 # Confused about DataSheet, need help find a cure to a problem.

I am glad I am making progress. Thank you both for the answers, so based on that lets see if I got PaulsC problem right this time.

I am still unsure about the voltage across the LED and ground.

But here we go. Essentially there is 12V across the Source and ground, which means that if I am powering the mosfet with 12V there is a difference of 0V which is below the threshold Vds(th). This Mofet will never turn on. I would need at least 14V to make it start to turn on which is not ideal. please please correct me if I am wrong or please tell me if I am right. I am still unsure about the voltage across the LED and ground.

If I am correct, I would like to post a schematic in bit to make sure I am on the right track if its okay with you guys.

Seems to me you are using the wrong type of MOSFET there.
N-channel MOSFETs “like” to sink current from a load to Gnd.
P-channel MOSFETs “like” to source current to a load (to Gnd).

N-channel MOSFET can be turned fully on by Arduino if they have Logic Level gates (Vgs <= ~4.5V)
P-channel MOSET needs gate pulled to Vs (12V in this example) to turn off, and gate is pulled low (Vgs shown as a negative #, as Vg is less than Vs), so a simple NPN is used to perform that while buffering the Arduino pin from 12V.

IRF540 has a “Standard” level gate, needing Vgs = 10V to fully turn on.

Look where Vgs is called - it’s always shown at 10V.

Where Vgs is shown lower:

Page 2, Electrical Characteristics @ TJ = 25°C (unless otherwise specified):
VGS(th) Gate Threshold Voltage, 2V to 4.0V Max, VDS = VGS, ID = 250μA << hardly any current flow

Figure 1,2,3 do show it turning on with Vgs = 4.5V, but not nearly as much as with Vgs at higher voltages, and only brief pulses.

There you go. I think you got it. The LED Vf=2V is just a typical value used in this case to test out a theory. The actual value depends on the LED and conditions in use (found in the datasheet).

One thing for me to point out since others may be confused: The schematic I posted, the values represented, and the voltages calculated by Crullier are for study only! It won't actually work!

Just picking nits here, but the P-channel mosfet looks like this in a schematic: Thank you everyone. I will post a schematic when I get home tonight,

And you know what you are doing... imagine mine. =) Actually I am pretty clear now.

Actually I am pretty clean now.

Why, did you just take a bath ?

hahaha, :D yeah that too.

Which circuit are you building? CrossRoads’? I hope so.

The one you gave here won’t work:

It will require you to drive it with 17V if it is a logic level MOSFET, or 22V if it is a 10VGS MOSFET. That’s because the gate voltage must be 5V (logic level) or 10V (regular) above the voltage on the Source.

polymorph:
Which circuit are you building? CrossRoads’? I hope so.

The one you gave here won’t work:
http://forum.arduino.cc/index.php?topic=295165.msg2066072#msg2066072

It will require you to drive it with 17V if it is a logic level MOSFET, or 22V if it is a 10VGS MOSFET. That’s because the gate voltage must be 5V (logic level) or 10V (regular) above the voltage on the Source.

those schematics were provided to me to help me understand what you just posted.

Hi guys, been waiting or some components (mosfets) to make this work as well as a darlington array as an alternative.

I came a cross the schematic below and did some reading and watched some videos on the zeener diode below. I understand that R1 is limiting the current for the LED inside the opto, and I also understand that R2 sets to voltage drop for the Zenner to work and let only 4.7v pass to the opto. However I have not been able to find information on how to "size" that resistor or how it related to the zeener (other than what I mentioned).

In this circuit Vcc is 12V. Also,

1- I need some guidance with respect to a schematics drafting software with simulation. The past week and I have been using LTspice which is free, but does not have many components and seems very "manual" to use. Is there an industry standard or something a little bit more advance even if its a paid version?

2- I noticed that many times when using a transistor to turn on another transistor, depending on what type of transistor the 2nd transistor is (NPN, or PNP) it needs to be either placed at the collector or emitter of the first transistor. Is this a correct statement?

I noticed that many times when using a transistor to turn on another transistor, depending on what type of transistor the 2nd transistor is (NPN, or PNP) it needs to be either placed at the collector or emitter of the first transistor. Is this a correct statement?

In a vague hand waving sort of way it is. But it is not very rigorous and open to misinterpretation.

Is there an industry standard

Yes it is called spice.

how to "size" that resistor or how it related to the zeener

You need to run the zenner at some current to get well into the knee region. You can get this current from the curves in the data sheet. Then you need some current for the LED. Add these currents together to see what needs to flow through this resistor. Then the voltage across the resistor is 12V - 4.7V = 7.3V So you know the voltage and current therefore you can calculate the resistor value from ohms law.

The calculate how much power will be dissipated in the resistor which is V times I ( current ) and choose a resistor of roughly twice that rating.

Grumpy_Mike: In a vague hand waving sort of way it is. But it is not very rigorous and open to misinterpretation. Yes it is called spice. You need to run the zenner at some current to get well into the knee region. You can get this current from the curves in the data sheet. Then you need some current for the LED. Add these currents together to see what needs to flow through this resistor. Then the voltage across the resistor is 12V - 4.7V = 7.3V So you know the voltage and current therefore you can calculate the resistor value from ohms law.

The calculate how much power will be dissipated in the resistor which is V times I ( current ) and choose a resistor of roughly twice that rating.

....But it is not very rigorous and open to misinterpretation...

Can you elaborate a little?

Yes it is called spice.

I guess my question is: Is there an industry standard Spice software? for instance in my professional field we have autocad (well there is better stuff out now.)

You need to run the zenner at some current to get well into the knee region. You can get this current from the curves in the data sheet.

So you are saying it is reverse current flow that makes the zeener reach its zeener voltage, as opposed to that I thought I understood which was a specific reverse voltage. got it.

I think I got it Mike,

Check this out. I looked up the sheet of the zeener. I used 0.200 W found under the testing condition and went on from there.

I am unsure on your last statement:

The calculate how much power will be dissipated in the resistor which is V times I ( current ) and choose a resistor of roughly twice that rating.

W = 12V * 0.044mA I get aprox 1/2W. Are you saying then to get a 1W "166omh" resistor (I need to learn standard values sorry).

200 W is way way over the top for running an opto isolator.

Yes if you are going to burn 0.25 W then get a 1/2 watt resistor.

There are many ways to interprate those words about transistors. You have to specify how you are connecting them, your words were ambitious and can only be interprets if you know the soloution in the first place.

Sorry it was a typo, the cutsheet says 200mW

Well If I got the math for the power dissipation I get 1/2W and if I double that I need a 1W resistor. Does that seem right? (sound too big to me)

Is there a problem with 12V? Is it actually a range of voltage?

If it is regulated, why use a zener? Just size the resistor for 12V and the LED in the optocoupler.

(12-1.5)/0.015 = 700 therefore 680 ohms

P = V^2 / R P = 10.5^2 / 680 = 162mW therefore 500mW or 1/2W resistor because you -always- use no less than double for resistor ratings. Or use a couple of 330 or 360 ohm resistors in series and you can use 1/4W.

If it is regulated, why use a zener? Just size the resistor for 12V and the LED in the optocoupler.

Thank polymorph, Yes its regulated. What you are saying make perfect sense. In reality the voltage can be between 12v and 13.4v but should not make much of a difference.