What modification is needed for this circuit work correctly?

[kyru27]

I've mounted the circuit whose schematic and simulation of execution is shown in the video inside the zip I'm attaching.

According to the place I got it from it should make the motor to work when 5 V are being inputted and make the motor to not work when 0 V, but as seen in the video the motor works all the time.

It's not possible for me to connect with anyone who did that schematic, so I'm posting it here to see if anyone can guess what could be faulty.

I'm more of a programmer than an electronics engineer (in fact I don't have the slightest idea about how an unipolar transistor works), so if possible I'd rather get what connections should be added or eliminated in a straightforwad way than being guided about how unipolar transistors work so I can design the circuit myself, but if necessary I'll stick into that.

Regards.

 

[Diver300]

You need to connect the reference for the 0 / 5 V signal to the negative of the battery. Otherwise the circuit is OK

It seems a strange choice of MOSFET. It is rated at 240 V and 0.9 A. The on-state resistance is over 4 Ohms, which is a lot for driving a motor.

If you have a MOSFETs with lower voltage rating, a higher current rating and lower on-state resistance would be available for the same money.

 

[kyru27]

I'm sorry, I put a wrong transistor, the "44" in its name and the fact that it worked in simulation the same as for real one brought be to error and I ended putting that one, anyway inputting the 5 V reference to the negative didn't change the result of the simulation, although it may be because what is failing is the simulation itseld.

The real transistor is a AUIRRFZ44N (https://www.electro-tech-online.com/custompdfs/2012/08/auirfz44n.pdf), That transistor is not in the program shown on the video or in proteus... so I'm not able to simulate it for the moment.

Where would the circuit be needed to be modified to achieve this behaviour I told at first with this new transistor?

Regards.

 

[Diver300]

How much current does the motor need?

The problem with the AUIRRFZ44N is that the gate threshold voltage is quite large, 2 - 4 V, so your circuit does not provide enough voltage to be sure of turning it on completely.

 

[kyru27]

How much current does the motor need?

The problem with the AUIRRFZ44N is that the gate threshold voltage is quite large, 2 - 4 V, so your circuit does not provide enough voltage to be sure of turning it on completely.

Nothing is said about current in its "datasheet" which I append, guess it can work with very low current.

Anyway if you tell me how to connect it, I'd buy one transistor with less gate thresold to solve the problem.

Thanks.

 

[Diver300]

A motor like that will only take about 1 A, so you don't need more than about 4.5 V to turn on the AUIRRFZ44N. However, your original circuit will only supply about 3.4 V to the gate of the MOSFET when the signal is at 5 V.

I suggest that you change the 10k resistor to 1M.

 

[kyru27]

I'll give it a try when I buy the resistor.

Anyway I guess as you aren't telling anything more about modyfing the circuit, the one shown on video + the 0 or 5 V input on the negative of the battery would be right (barring the 10 K resistor which will be replaced by the 1M one and the different transistor model). Would it?

Thanks.

 

[Diver300]

The basic circuit from the video seems correct, as long as the reference for the 0 or 5 V is connected to the battery negative. Replacing the 10k with 1M will mean that the gate voltage is almost exactly 5 V when the control voltage is 5V

 

[Boncuk]

The real transistor is a AUIRRFZ44N (https://www.electro-tech-online.com/custompdfs/2012/08/auirfz44n.pdf), That transistor is not in the program shown on the video or in proteus... so I'm not able to simulate it for the moment.

Where would the circuit be needed to be modified to achieve this behaviour I told at first with this new transistor?

Regards.

The transistor you selected is available in Proteus ISIS. Here are types available: IRFZ44E, IRFZ44N, IRFZ44NS, IRFZ44Z, IRFZ44ZL, AND IRFZ44ZS. They just differ in package.

To get a 9V motor to run at 9V is a bit difficult using a standard type MosFet transistor. MosFet transistors conduct fully at a VGS of 10V. If you want the motor to run at 9V it won't attain full rpm at all.

Your next better choice should be a 2N3055. Just put the base voltage directly on the base pin using a pot. Depending on the motor requirements you might use another NPN transistor like BD139 which can be controlled directly as well.

Boncuk

 

[Boncuk]

I'll give it a try when I buy the resistor.

Anyway I guess as you aren't telling anything more about modyfing the circuit, the one shown on video + the 0 or 5 V input on the negative of the battery would be right (barring the 10 K resistor which will be replaced by the 1M one and the different transistor model). Would it?

Thanks.

You might as well forget about the 10K (1M) resistor. It's purpose is quick discharge of the gate.

Boncuk

 

[Diver300]

The downside of using a 2N3055 is that the gain is low. You may need a lot of current to drive the base, and I don't know how much current the 0 / 5 V source can provide. If it is going to be a microcontroller output, the current can't be more than around 10 mA, and less for some microcontrollers. The 18 Ω resistor means that the circuit is looking for 240 mA.

The advantage of having some resistor in the 10K position when using a MOSFET is that the resistor will cause the MOSFET to turn off if the 0 / 5 V source is disconnected. Without the resistor, the gate voltage will stay at what it was last left at, or leakage currents could take it to any voltage, and the motor behaviour is unpredictable when the 0 / 5 V source is removed.

There is no problem running a MOSFET with a 9 V supply. Although the specifications of many MOSFETs are quoted at 10V Vgs, there is hardly any loss of performance at 9V. The problem with low gate voltages comes from non logic-level MOSFETs, such as the AUIRRFZ44N, where the turn-on threshold is quoted at 2 - 4 volts. They are not fully on at 5 V, although probably enough to run that small motor. The original circuit with the 4.7kΩ and the 10kΩ resistor would have only provided 3.4 V, so the AUIRRFZ44N might not turn on at all.

If a logic-level MOSFET is used, there is no problem at all using a 5V Vgs, because the turn-on threshold is smaller, maybe 1.5 - 2.5 V, and logic-level MOSFETs are fully on with a 5V Vgs.