Hello,
I want to control a rgb led strip with my arduino. I saw that the best way to do this, is with a mosfets(irfz44 because I have them). But could someone explain to me why there are 2 resistors before the gain input.
How I see it: it is a voltage divider, but why is that?
The mosfet gate is drawn incorrectly. It's too easy to confuse this with a BJT.
Could you explain what is wrong?
Edit:
Do you mean it should be drawn like this?
The 1k is to limit current drawn from your arduino and the 22k is to hold the gate down to ground when the arduino input is open (low)
JackStardust:
Could you explain what is wrong?Edit:
Do you mean it should be drawn like this?
No, it should be drawn like it is always drawn.
Could you be a bit more precise? Give an example ?
The first was a bit BJT like indeed. But the second is okay enough for a hand drawing I would say.
But you can leave out the resistor for switching small mosfets / low gate capacity. The IRFZ44 has some so a resistor is nice. But even here I don't think driving it without resistor will give you any trouble. So 1k is pretty high, 100ohm should work as fine and limits the absolute inrush to 50mA.
The IRFZ44 is not a "logic" mosfet.
Some will work with 5volt drive, others will not.
Leo..
Which mosfet do you advise?
It's indeed not a logic level. But if you only try to drive 1A instead of 50A you should be fine according to fig 2 and fig 3. So if that's the mosfet you have, use it I would say.
Google "Logic Level Fet".
Some have an "L" in the model nr.
e.g FQP30N06L
I agree with septillion, You could be fine with small currents.
Leo..
septillion:
It's indeed not a logic level. But if you only try to drive 1A instead of 50A you should be fine according to fig 2 and fig 3. So if that's the mosfet you have, use it I would say.
No, that's not what happens, if you drive a 10V MOSFET with 5V its on resistance could be anything from very
low to very high, will depend on temperature, age, which batch the device came from - its just
ill-defined and unreliable. The key thing about MOSFET switching is that there is a plateau in the gate
voltage/charge curve, and to switch on you need to be well above that plateau voltage. Otherwise
you are mainly runnning on leakage current really... Every MOSFET datasheet shows a "typical" gate
charge curve - expect the plateau to be about 2.5V for a logic level MOSFET, around 6V for a non-logic-level.
This nothing to do with the threshold voltage, that only defines the hard-off gate voltage.
Use a logic-level MOSFET if you are driving the gate with 5V, then it will work reliably.
If you switch up those resistors you'll get a slightly better result.
As you've shown, the input is part of a voltage ladder reducing the Gate voltage (bias).
In the attached example, that is eliminated.
I would use a value < 1kohm (<< 1 kohm).
It's true you don't pass that plateau. But with 1A on a 50A mosfet I think the losses because it's not 100% switched on are marginal.
JackStardust:
Could you be a bit more precise? Give an example ?
Google "mosfet symbol"
Here's the datasheet, you can see how the symbol is drawn.
http://www.irf.com/product-info/datasheets/data/irfz44zpbf.pdf
