Need Help Understanding this Circuit!

[cremaster]

Hi everyone, here is the circuit:

http://www.kerrywong.com/blog/wp-content/uploads/2010/02/controllercircuit.png

from this page

Kerry D. Wong » Blog Archive » A Parallel Port Stepper Motor Driver With Discrete Components

I want to build a similar circuit to control a DC motor with my parallel port, but I have real difficulty understanding the operation of the MOSFETS.

Why is the gate of the each MOSFET connected to the source? I thought the MOSFET was supposed to isolate the controlling portion of the circuit from the controlled. How does one create voltage on the gate when there is no place for the voltage to go? I would have expected a MOSFET to have four terminals, one pair of wires for a small current to flow through causing a large current to flow through the other two wires. Also, why does this even work, wouldnt the current simply flow through the diode, turn right, through the resistor and out to ground bypassing the gate?

Could someone please explain the function of the MOSFETS in terms of electron flow, in this particular circuit?

Thank you!

 

[MikeMl]

Forget electron flow. Real current flows from Vcc, through the winding, into the drain of the FET, out the source to ground and happens only when the gate of the FET is at ~5V. If the gate is at ~0V, the FET is off and no current flows.

This is a very poor circuit. Most parallel-ports do not pull high enough (~3.3V) to turn on the FETs. The diodes in the circuit make this even worse, because of their forward drop. The gates are simply not driven high enough to properly turn on the FETs. Mr Wong is naive, or inexperienced. The 100K resistor from gate to source is there only to guarantee that the FETs are Off if the cable is disconnected from the PP.

 

[crutschow]

Why is the gate of the each MOSFET connected to the source? I thought the MOSFET was supposed to isolate the controlling portion of the circuit from the controlled.

Don't know where you got that idea. MOSFETs (or any transistor for that matter) do not provide isolation between input and output. They provide power gain between input and output but not isolation. In fact they will not work if the input control voltage is isolated from the MOSFET. The common for the control voltage must be common with the MOSFET source for typical operation of the transistor.

 

[crutschow]

...How does one create voltage on the gate when there is no place for the voltage to go?
...Also, why does this even work, wouldnt the current simply flow through the diode, turn right, through the resistor and out to ground bypassing the gate?

You seem to have an erroneous perception of voltage and current. Voltage can create a current, but you don't need a current to have a voltage. You don't need a "place for the voltage to go". Think of a battery. It has voltage even when no current is flowing.

In the circuit, the voltage will go through the diode, and appear across the resistor and be on the gate at the same time. The voltage does not bypass the gate.

 

[cremaster]

Thanks for the quick replies. I wont use this circuit then.

Poorness of design aside, would all the ground symbols in the circuit be common? The ground pin on my parallel port, would that be connected with the four ground symbols as well as the negative side of Vcc? Or would I only use one or the other?

 

[cremaster]

@crutschow: Ahh I see, I knew I was thinking about it incorrectly. But because there is a circuit there will be a small current right? Depending on the resistance? So if I wanted to measure how much voltage is on the gate, which two points would I choose? If I put one end of a multimeter on the gate itself and the other on the source would that be the voltage im interested in? Or is it just the voltage between the actual pin of the port and the ground pin?

 

[audioguru]

All the grounds are connected together.

 

[cremaster]

You seem to have an erroneous perception of voltage and current. Voltage can create a current, but you don't need a current to have a voltage. You don't need a "place for the voltage to go". Think of a battery. It has voltage even when no current is flowing.

In the circuit, the voltage will go through the diode, and appear across the resistor and be on the gate at the same time. The voltage does not bypass the gate.

Ahhh, I see, I was thinking about it incorrectly. So its the voltage on the gate that determines whether current can flow through the transistor. But since there is voltage and a complete circuit, a small current will still be flowing, right? How much current would be determined by the resistor? It couldnt be JUST voltage right?

Also, if I wanted to measure the voltage on the gate, which two points of the circuit would I measure? With a multimeter for instance. If I put one side of my multimeter on the gate itself, and one on the source or drain, what would that say with regard to voltage?

 

[cremaster]

"How much current would be determined by the resistor?"

I said that in a confusing way, what I meant was: "There would still be a current flowing, and how much current would be determined by the resistance, right?"

 

[crutschow]

Ahhh, I see, I was thinking about it incorrectly. So its the voltage on the gate that determines whether current can flow through the transistor. But since there is voltage and a complete circuit, a small current will still be flowing, right? How much current would be determined by the resistor? It couldnt be JUST voltage right?

Also, if I wanted to measure the voltage on the gate, which two points of the circuit would I measure? With a multimeter for instance. If I put one side of my multimeter on the gate itself, and one on the source or drain, what would that say with regard to voltage?

Yes a current flows though the resistor as determined by Ohm's law, I = V/R. If you don't know Ohm's law, please study it. It's one of the most fundamental laws of electrical circuits.

The gate voltage that is specified in the data sheets is measured with respect to the source terminal. It's the gate-to-source voltage that determines the transistor conduction state.

 

[cremaster]

All the grounds are connected together.

Is this always the case?

 

[audioguru]

If the grounds are not connected together then the circuit will not work.

 

[cremaster]

If the grounds are not connected together then the circuit will not work.

Is this always the case?

 

[audioguru]

Every circuit is different but most circuits do not work if the grounds are not connected together.