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555 motor PWM speed controller

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boylesg

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I am trying to use this circuit to control my 12V DC engine:
https://i.stack.imgur.com/gv9jz.png


I am using IRD630A as my FET.
According to my multimeter there is a square wave happening with a frequency of about 75Hz and duty cycle varying between 20% and 90%.
However there is not noticeable change in the pitch of speed of the engine.
I suspect the problem is that the FET gate is not discharging between pulses and hence the FET keeps conducting between pulses.
Why? Because at one stage I forgot to put my 555 in the IC holder, connected up my engine and it was running with a floating FET gate. I had to touch the FET gate to GND in order to stop the engine.
So if that is my problem then what do I need to do to make the gate discharge completely between pulses?
 

I recommend you use something better like one of the older
smaller Unitrode parts (UC35xx voltage mode, UC38xx current
mode) which have drivers suited to at least the lower end of
power FET gate characteristics.

A logic level FET might have a hard time turning off with
a 555 timer's output low voltage.

"Suspecting" is not "debugging". 'scope >> conjecture.
 

Hi,

Also, I think it's habitual to place a resistor to ground from the FET gate so it is definitely tied to ground when off instead of pretty floaty.

Have you done the look and learn experiment of having no gate resistor on a PMOS and an NMOS driving an LED or something else very simple? They never turn off. The gate resistor ties them to a ... known "off" voltage level/anchors them off, at least that's how I could explain it.
 

Use a CMOS 555 i.e. a modern one, put a 10k res from gate to gnd, do not allow static electricity to upset the fet or the 555.

The fet needs to be at least 3 x the max motor current, and on a heatsink ...
 

try first by inserting a diode between pin3 and 47 ohm(Anode at pin3).

Then the ground discharging resistor.

If possible change to PMOS FET (in which case your duty cycle will be different).
 

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