Amplify Square Wave with Simple Mofset Circuit

Question

I am a hobbyist. I have a square wave being generated by a 5 volt Arduino. (I tried a 555 circuit but I couldn't get it at an exact square wave.)

The Arduino square wave runs from about 2 or 3 volts to 0 volts. I want to amplify that positive pulse to 12 volts with a mosfet (741) to pulse an electromagnet. The electromagnet is a 12vdc and should run at or below 250 ma. I tried the following setup but am getting a weird signal, kind of like a square wave with rounded shoulders at around 1 volt and a peak in the middle that goes up to about 7 volts. I expected to see the exact same square wave that went in, but coming out at 12v+ instead of 3. am I doing something wrong?

Edit: Do you think the above circuit is correct?

Answer 1

Strange circuit: -

If it is a P channel MOSFET then that's OK because source connects to +12 volts (via a resistor whose value is unspecified) and the drain connects to the load BUT, to properly drive the gate, the gate levels have to be 12 volts (MOSFET off) or gate at say 6 volts or less to turn the MOSFET on. Clearly this is not achieved with the Arduino output.

If the MOSFET is N channel then the body diode will conduct and the load will be permanently powered by about 11 volts minus the volt drop across the resistor in series with the source.

Neither will do what I think you want.

Answer 2

The gate voltage drive from your microcontroller (0-to-5v) is perhaps too feeble to properly drive the IRF741 MOSfet's gate fully ON:

Although the spec sheet only specifies +4v on the gate, you may only get a fraction of a milliamp current through the MOSfet, as well as your load. At +5v gate voltage, you'll get more current, but you don't know how much. With +10v on the gate, the MOSfet is guaranteed to be fully ON, and current flow through your electromagnet will be limited by the electromagnet's resistance. You'd be safer using a MOSfet classified as logic-level, instead of this one, since your microcontroller can only supply +5v.

Since you are driving an inductive load, you must also include a diode to catch the huge flyback pulse voltage when you turn the MOSfet off. Your MOSfet can only withstand about 30v, and the flyback pulse will likely be many times that voltage for a short instant....but an instant is all it takes to break down your MOSfet. The diode goes across the electromagnet, with cathode towards the +12v supply.

Answer 3

Not sure how your circuit is wired. As the second one is drawn, -12V is on the Drain. If you flip 12V battery around, it should work.

I'd use the circuit from Switching Test Circuit (Figure 9). Switch your arduino output for pulse generator.

Add a \$R_{GS}\$ to bias it off, while Arduino is being configured.

Edit...

The Duty Cycle of 555 timer is:

$$ D = \frac {R_B} {R_A + 2 R_B} $$

so you cannot get a 50% duty cycle. There are ways, but you cannot get there just with a 555.

Answer 4

Wouldn't it be against the Human Rights of said migrants if they do restrict free-movement?

Your electromagnet isn't going to care if the square wave has a rise time of 1ns or 1ms. So a 555 will work just fine.

As to a MCU based approach, it will work as well. In your particular case, you will need to make sure that there is sufficient swing from the Arduino output. If the high is 2 or even on a 5v Arduino, there. ISS something seriously wrong. Fix that first.

Then you will need a MOSFET that goes full open on 5v output.

If you don't have one, get a driver then.