[SOLVED] Switch on a BJT which switches on a MOSFET

[Just-s]

Note that I don't have any experience in electronics so general tips and explanations are also appreciated.

The goal is switch a 5v 1A DC motor with a logic level signal(eventually a microcontroller) powered by a 5v power supply.

To do this I originally got a IRF740 MOSFET. Since then I learned that at 4.5v Vgs it merely begins to switch on, but it is not enough to allow a 1A drain source current.

I know there exist "logic level MOSFETs" which are made exactly for that, but I am trying to work with the components I currently have.

I also have some BJTs and while none of them are capable of a 1A collector-emitter current I thought it should be possible to switch to switch the MOSFET with it.

The idea is: the MOSFETs gate is connected to the BJTs(2N3904) collector. The low current 3.3V input voltage is connected to the BJTs base. Emitter is connected to common negative rail.

However the MOSFET does not seem to be switched on. If i replace the MOSFET with an LED it lights up, so I'm confident that i got that part of the circuit right.

Additional questions:
1. When the BJT is off, I measured the collector/emitter voltage, it was around 4 volts. Is it normal? Can a voltage exist without current?
2. As I understood, the MOSFET is a voltage operated device, does that mean the current at it's gate doesn't matter at all/is not necessary?

 

[barry]

Hi,

I'm so sorry for the OP for all the off topic nonsense written here.

The only thing I can agree just is: use a diode in parallel to the motor to suppress a rather low energy pulse at switch OFF.

Most written stuff only applies when a motor is
* driven by half bridge or full bridge (which is nit the case here, we just have one transistor to GND)
* and additionally electrical braking is used

Klau

Some people just feel the need to show off their knowledge, whether it’s pertinent to the issue or not.

There is something called elegance in engineering which implies ingenuity and simplicity. Some folks here are solving problems which have nothing to do with the OP’s original question: how to turn on a 5V 1Amp motor from a uP.

Using a voltage doubler is elegant (ingenious and simple); using the proper MOSFET is more elegant. The rest of the stuff here, not so much.

 

[D.A.(Tony)Stewart]

Some people just feel the need to show off their knowledge, whether it’s pertinent to the issue or not.

There is something called elegance in engineering which implies ingenuity and simplicity. Some folks here are solving problems which have nothing to do with the OP’s original question: how to turn on a 5V 1Amp motor from a uP.

Using a voltage doubler is elegant (ingenious and simple); using the proper MOSFET is more elegant. The rest of the stuff here, not so much.

I guess the assumptions need to be explained for the few and the oversight on a property flyback diode are common even with experts. The other day I read a student used a 1A diode for 1A 5V motor which was > 1kV spike without it *The logic level level MOSFET switch has no protection for this.* He was using 450 Hz PWM to control the acceleration and start current to keep it below 5A otherwise his power supply would shut down. That worked all year then had the brilliant idea from someone to make that silent to make that 15 kHz and then Diode went up in thick smelly epoxy smoke , which of course is carcinogenic at half speed.

FWIW He was using the proper 8 mohm logic level NFET and a good diode. 1N4005.

Barry et al. What was the problem ? First it was just a switch , then a variable speed switch.

Most people make too many assumptions that it's just that simple with unknown kinetic energy and capacitance..

 

[barry]

Ok, I'll play your game.

First of all, the OP is NOT talking about PWM. Secondly, you don't seem to accept that fact.

The average current through the flyback diode is much greater with a 15KHz PWM than with a system where the motor is turned on and off very infrequently, which is presumably what the OP has in mind. And THAT'S why the diode failed with the higher frequency PWM. Which is irrelevant to this topic.

This is like someone saying 'I want to ride my bike through the park' and you say 'You shouldn't do that because I know someone who rode their bike on the freeway and got crushed by a semi.'