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Author Topic: DC Motor speed control  (Read 5527 times)
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It's only necessary to use a mosfet driver if you are switching the mosfet at a frequency much higher than the standard frequency used by the Arduinio PWM pins. So you don't need to use one.
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Okay my project leader told me that i have to somehow theoretically prove that arduino can drive those mosfets. As i understand apart from voltage and current frequency of pwm and capacity of mosfet is also important. What is frequency of PWM? is it clock frequency or something?
Basically he wants some calculations from me. Like after which frequency impendance will be too big and will hinder the signal... not like i understand what this information will give me and how can there be impendance when its dc.
Sorry if some things may sound real stupid...its just that i do not have full understanding yet.
Will PWM frequency and current be enough for 4 motors?
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See http://arduino.cc/forum/index.php/topic,105387.msg792327.html#msg792327 for how to estimate the mosfet switching time, and http://arduino.cc/en/Reference/analogWrite for the Arduino PWM frequency.
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Hi again!
Is arduino able to control 4 motors???
I was testing today the mosfets i got... i did not plug in arduino but started with 3v battery in place of arduino and it did not work, well it also can be because i was in a hurry and could make a mistake, but i think i should ask if it should have worked with battery 3v in place of arduino and 9v on the motor as it was on the scheme...
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Quote
but i think i should ask if it should have worked with battery 3v in place of arduino and 9v on the motor as it was on the scheme...
Does the data sheet for that MOSFET say they should turn-on with 3V on the gate?  Do you have a common-ground between the battery and 9V power supply?
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well i did connect grounds....
and treshold voltage seems to be 2V and current 250uA....
Well im asking since maybe i dont know something that i should...well i am pretty sure that i do not know some things that i should in order to complete this project myself... but i hope that at the end of it i will have learned those things
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If i understand correctly then there should be no problems with  controlling 4 motors, since current that arduino can provide is 40mA and mosfet needs 250uA, but frequency will drop?
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since current that arduino can provide is 40mA
That is the absolute maximum before damage starts. It is recommended, for long term safety, that you don't exceed 25mA.
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1. Threshold voltage is the voltage at which the mosfet starts to conduct, not the voltage at which it is fully on. To test the mosfet switching the motor, you need to apply at least 4.5v to the gate for typical logic level mosfets, and 10v for other types. So a 3v battery is likely to be insufficient.

2. The mosfet does not take 250uA at the gate, that figure is the drain current at which the gate threshold voltage was measured. The gate of a mosfet looks like a capacitor. When the Arduino pin driving it changes state, the capacitor charges or discharges through the pin. The purpose of the series resistor is to limit the peak charge/discharge current to 40mA or less.
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Hi again!
I dont know why...but this does not work...i use as a battery on scheme 9V and in place of arduino 9v battery as well but i tried it with arduino aswell. Battery that is battery on scheme heats up. Well i connect mosfet 1 pin is gate 2 is drain 3 is source as in datasheet. Basically like that http://circuits.datasheetdir.com/97/IRF520-pinout.jpg . I am really frustrated.... 3 days until the day that i have to hand my work over and the scheme does not work....
I made some photos of it if it will help u to help me please use it:D
http://oops.ee/wp-content/uploads/2012/05/DSCN3634.jpg
http://oops.ee/wp-content/uploads/2012/05/DSCN3635.jpg
http://oops.ee/wp-content/uploads/2012/05/DSCN3636.jpg
http://oops.ee/wp-content/uploads/2012/05/DSCN3637.jpg
http://oops.ee/wp-content/uploads/2012/05/DSCN3638.jpg
http://oops.ee/wp-content/uploads/2012/05/DSCN3639.jpg
http://oops.ee/wp-content/uploads/2012/05/DSCN3640.jpg
http://oops.ee/wp-content/uploads/2012/05/DSCN3641.jpg
http://oops.ee/wp-content/uploads/2012/05/DSCN3642.jpg
http://oops.ee/wp-content/uploads/2012/05/DSCN3643.jpg
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That motor looks to me too big for a 9v battery to drive. I think the mosfet is turning on but the battery is unable to provide enough current to start the motor, so the battery is just getting hot. Also the connections made to the battery using sticky tape may not be connecting very well. To check that the mosfet isn't the problem, bypass the mosfet by connecting a jumper wire between the source and drain terminals.

Try using a battery pack made from 4 or more alkaline or lithium (or fully charged NiMh) AA cells to power the motor instead. Driving the mosfet gate from a 9v battery is OK.
« Last Edit: May 19, 2012, 03:27:28 am by dc42 » Logged

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well if i connect motor and battery directly, motor works.... Mybe mosfet applies some resistance?
About sticky tape... we doublechecked it and it outputs right voltage:SSS
In 3 hours ill be able to check everything again... i will write here again when the time comes 
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I just noticed from your photos that your 100 ohm resistor isn't properly connected. One end is correctly connected to the mosfet gate terminal, but the other end is plugged into the wrong row of the breadboard, so it isn't connected to anything.

Remember that mosfets are static sensitive devices, so don't touch that resistor until you are touching the source terminal of the mosfet (or something else connected to ground) with your other hand.
« Last Edit: May 19, 2012, 05:20:45 am by dc42 » Logged

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Yeah.. thx a lot
Sadly it didnt fix the problem...somehow no voltage is going to the drain(collector)
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I can say this much that battery goes dry very fast....can it be that diode is dead? Or why else can batteries voltage drop in some minutes...
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