will be switching it instantly, and at 0hz.

right?
 

(*steve*)

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Well, maybe not at zero Hz, but at least at a very low frequency

And your switching will be fast enough for that frequency :)
 

Xenobius

May 15, 2012
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Dear friends,

I have finally built this circuit (as attached) and I am having a lot of problems. To start with, I am going to tell you what the points are.

I have a SPDT relay which is taking 12VDC and outputing Either 12VTEST (NC) (Green) or 12VLIVE(NO) (Red)

I1 is a 5v logic that I am trying to read
O1 is a 5v Logic that I am inputing
T1 is a fuse that I am blowing when I give 5v on O1 (connected to ground)
Bi Led: This should turn on Green when Test and Red when Live

Problem1: With relay set to NC & without fuse, I get 12V on Test and 8V on Live!
Problem2: With relay set to NC & without fuse, the green led is dimmly lit.


I have no clue what is wrong with Problem1 but for Problem 2, I think the 12vtest is passing from R4, into R12 and into R13 and reaches ground...

Please help as I am a little lost.
Thanks as always
 

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(*steve*)

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Are you saying that the two pins labelled T1 go to the ends of a fuse? Or are they connected together, and there is a fuse from here, to the ignitor, then to ground? What is the fuse rated at?

In the former case above, there's no reason why a fuse there should blow. Sounds like a wiring error. In the latter case... dunno.

Change the 1k resistor in the cathode of the bicolour LED to 2 1k resistors in the anodes of the LEDs.

The previous discussion was about the mosfet choice. I think this is the first time you've discussed what the other connections (other than the input) do.

Problem 1 and 2 are the same. The small current through the LEDs causes a voltage to appear on T1 (assuming both T1's connected together). Connecting an ignitor (low impedance between T1 and ground?) should cause the voltage to drop to almost nothing and the LEDs to reach an appropriate brightness.
 

KrisBlueNZ

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As Steve said, you need to clarify this "T1" business. You have two points marked "T1" in your schematic. Normally this would mean they're connected together. If not, you should draw how they are connected. You also need to draw the fuse, and the igniter. Draw the whole thing so we can see how it fits together. You could also draw the TEST/LIVE relay for clarification.

You're right about the LED current flowing through the other resistors. Why don't you just return the LED cathode to ground? As Steve says, use separate resistors in the anodes, not a single common one in the cathode. If you return the LED to ground, it will indicate either red or green, according to whether the system is in test or live mode, at all times. As you have it connected at the moment, it looks like it's supposed to go out when the output is activated. Which behaviour do you want?

The drive to the MOSFET is right, but it would be a good idea to put the part number of the MOSFET on the schematic.

I'm not sure about the 12 ohm resistor between the MOSFET and the igniter. Are you trying to limit the current to 1A? That resistor will limit the drain current to 1A if the bottom end of it is shorted to ground. If you're driving the igniter from it, then the 12V will be split between the resistor and the igniter. Assuming the igniter is rated for 12V and draws 1A, it will have 12 ohms resistance as well, so the resistor and the igniter will each get 6V across them, and the current will drop to 0.5A. I don't think that's what you want. If the igniter wants 12V across it, you need to remove that resistor. You should have a fuse, say 2A, but not the resistor.
 

Xenobius

May 15, 2012
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Right T1 is connected together and as for the fuse, I was referring to the ignitor. So basically T1's are connected together and the ignitor is connected from Ground to T1.

Appologies for mentioning a fuse but I figured that ignitors are also called fuses sometimes. Regarding the LEDs I will change the single 1k resistor to 2 on the anode however when the relay switches on, voltage should appear on 12V Live not 12V Test and it is giving me around 8v on 12v live and 12v on test as if the relay didn't switch...

I tested the relay and it actually is switching. I also noticed that for a micro second, the leds switch to red and back to green....

Could it be that the Mosfet is shorted? How do I test a mosfet?
 

(*steve*)

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If an ignitor is also called a fuse, then this means it is DESIGNED to go open circuit.

remove the 12 ohm resistor and test again. If the mosfet is shorted then this will effectively remove it from the circuit.

You'll never get 12V on T1 in test due to the voltage drop across the LEDs and the voltage divider action of other resistors in the circuit (unless the MOSFET is on)
 

Xenobius

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If an ignitor is also called a fuse, then this means it is DESIGNED to go open circuit.

remove the 12 ohm resistor and test again. If the mosfet is shorted then this will effectively remove it from the circuit.

You'll never get 12V on T1 in test due to the voltage drop across the LEDs and the voltage divider action of other resistors in the circuit (unless the MOSFET is on)

Thanks Steve. The 12ohm resistor will stop the "infinite amount of current" that will flow into the fuse until it actually blows and goes open circuit. I had confirmed this in this same thread earlier. The mosfet is designed to handle 5Amps and the 12Ohms resistor will limit this current to a maximum of 1A. The ignitor is designed to blow between 800mA to 1A.

As for T1 not getting 12V in Test, I don't mind. What worries me is the fact that I have 8V on Live during Test! I'm not sure if its a wiring problem (I doubt it because the software I use will tell you when you short something) and this is currently soldered on a PCB not a breadboard so its kinda difficult to have a short. I will however check again of coarse.
 

(*steve*)

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The 8V you see in Live is due to the fact that the ignitor isn't there. Wire one up and that voltage will drop to almost zero (if it doesn't, you have a wiring error).

When I said remove the 12 ohm resistor, I meant to take it out, leave it open, unsolder it, etc, not replace it with a link. Once you have done this, I presume the voltage will still be around 8V without the ignitor there.

For simple testing, you could replace the ignitor with a 100 ohm resistor. This will be a low enough resistance that the circuit works as designed, but high enough to prevent any really serious current from flowing.

You should find the voltage across the 100 ohm resistor in test (or active, but not firing) mode to be about 1 volt, and when active and firing, between 10 and 11 volts.
 

Xenobius

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This is great news! I didnt try the ignitor because I wanted to move one step at a time!!
THANKS A LOT... Will post my findings tonight
:D
 

Xenobius

May 15, 2012
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Hi all,

Ok so here is what happened so far. The Relay was wired wrongly and is now switching properly. The 8v still appear on the LIVE however this disappears once I connect the ignitor.

I actually managed to blow up some fuses using a button (for now) and attached is the result.
Channel 1 is the fuse blowing and Channel 2 is the button being pressed.... there is an awful delay of 102mS from the time I press the button to the time it triggers... but this is not an issue for now.

Regarding the dimmly lit LEDs, this happens both when the leds are green and red however I didnt have the time to change the single 1k resistor from catode to 2x 1k resistors on the annode.. I will do this tonight... do you really think it makes a difference however?

Thanks all for helping me a fullproof circuit :D
 

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(*steve*)

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If your two 12V sources cannot be simultaneously powered, then moving the resistor will not make a difference (in fact, in that case it's in the right place).

The dimly lit LEDs are due to the high impedance path to ground you supply through the 30k of resistors.

How is that switch connected? The mosfet appears to be turning on rapidly (I assume the top trace is current into the circuit? or maybe across R10?

Actually both traces look a bit odd. Where do you have the probes, and where is your ground connected?
 

Xenobius

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Ok so is there a solution to totally turn the leds off? I'm assuming I will need a transistor somewhere right?

Also the probes are connected across the switch (which is pullup by default) and across the ignitor (ground to mosfet)

What's wierd exactly? I barely know how to use the oscilloscope but I assumed the setting were good :D
 

(*steve*)

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What is weird is that I don't see a low to high to low transition as you press the button, but a falling edge when the button is pressed, followed by a decay, then a rising edge when it switches off.

Looks like there's a capacitor in there...

Connect the ground lead(s) to the ground in your circuit (make sure it's compatible with the scope's ground) and the probes to O1 and T1 (or O1 and I1). Both I1 and T1 are interesting, but initially I'd like to compare them with O1. Later it would be interesting to look at I1 vs T1
 
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