MOSFET query

[vuchris]

I've designed the following circuit:

**broken link removed**

note:
IGNORE THE REGULATORS UP THE TOP, JUST TAKE THE VOLTAGES AS THEY ARE IN RED.

heres what it does:

the switches J4 and J3 are representing a PIC microprocessor which determines the direction of a DC motor and spits out logic 1 accordingly. J3 is logic 1 when the motor is turning clockwise, J4 is logic 1 when the motor is turning counter clock wise.

Heres the problem:
Why is it that I only see .5V at the source of Q2 and Q4 when the corresponding switch is closed (this is in reality too, not just in simulation). But when I remove Q1 and Q3 I see the more correct voltage of around
+9.2V.

 

[audioguru]

Didn't I already talk about your faulty circuit on another website??

 

[Optikon]

Your circuit doesnt work.

You dont have this stuff wired up right I'm afraid. You need to have several volts Vgs for your mosfets to be "ON". Check that.
Q1 looks like its gate is shorted to its source - that will *NEVER* turn on.. (schematic lines that end in a "T" are CONNECTED)


Why dont you use an analog switch for all your resistor switching instead of these discrete fets? Much easier!

Also, why are you using the ancient & crummy 741 opamp? Is this for a school assignmnet?

 

[audioguru]

These are N-channel Mosfets. Q1 and Q3 are upside-down.
If they are connected properly then they still won't completely turn on because they need 10V gate-source voltage to fully turn on.

The huge schematic is a nightmare of short-circuits.

 

[vuchris]

thank you for the replies.

Optikon:

its not shorted to its source when the switch is down.;

I'm using the 741 because i had it lying around, this is just a prototype.

No its not a school assignment.

I can't use an analog switch because the process is fully automated.

Please note; this circuit works FINE if I remove q1 and q3 from the circuit.

The idea of q1 and q3 is to ground the other terminal of the op amp. ie. if Q2 is on, Q3 will ground the other terminal.

audioguru
theyre not upside down, the idea of q3 and q1 are to provide a ground path to the op amp (as said above).

 

[audioguru]

I don't see a motor in your circuit.
I don't know why you have Mosfets that need 10V gate to source when the circuit doesn't provide enough gate voltage.
I don't know why two Mosfets are source-followers that need 20V on their gates to turn on but you feed the gates only 5V.
I don't know why two Mosfets are upside-down.
I don't know why there is an opamp that does nothing.

 

[Optikon]

thank you for the replies.

Optikon:

its not shorted to its source when the switch is down.;

I'm using the 741 because i had it lying around, this is just a prototype.

No its not a school assignment.

I can't use an analog switch because the process is fully automated.

Please note; this circuit works FINE if I remove q1 and q3 from the circuit.

The idea of q1 and q3 is to ground the other terminal of the op amp. ie. if Q2 is on, Q3 will ground the other terminal.

audioguru
theyre not upside down, the idea of q3 and q1 are to provide a ground path to the op amp (as said above).

You need to swap the source & drain pins on Q1 & Q3. Look those up in the datasheet and flip them around.

I don't understand your comment about what a fully automated process has to do with an analog switch?? What I am talking about here is replacing all these mosfets with a single chip that contains JFETS that do the same kind of switching. Look up an analog switch such as DG444 etc.. to see what I am talking about. This application is perfect for an analog switch (a bunch of discrete mosfets is clumsy IMO).

 

[specie]

I don't see a motor in your circuit.
I don't know why you have Mosfets that need 10V gate to source when the circuit doesn't provide enough gate voltage.
I don't know why two Mosfets are source-followers that need 20V on their gates to turn on but you feed the gates only 5V.
I don't know why two Mosfets are upside-down.
I don't know why there is an opamp that does nothing.

Hey vuchris,
I guess you have to answer the questions above before you get a reasonable answer. Circuit does not make sense.