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Topic: servo and mosfet (Read 526 times) previous topic - next topic

Trickyrick

Jul 20, 2018, 11:28 pm Last Edit: Jul 20, 2018, 11:31 pm by Trickyrick
Im trying to power a 5v servo through a IRF630 N channel mosfet
When pin 22 of arduino goes high its connected to the Gate of the mosfet.
Not going to well
Using Mega 2560
Servo is MG996R 55g Gear Servo Motor


AWOL

Quote
Not going to well
Did you mean drain?
Gate?
Source?
"Pete, it's a fool (who) looks for logic in the chambers of the human heart." Ulysses Everett McGill.
Do not send technical questions via personal messaging - they will be ignored.
I speak for myself, not Arduino.

Coding Badly


Is that a logic level MOSFET?

Quote
Not going to well
How else will you get water?

Or make a wish?


Trickyrick

#3
Jul 20, 2018, 11:35 pm Last Edit: Jul 20, 2018, 11:39 pm by Trickyrick
I tried looking that up
I cant tell if it is a logic level
IRF630
Data sheet for the mosfet
https://4donline.ihs.com/images/VipMasterIC/IC/PHGL/PHGLS20230/PHGLS20230-1.pdf?hkey=EF798316E3902B6ED9A73243A3159BB0

Coding Badly


Wow.  I realize Google is occasionally not available but such a thing is becoming so rare as to be nonexistent.  I am sorry to hear about your internet troubles. 

When Google is once again available for you this will be helpful...

https://www.google.com/search?q=%22IRF630%22+logic+level

...the fourth hit has the answer.


Trickyrick

I saw a few of those web sites Im not sure how to tell if a mosfet is logic level

Wawa

#6
Jul 21, 2018, 12:23 am Last Edit: Jul 21, 2018, 12:24 am by Wawa
No, the IRF630 is not logic level.
But you shouldn't switch a servo low-side anyway.
By switching (disconnecting) the ground wire of the servo, ground becomes 6volt.
And the control wire, assuming it's connected to the Arduino, becomes negative 6volt.
I don't think that's healthy for that control wire and/or the Arduino pin.

Try switching high-side (disconnect 6volt power), with a p-channel fet.
Third diagram here.
Assuming you force the Arduino control pin LOW before disconnecting power, otherwise you still have problems.
Leo..

bluejets

Why turn servo power on and off in the first place?

Trickyrick

Thanks Wawa
To answer Bluejets question.  The servo's will be at rest for long periods of time before use so I thought that I could prolong there life by cutting power when there not in use.  I noticed that one out of three would make a lot of humming noise when not in use.
Wawa can I not do the same thing with an N channel (arduino pin high mosfet on)

Wawa

Wawa can I not do the same thing with an N channel (arduino pin high mosfet on)
No, unless you have >12volt available.
The gate needs to be 5-10volt higher than the source to turn an n-channel fet fully on.
And the source is servo supply (6volt).
Leo..

Trickyrick

Thanks Im going to use a FQP30N06L N channel mosfet it only requires 4 volts for the gate to be open
Thanks for your help

Wawa

I'm affraid you don't understand a source follower.

To turn this n-fet on, you need 4volt more than the source, which is servo supply.
So you need >10volt at the gate if servo supply is 6volt.

Use a p-channel fet for high-side switching.
Leo..

bluejets

#12
Jul 23, 2018, 01:26 am Last Edit: Jul 23, 2018, 01:28 am by bluejets
Thanks Wawa
To answer Bluejets question.  The servo's will be at rest for long periods of time before use so I thought that I could prolong there life by cutting power when there not in use.  I noticed that one out of three would make a lot of humming noise when not in use.
Wawa can I not do the same thing with an N channel (arduino pin high mosfet on)

Humming noise would more than likely be external load pushing against the servo mechanism and the servo trying to correct.
Either that or elcheapo servos.
I would not imagine a stationary servo would be a large consumer of power or that any life would be extracted due to it being on.

Trickyrick

Sorry Leo
I posted over on electronics.stackexchange

https://electronics.stackexchange.com/questions/386966/mosfet-with-arduino

I got a different answer there so I'm not sure what to do.
This is one of the replies I got there

"You also suggest an alternative of a FQP30N06L. Looking at the equivalent diagram for that MOSFET, note that the bottom line on the chart is for 3V... it doesn't work particularly well for 3V (although it still manages to handle 10A, but that's substantially lower than it can manage for just slightly higher gate voltages), but for 3.5V it's doing reasonably well, and by 4.5V it's managing to handle approximately 14A for V_DS > 1.2V. This will work fine for your application."

So I take it that this answer is wrong because there not taking into consideration the voltage Im using to operate the sero 5v

Wawa

Bottom line is that you can't use an n-channel fet (FQP30N06L) to switch high-side (the supply wire).
At least not without additional circuitry.
Leo..

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