I'm trying to build simple line-following car using mosfet and ldr
in the sim, the drain source is always open, the ldr and vr doesn't affect the gate.
I tried npn transistor and still getting the same problem.
I'm using irf530
the battery is 3-9v
Hi,
as per recommendations, post information so we can help.
For example:
What MOSFET are you using?
What voltage powers this circuit?
What is this component inside the red circle?
it's potentiometer
Will the current that feeds the motor pass through this potentiometer?
What is the motor current and what is the dissipation of this potentiometer?
the pot is part of voltage divider to control the gate voltage which control the point that ldr sense the black line,
the motor current is around 1A when it has load on it
Where is ground tied to the circuit?
Or shall I ask, are you sure a ground path is defined?
What voltage are you applying? IRF530 Vgs th is 4V.
What do you mean?
Where are your gnd and 5V supply points?
Tom.... 
Hi,
What do you expect your circuit to do on the simulation.
Put the mouse cursor on the Gate and read the voltage, do the same on the Drain and the Source
In this case Gate voltage is 3.96Vdc
Change the values around the pot to see what effect it has.
The motor is in series with 470R which will limit the current to about 10mA even if there is no resistance in the other components. I doubt if that is enough to make the motor turn.
FETs are very temperature sensitive so a heat sink is probably required although obviously that depends to some extent on the device. Warm to the touch is enough to cause a significant increase in on resistance. Hot to the touch and the on resistance could be several ohms. There will be a graph on the device data sheet. The absolute maximum current rating is with an infinite heat sink so the maximum current rating needs to be a lot larger, like a few times larger, than the expected current.
Brushed motors generate lots of interference so it is advisable to put some capacitance in parallel with them as close as possible to the motor terminals. I usually use two capacitors in parallel, an electrolytic to short out the low frequency stuff and a disc ceramic for the high frequencies. Other capacitor types are available. Also some capacitance, say 0.1uF disc ceramic, between 5V and 0V close to the electronics.
For my current project I am using an Arduino and with an opto-isolator between it and the motor. If it needed proportional control then I would use one of the mark-space pins to control it. Brushed motors behave better, particularly at low speeds, when controlled that way. That is because the higher voltages in pulses are less affected by the commutator infelicities than is a steady low voltage. Incidentally, that is even more true for model railways controlled by the track voltage. There you have the additional problem of getting current in and out of the wheels.
PS. For trouble shooting measure the voltage from ground to each of the points along the current path through the motor. That will tell you where the problem is.
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