turning mosfet gate to zero

Thread Starter

captoro

Joined Jun 21, 2009
151
Hello.

I posted my schematic of switching mosfet. Q1 to Q3.
These continuously switches at 50% duty cycle.
I added Q4 so I can stop the switching at gate Q1 to Q3. My design is not working. Anyone knows why?

Ken
 

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dl324

Joined Mar 30, 2015
14,680
I posted my schematic of switching mosfet. Q1 to Q3.
These continuously switches at 50% duty cycle.
I added Q4 so I can stop the switching at gate Q1 to Q3. My design is not working. Anyone knows why?
Your schematic doesn't have any MOSFETs and Q1-3 will die when you turn them on.
clip.jpg
 

jpanhalt

Joined Jan 18, 2008
11,088
You appear to have the gates set to about 1/3 Vcc as a minimum. View it as a voltage divider with 100 Ω in the top leg and 100 Ω ||100 Ω (i.e., 50 Ω) in the bottom leg. Get rid of the pullup and increase the discharge to at least 1k (10k may be more common).

1603748968069.png
 

Thread Starter

captoro

Joined Jun 21, 2009
151
You appear to have the gates set to about 1/3 Vcc as a minimum. View it as a voltage divider with 100 Ω in the top leg and 100 Ω ||100 Ω (i.e., 50 Ω) in the bottom leg. Get rid of the pullup and increase the discharge to at least 1k (10k may be more common).

View attachment 220707
The top part with the three mosfets works. I do get an output. When I add the Q4 mosfet. I don't see any difference. It does not affect the gate of the three mosfets .
 

Papabravo

Joined Feb 24, 2006
18,786
The icons are incorrect . But at Q's are kosfets and not BJT
As drawn they will still die if they are turned on, creating a virtual short between +24VDC and GND. Using the wrong symbol is pretty sloppy engineering. There really is no excuse for it. Publishing that schematic on a worldwide forum exposes you to all manner of ridicule and abuse.
 

dl324

Joined Mar 30, 2015
14,680
The icons are incorrect . But at Q's are kosfets and not BJT
Then why did you use the wrong symbols? MOSFET is an acronym and should be all caps (ignoring your spelling).

Why do you have R43-45? What are they being driven from? What is the power supply on R44?
clipimage.jpg
 

Thread Starter

captoro

Joined Jun 21, 2009
151
The top part with the three mosfets works. I do get an output. When I add the Q4 mosfet. I don't see any difference. It does not affect the gate of the three mosfets .
Let me me post an updated schematic. Sorry for the confusion.
 

dl324

Joined Mar 30, 2015
14,680
Let me me post an updated schematic. Sorry for the confusion.
Don't bother making more posts until you have an accurate schematic that gives the required information (like supply voltage for R44 and information about the driving signal).
 

dl324

Joined Mar 30, 2015
14,680
Here is my updated circuit with MOSFETS. When Q4 goes high. I would of thought the gates on Q1 to Q3 to be grounded. Therefore not conducting. But it is.
clipimage.jpg
Next time do a screen capture or have LTspice save the output to a graphics file.

When Q4 what goes high? Why don't you connect the drain directly to the gates of Q1-3?
 

Thread Starter

captoro

Joined Jun 21, 2009
151
Even if I lose R4, I get same result. I did replace R1 to 50k to get better voltage out. But Q4 is still Not affecting the Q1-3.
If I ground Q1-3 directly. Then the MOSFETS does not engage. Wondering why I cannot do the same ..meaning grounding a
gate with another MOSFET.
Basically V2 creates a constant signal. And V3 acts as an interrupt.
 
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dl324

Joined Mar 30, 2015
14,680
If I ground Q1-3 directly. Then the MOSFETS does not engage. Wondering why I cannot do the same ..meaning grounding a
gate with another MOSFET.
Are these simulation results? You're using a generic N channel MOSFET. What is it's threshold voltage and on resistance at 10V?
 

Thread Starter

captoro

Joined Jun 21, 2009
151
Are these simulation results? You're using a generic N channel MOSFET. What is it's threshold voltage and on resistance at 10V?
I was initialy using a breadboard. It was not working no I decided to simulate it on LTSpice. Thought that was a simple circuit. I am using a generic MOSFET. Using a 12 Vgs.
 

click_here

Joined Sep 22, 2020
544
If I ground Q1-3 directly. Then the MOSFETS does not engage. Wondering why I cannot do the same ..meaning grounding a
gate with another MOSFET.
Basically V2 creates a constant signal. And V3 acts as an interrupt.
Because the gates are not being grounded - Their inputs are in a voltage divider with those R2/R4 resistors.
 

djsfantasi

Joined Apr 11, 2010
8,390
Isn’t R2 and R4 forming a voltage divider? Thence, when Q4 is on, the other MOSFETs are receiving a 0-6V sine wave signal. Depending on the model, that may be sufficient to turn on the MOSFETs.

Try a different model and increase R2 or decrease R4.
 
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