Boost Converter Voltage Drop with MOSFET

Using Arduino General Electronics
1

Hi,
I'm trying to run a motor on software controlled speeds. I've figured out, I can easily achieve this by using a MOSFET and a PWM signal.
The motor is quite small. I've figured out that without load on 5V it needs about 120 mA when spun up. My idea is to provide either 9 or 12 V to it and control the speed through the PWM signal. I want to get the power from the USB port. I assume it should supply enough power for the motor with it's small load even on 12 V.

I bought a boost converter to get my 12 volts. It's connected to VIN and GND of an Arduino Nano (compatible). I think that's the right approach to avoid putting stress on the voltage regulator. A future idea might be to connect a battery so my device is portable.

The MOSFET is an IFR520 (the usual MOS Module). Let's assume I'm providing a PWM signal of 128/256 pulses so the motor runs at 6V effectively. The voltage at the terminals looks fine when nothing is connected. My multimeter shows 12 Volts. But when I wire up the motor the voltage drops to about 0.3V which is far less than the motor needs to even get moving.
Again without load I can observe that the MOSFETs V+ and my USB's GND give me the same voltage so I suspect that there's something I don't understand about the boost module and the MOSFET's gate. When I connect a few batteries on the as source for the MOSFET, it works fine. That's why I think that I have a misunderstanding how the MOSFET works when it's switching the potential of its own circuit (the boost module isn't isolated). Is it even possible? Another general question about the IFR520: How does it even work, when I didn't close the loop of the circuit when I was using the battery? As far as I can see the GND of the MOS module is only used to close the circuit so the integrated LED can work.

Here's a drawing of my setup:

2023-12-25-Notiz-23-45
2023-12-25-Notiz-23-452152×828 142 KB

With future sight I've considered to immediately boost the voltage of the battery output to 12V and connect that to VIN of the Arduino so the voltage regulator converts it back to 5 volts and I can just say my device runs at 12V (so I could also just use any 9-12 V DC power supply).

Please help me finding my mistake.

2

Every time they start moving, brushed DC motors typically draw up to 10X the free running current. In your case, the startup/stall current could be 1 Ampere or higher at 5V. With a 12V power supply, it will be over 2 Amperes. The power supply must be able to easily handle that.

Furthermore, the IRF520 is not a logic level MOSFET, and cannot be fully turned on by an Arduino output. Use a logic level MOSFET instead.

2 Likes
3

Wow! Thanks for the quick reply!

Every time they start moving, brushed DC motors typically draw up to 10X the free running current. In your case, the startup/stall current could be 1 Ampere or higher at 5V. With a 12V power supply, it will be over 2 Amperes. The power supply must be able to easily handle that.

Stupid short test: 5V and help the motor "starting" with my fingers. If the voltage of a fan is high enough, this can be enough to keep it spinning. I've cross tested it with a single AA rechargeable battery. It is strong enough start the motor. It should be able to provide 2A, so yeah. Understandable.

Furthermore, the IRF520 is not a logic level MOSFET, and cannot be fully turned on by an Arduino output. Use a logic level MOSFET instead.

I'm not sure, why the IFR520 is sold as Arduino module if it wants up to 20V on the gate. I've read about that. But I'm still wondering why it works fine when I use it to run the motor from a different current source.

I think, you're right about the current. I just got confused because of the the measurement between the boost modules V+ and my Arduinos GND which is very similar to the voltage I can measure between MOS Module's V+ and V-.

If you say, all my further thoughts can be binned and it's really just about the current, I'll have to reconsider my power source.

4

The sellers may not know the difference, but if they do, they plan on buyers that don't.

2 Likes
5

This topic was automatically closed 180 days after the last reply. New replies are no longer allowed.