# Is there a connection between Drain-Source voltage and Gate voltage?

I have an IRF530 n-channel MOSFET with me which I intend to use to control speed of a 450mA motor from my AVR controller. The controller can either be that can either run at 5V or 3.3V. I intend to PWM the voltage at the gate of MOSFET to see how the speed of motor reacts to that.

But before doing that just to do a sanity check I hooked up the power supply module to my breadboard that supplies both 5V and 3.3V. I checked the current rating of the power supply module and it said it can deliver up to 700mA of current. So the power from rail (5V) goes to motor, from there it goes to my multimeter and then at the drain of the MOSFET. The source of the MOSFET was connected to Ground. I then gave a stable 5V to the gate of the MOSFET. The current flow stabilized at around 280mA. I then changed the voltage at the gate to 3.3V (drain to source voltage stayed at 5V). The current flow stabilized at 240mA this time. If I remove the MOSFET and just directly connect the motor to the power supply, the current flow stabilized somewhere around 360mA.

Now I have this question: 1. When the supply is capable of delivering up to 700mA why just ~300mA flow through? The power supply module is fed by a wall wart adapter which says maximum current of 500mA. But still it could go higher but it didn't? IRL530 datasheet says it can deliver up to 22A of current so the limiting factor shouldn't be the MOSFET. The graph in the IRL530 datasheet says that at 5V gate voltage and 5V drain-source voltage close to 22A should be able to flow through. But I am new to MOSFETs. Is there a connection between the gate voltage and drain-source voltage? What is limiting the current flow here?

The problem is that 3.3V and even 5V is not enough gate drive voltage to turn the IRF530 on to the desired degree to run a motor, it requires around 10V to be fully on. At 4V you can count on only 250 microamperes and to get the advertised Rds(on) of 0.16$$\\Omega\$$ (at 25°C, more when hot) you should give it 10V.

Choose another MOSFET- a "logic level" type that has specified Rds(on) with your desired drive voltage (or less Vgs would be okay too). Refer to the datasheet:

• Do logic level MOSFET come for power usage? Could you recommend any so that I could have a look at datasheet to understand basic differences. I am very new to MOSFETs so don't know any commonly used part numbers – A. Munir Apr 6 at 19:48
• IRLZ44N is suitable for 5V drive. You should do a parametric search at a distributor such as Digikey for Rds(on) and the voltage at which it is specified then drill down to datasheets. There are a lot of logic level drive MOSFETs, but most of the ones suitable to your type of application are surface mount types. You can even find 2.5V or less types in SMT. – Spehro Pefhany Apr 6 at 19:51

If I remove the MOSFET and just directly connect the motor to the power supply, the current flow stabilized somewhere around 360mA.

Now I have this question: 1. When the supply is capable of delivering up to 700mA why just ~300mA flow through? The power supply module is fed by a wall wart adapter which says maximum current of 500mA. But still it could go higher but it didn't?

To add to Spehro answer, If you remove the MOSFET and directly connect to the power supply, the power supply is a constant voltage source. Thus the motor/loads will determines the current. If you put a load on its rotor, such as your hand, the current will be increased.

There are many types of motor modelling which help you understand better its relation against the current.
For a typical Permanent DC Brushed Motor with Load, you can read here: "LESSON 14: TRANSFER FUNCTIONS OF DC MOTORS ET 438a Automatic Control Systems Technology"

Those are some screenshot of the slides. It is the transfer function of motor speed per armature voltage. The current relation is intrinsically exist in the transfer function, explained in earlier slide. It will be explicitly clear if state-space model is used instead (with current as one of its state variable).