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Driving power MOSFETs

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danielsmusic

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I'd like to experiment with MOSFETs and bipolar transistors to drive a motor via CMOS chips.
I will be arranging them both in H-bridge configuration.
Ever since I started experimenting with logic ICs I've never actually been able to get a logic chip to drive any sort of transistor. My understanding of a MOSFET is that I will need a higher voltage at the gate than the drain to fully saturate it. Whereas, with a bipolar transistor, you need a few 100mAs...which is out of the question.
So, could I bring them both together and use a MOSFET with a resistor at the source to drive the base of a bipolar transistor? Would it not be possible to use either of them on their own?

All my attempts have failed, even with very small bipolar transistors. The transistors I have are TIP2955 and TIP3055. I've not decided on the MOSFET. I would like to also drive the motor directly with a MOSFET. As I believe that the switching time is much faster (for PWM). The motor is 12V and the chip is driven by 5V.

Could someone give me a crash course with maybe some drawings? I would appreciate it.
 
I'd like to actually experiment with the transistors, I'm not building something I will "use", just something to play with.
 
Ordinary Cmos logic ICs like the CD4xxx family operate perfectly with a 12V supply. Then they directly drive ordinary Mosfets at a low frequency.
If your Cmos is a 5V micro-controller or 5V 74HCxxxx high speed Cmos then logic level Mosfets are needed.

Each Mosfet should have a 47 ohm resistor in series with its gate mounted at its gate to avoid high frequency oscillation.
 
I've got some **broken link removed** to play with.
I'm using 74HCxxx chips.
I will be operating it at a fairly low frequency, around the 100 - 1000Hz range.

Here is my proposed circuit:
 

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The Mosfet you selected will work fine when driven from the 5V output of the 74HC IC. It won't need a heatsink if the max current is less than 9A.
 
I built a nice circuit with just the MOSFETs to drive a motor using this:
A Simple PWM Circuit Based on the 555 Timer
The whining motor made me happy.

Now, I want to use the MOSFET to drive the base of my transistors, they need (i think) 1 amp to saturate. If I wired the source directly to the base would I fry the MOSFET and the transistor at the same time because there's no resistance in the base? If so, how could I limit the current?
 

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Of course the current in the Mosfet and in the base of the transistor will be extremely high.
Why use the transistor when the Mosfet can drive the motor all by itself?
 
Again, it's experiments.I'd like to build a H-bridge, but only have 1 MOSFET and 20 transistors at the moment.

How would I limit the current to the transistor's base without "un-saturating" it? Could I limit the current to the gate of the MOSFET?
 
A 2N3055 transistor has a max collector saturation voltage loss of 2V when its base current is 1A and its collector current is 6A. It needs a driver transistor or two driver transistors.
An H-bridge needs two NPN and two PNP power transistors.
 
What is the max continuous current of the motor in your application?
 
What is the max continuous current of the motor in your application?

I have no idea, all I could say is that it's quite hefty. A single STP55NF06L MOSFET can drive it without a load and it doesn't get warm.
 
Why are you using both a MOSFET and a BJT?

It's a silly idea, just use a MOSFET on its own.
 
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