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16  Using Arduino / General Electronics / Re: Burned FET on: May 05, 2013, 02:12:43 pm

I agree.  It is difficult to cause a power MOSFET to go into thermal runaway, under normal conditions, that is.  But operating a TO220 MOSFET at near a watt where it is not a major source of resistance in the circuit and providing no cooling at all is one way this can be achieved.

Looking at the normalized Rds(on) vs. temperature curve for an IRL540 and taking it piece by piece shows us that at 850mw, the temperature of the 540 will want to rise by 52 degrees in free air at an ambient of 25 degrees.  At 77 degrees (25 + 52) the Rds will rise by a factor 30%, increasing the dissipation to 1.1w, causing the temperature to rise to 93, causing the dissipation to go to 1.4W, causing the temp to rise to 111, causing....The curve actually just gets steeper and steeper.

And all that assumes free air with a fixed ambient temperature of 25.  Sealed in a plastic box, this will happen much quicker as the ambient temperature will rise, being heated by the MOSFET.

The self-stabilizing effect becomes prominent when the MOSFET itself is a major contributor to the circuit's overall resistance.  Then a 1.5 factor increase in Rds will reduce the overall current dramatically and actually lower the dissipation of the device.

The fact that the load in this case is the major contributor to the circuits resistance makes the matter worse since the IRL540 going from .08 ohms to .12 ohms will have little effect on the drain-source current, but a large effect on the power dissipated by the device.

Actually, the load here is quite likely going to be a constant current load too.
17  Using Arduino / General Electronics / Re: Burned FET on: May 05, 2013, 12:32:17 pm
I saw that maximal Gate-to-Source voltage for IRL2505 is 16V while for IRL540 is 10V. Since i am powering the whole circuit with a little bit more than 12V, this could be problem?

No, the problem was you were dissipating about 800mW with the IRL540.  Since it was in a sealed plastic box, this allowed it to get hot.  Too hot.  Once it get's hot enough the Rds goes up rapidly, increasing the dissipation and making it even hotter.   Thermal runaway ensues and the device burns up.  This can happen really quickly.
18  Using Arduino / General Electronics / Re: Burned FET on: May 05, 2013, 12:17:29 pm
Now i putted another FET (IRL2505) in the same circuit and after more than hour it's almost completely cold. Without heatsink on it, everything is in completely sealed plastic box.

You should not have a problem with the IRL2505.  The dissipation would be around 100mw.
19  Using Arduino / General Electronics / Re: Burned FET on: May 04, 2013, 09:02:12 am
That's almost a watt across the MOSFET.  A TO220 package can usually handle a watt or two with lots of cool air flow.  Was the MOSFET covered up or kept from free airflow?  You might try a little heat sink on it to help it out and make sure it has sufficient airflow.
20  Using Arduino / General Electronics / Re: Led light replacement of a 300wat halogen on: May 03, 2013, 08:33:55 am
Sorry to continue the hijack.  I just ordered two LED bulbs from the far east as possible replacements for CFL type bulbs since, as BillO pointed out, they don't last long, especially when installed base up.  They are in the 10W/1000 lumen range and intended to replace the typical 18W CFL.  One is a corn-cob type (~$6) the other is a globe type (~$10).  If they work out (and that is by no means a foregone conclusion) they would be worth the cost.  We'll have to see.
21  Using Arduino / General Electronics / Re: Can we use 3.579545MHz crystal instead of 3.57MHz on: May 01, 2013, 06:47:41 pm
I agree with florestra.  Too much input causes clipping which creates too many high amplitude harmonics and just confuses the situation.  Make sure your input is low enough and fairly sinusoidal.
22  Using Arduino / Project Guidance / Re: how to use a digital tachometer with an arduino on: April 30, 2013, 04:37:20 pm
It would probably be easier to do without the laser tach.  Just use an LED and phototransistor.

1) super-bright crystal clear LED fed with 5V through  180 ohm resistor
2) Phototransistor fed with a 100K ohm resistor from 5V to collector, emitter to ground
3) Take output of collector - resistor junction to Arduino input.
4) Place both LED and phototransistor next to each other pointing in the same direction with a bit of back tape between them.
5) Point them at the fan  blade, from  short distance (2cm-3cm)
6) Read the pulses coming off the phototransistor

Something like that.
23  Using Arduino / Project Guidance / Re: motrcycle turn signal on: April 30, 2013, 04:28:00 pm
Generally you will need to provide some sort of voltage divider for each signal you want to bring to the Arduino.  To determine the values of the resistors you'd first need to know the maximum voltage your bike's electrical system will get up to (but just a 10K trimpot would do).  What are you wanting to do with the Arduino?
24  Using Arduino / General Electronics / Re: IR Beam Interrupt on: April 30, 2013, 07:25:47 am
This project does something similar, It has an IR Transmitter fitted and the Arduino looks for the presence of the IR to count a lap

Seems to be the wrong link?  It's about controlling multiple servos from a RC transmitter.
25  Using Arduino / General Electronics / Re: IR Beam Interrupt on: April 30, 2013, 07:23:45 am
Hi BillO,
I really thank your information. In my case, I imagine first I need to know a little bit more, I only need to measure "the interruption". But as you say, power need to be considerated┬┐ The entrance where the door is located, only has 2 or 3 meters.... I understand there will be two parts, the transmitter and the receiver, right┬┐

Thanks so much,

Yes, you will need a source and a receiver.  The source could just be an IR LED with a current limiting resistor.  They come on various continuous power ratings (30mW to 100mW are easy to find).  That circuit given to you is for a typical receiver.   For 3m you would probably need a more powerful source and/or a more sensitive receiver.  Please do some research via google.  There is a lot of information out there about this.
26  Using Arduino / General Electronics / Re: How to get a 3.6V supply from 5V? on: April 28, 2013, 08:14:27 am
With the zener and a 15 ohm resistor you'd be burning off nearly a watt of power.  I'd strongly suggest the voltage regulator circuit (first one) that runaway pancake suggested.  Use 4.3V-4.4V zener.  The rest of the values look good but you could use any resistor from about 500 to 2K and the cap could be almost any value (10uF - 330uF, whatever).
27  Using Arduino / General Electronics / Re: Position of Back EMF diode on Relay on: April 27, 2013, 11:18:03 am
If the relay is remote from the driving circuitry, I usually try to solder the diode directly across the coil of the relay, regardless of whether the drive circuitry also has a diode.  Without the diode at the relay long wires will cause a lot of broad spectrum radiation which is not only (strictly speaking) illegal but undesirable too.

I know a guy that used a large relay to control some fish tank heaters for a large tank,  He had the relay about 3 feet from the controller and did not have a diode across the coil at the relay.  However, there was one in the controller.  The radiated "pop" was so bad it would cause his wireless phones to disconnect if they were used in the same room.  Sure, it may be that his wireless phones were cheap, but the problem went away after we soldered a 1N4001 across the relay coil.  $0.10 solution.
28  Using Arduino / General Electronics / Re: How to get a 3.6V supply from 5V? on: April 27, 2013, 11:00:05 am
I've tried putting 2 diodes together which should drop 1.4V in theory but in practice it was all over the place, anywhere between 4.5V and 3.0V depending on what the device was doing.

Wow, this device sure has variable current needs.

Before anyone can give you truly meaningful advice, we'd need to know a little more.  Can you tell us:

1) What the device is (link to spec sheet would be nice)?
2) Voltage tolerance of the device?
3) Current requirements of eh device?
4) Any other hard facts that might be pertinent to your use of the device?
29  Using Arduino / General Electronics / Re: Begginer Electronic Book? on: April 19, 2013, 09:47:05 pm

I gotta laugh about this, sorry.  In another post recently it was suggested that a newbie guide might be a good idea, however, the idea was soundly squashed.  Now here we have someone looking for just such a thing.  (Sighhhh...)

Oh well.  Try  They have a on-line book about electronics there.  It's not specific to what is done here, and you'd have to dig pretty deep to find the basic rules and pertinent tips 'n' tricks, but most of what you need to learn is there, plus a ton more.

30  Using Arduino / General Electronics / Re: Best way to control the direction of motors on: April 19, 2013, 09:31:39 pm
Why not consider something like a L293D?  They can handle the current you are talking about and are pretty cheap.  They were designed for just this sort of application.
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