Problem: Motor always activated with mosfet

Thread Starter

isfan

Joined Nov 1, 2020
5
Hello, good morning, to see if someone can see the diagram that I attached and can you tell me if you see any problem?

I have a mosfet to turn a small 3.3v motor on and off

Vcc and gnd power supply is 3.3v

the question is that the Motor + and motor- pins are giving me whenever the motor is on
when I set the Motor-on pin to 1 (3.3v) the motor seems to pick up more power, when I set it to 0 it stays on ...
The idea would be to turn off the engine or turn it on by applying a 0 or a 1 ...

any ideal?
 

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ericgibbs

Joined Jan 29, 2010
13,597
hi isfan,
Welcome to AAC.
If you disconnect pin 7 or remove the 1k does the motor still run, if yes, check the MOS pin out wiring., or the MOS has failed.
Do you have a protection diode across the motor.?
E
 

Thread Starter

isfan

Joined Nov 1, 2020
5
Hello thanks for the quick response
If pin 7 is open, I have disconnected it and the motor is still active ...
The motor does not have a protection diode.
 

Thread Starter

isfan

Joined Nov 1, 2020
5
around 200mA, also the question is that it is always active, I put a led instead of a motor and it is always on.
I will have to review some things.
 

ericgibbs

Joined Jan 29, 2010
13,597
hi,
I guess you know a motor when starting or stalled can draw upto 5 to 10 times the normal running current.?
Also you must have say a diode across the motor that will provide over voltage protection for the MOS, when the motor is switched Off.

I would say your BS138 is damaged.
E

Edit:
Measure the resistance of the motor.?
 
Last edited:

Audioguru again

Joined Oct 21, 2019
3,509
A melted Mosfet is turned on all the time.
Not enough gate voltage to turn on the Mosfet completely causes the tiny Mosfet to get too hot.

The Mosfet used was a bad choice. There are tiny Mosfets that conduct a few amps when the gate voltage is fairly low.
 

Alec_t

Joined Sep 17, 2013
12,198
Assuming your MOSFET is connected with the correct polarity it looks like it has a drain-source short-circuit (because it didn't have a protection diode across it). Bin it. Replace with a beefier device having a lower Rds(on).
 

Thread Starter

isfan

Joined Nov 1, 2020
5
hello thank you all for the answers the problem was the damaged mosfet ... once everything was verified ...

At 0.22A, assuming it is a maximum as a parameter to follow, with those 7ohms, it would be at 1.54W of which 0.36W at most the package supports.

Anyway ... surely the MOSFET is dead. The biggest problem is that we do not know the consumption of the engine.
But 220mA is already very little for any motor, and if it still did not reach that value, and it was only half 110mA (as a protection margin and having 2ohms), at the moment of making force (if you want a motor) the current would trip and goodbye MOSFET
 

ericgibbs

Joined Jan 29, 2010
13,597
hi isfan,
Do you have a resistance value for the motor.?
That would give you an indication of the Start and Stall current for the motor, I realise the BS138 has a typical high Rds On of 1 Ohm, which should be added to the motor resistance.

E
 

ericgibbs

Joined Jan 29, 2010
13,597
hi isfan,
Clip from ebay spec says =< 400mA running current, so at start or stalled it could be 5 times that value, ~ 2Amps.
And if the BS138 Rds On is 1R , W= I^2 * R = 4A * 1= 4Watts, which could enough to damage the BS138, even if applied for a short time.

Use a better Logic level ,MOSFET.

E
 

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