what happens when p-mosfet gate is left floating?

Papabravo

Joined Feb 24, 2006
14,412
Where did this circuit come from and what is the circuit supposed to be doing. I ask because there are a number of things that just don't make much sense to me. I need to know the values of the resistors. What is the purpose of ZD3? Why does Q18 have a circle around it and show internal resistors -- is it a Darlington perhaps. Is the base of Q18 connected to the junction R58 and C26?
 

Ian0

Joined Aug 7, 2020
490
Looks like a buck regulator to me. Q14 is a P-channel MOSFET, which is switched on/off by Q18.
ZD1 is a clamp to prevent over-voltage on the MOSFET gate.
>1.2V on input, Q18 base > 0.6V, Q18 is on, -11V between MOSFET gate and source limited by the zener. MOSFET is on.
<1V on input, Q18 base <0.5V, Q18 is off, 0V between MOSFET gate and source, MOSFET is off.
I would expect a PWM signal on Q18 base, to give an equivalent PWM on the MOSFET, which is filtered by L3/C19 to give an intermediate voltage on the output, EXCEPT that R42 @ 100k is MUCH too high for any decent speed of PWM.

The original question is "what happens when p-channel gate is left floating?". It's not left floating because R42 pulls it up to the source voltage if Q18 is not switched on. Remove R42 and the gate voltage will float about all over the place, and the MOSFET will switch partially on, and, if there is a load connected, dissipate a lot of heat and blow up.
 
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Bordodynov

Joined May 20, 2015
2,661
You mean it has a little A/D converter on the base and a DAC to control the collector and emitter! Wow -- ain't technology GRAND!
No, this is the way it is arranged, as it is drawn. In some of my projects I even used two "digital" transistors in one housing. Using them saves a lot of space on a printed circuit board and simplifies wiring.
 

Ian0

Joined Aug 7, 2020
490
You mean it has a little A/D converter on the base and a DAC to control the collector and emitter! Wow -- ain't technology GRAND!
No, it's got a serial interface so you can connect 16 of them to a single microprocessor output pin
OR
It's got a built in schmitt trigger, so that it can only be either on or off. (Now that would be useful, but I can't see a way of doing it with fewer than 4 pins, in which case I might as well use a 74HC1G14)
OR
It switches off as fast as a logic gate (74 series TTL if it's really lucky)
Or, most likely . .
It's what was originally called a Resistor Equipped Transistor, until they gave it to the new boy in marketing to try and get some of the MOSFET business . . . . . .as if I can't connect a BC847 to a logic gate if I wanted to.
 

Thread Starter

Younes Thabet

Joined Jan 9, 2019
17
Where did this circuit come from and what is the circuit supposed to be doing. I ask because there are a number of things that just don't make much sense to me. I need to know the values of the resistors. What is the purpose of ZD3? Why does Q18 have a circle around it and show internal resistors -- is it a Darlington perhaps. Is the base of Q18 connected to the junction R58 and C26?
This circuit to run a DC motor, Values of Res are shown in schematic, zener probably for assuring gate voltage of Q14, Q18 is resistor equipped transistor, base is connected to microcontroller.
 

Thread Starter

Younes Thabet

Joined Jan 9, 2019
17
Looks like a buck regulator to me. Q14 is a P-channel MOSFET, which is switched on/off by Q18.
ZD1 is a clamp to prevent over-voltage on the MOSFET gate.
>1.2V on input, Q18 base > 0.6V, Q18 is on, -11V between MOSFET gate and source limited by the zener. MOSFET is on.
<1V on input, Q18 base <0.5V, Q18 is off, 0V between MOSFET gate and source, MOSFET is off.
I would expect a PWM signal on Q18 base, to give an equivalent PWM on the MOSFET, which is filtered by L3/C19 to give an intermediate voltage on the output, EXCEPT that R42 @ 100k is MUCH too high for any decent speed of PWM.

The original question is "what happens when p-channel gate is left floating?". It's not left floating because R42 pulls it up to the source voltage if Q18 is not switched on. Remove R42 and the gate voltage will float about all over the place, and the MOSFET will switch partially on, and, if there is a load connected, dissipate a lot of heat and blow up.
Thank you for your answer..
Gate of Q14 can handle up to 20V .. what would happen if i remove zener !!?
what do you think would be right value for R42?
is C58 a decoupling cap!?
 
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Ian0

Joined Aug 7, 2020
490
C58 is a decoupling cap. It is to be hoped that there are MUCH bigger decoupling caps on that supply if you are going to speed control a motor with PWM.
What happens if you remove the zener? Nothing much.
Good value for R42 - about 220 ohms, if your “digital” transistor can stand it, and zero for R39. Depends a lot on the PWM frequency.
You can leave out L3 and C19, the value of L3 will be insignificant in comparison to the motor inductance.
But why not use a cheaper and more efficient n-channel MOSFET, and drive it with a proper MOSFET driver such as the MCP1401, by connecting the motor to the positive supply?
 

Papabravo

Joined Feb 24, 2006
14,412
This circuit to run a DC motor, Values of Res are shown in schematic, zener probably for assuring gate voltage of Q14, Q18 is resistor equipped transistor, base is connected to microcontroller.
I know the values of the Resistors are in the schematic, but they are TOO SMALL TO READ! I'd read them if I could but I can't.
 

Papabravo

Joined Feb 24, 2006
14,412
No, it's got a serial interface so you can connect 16 of them to a single microprocessor output pin
OR
It's got a built in schmitt trigger, so that it can only be either on or off. (Now that would be useful, but I can't see a way of doing it with fewer than 4 pins, in which case I might as well use a 74HC1G14)
OR
It switches off as fast as a logic gate (74 series TTL if it's really lucky)
Or, most likely . .
It's what was originally called a Resistor Equipped Transistor, until they gave it to the new boy in marketing to try and get some of the MOSFET business . . . . . .as if I can't connect a BC847 to a logic gate if I wanted to.
I certainly do enjoy a bit of wry humor. Two thumbs up!
 

Papabravo

Joined Feb 24, 2006
14,412
No, this is the way it is arranged, as it is drawn. In some of my projects I even used two "digital" transistors in one housing. Using them saves a lot of space on a printed circuit board and simplifies wiring.
The reason I mentioned it was that we used to draw Darlington transistors this way (not with the circle), especially when the values of the resistors were specified as in the old TI Power Data Book.
 

Ian0

Joined Aug 7, 2020
490
The reason I mentioned it was that we used to draw Darlington transistors this way (not with the circle), especially when the values of the resistors were specified as in the old TI Power Data Book.
So the good old TIP121 was a digital transistor all along, and we never knew it!
 

Thread Starter

Younes Thabet

Joined Jan 9, 2019
17
C58 is a decoupling cap. It is to be hoped that there are MUCH bigger decoupling caps on that supply if you are going to speed control a motor with PWM.
What happens if you remove the zener? Nothing much.
Good value for R42 - about 220 ohms, if your “digital” transistor can stand it, and zero for R39. Depends a lot on the PWM frequency.
You can leave out L3 and C19, the value of L3 will be insignificant in comparison to the motor inductance.
But why not use a cheaper and more efficient n-channel MOSFET, and drive it with a proper MOSFET driver such as the MCP1401, by connecting the motor to the positive supply?
Actually this schematic isn't mine.. I am just trying to reverse-engineer some old PCBs with my rusty electronic knowledge.
If I am ever going to redesign it I ll definitely go with an n-channel mosfet for sure...
Do you think going with an n-channel mosfet and its driver would be a cheaper solution than this one?
 

ronsimpson

Joined Oct 7, 2019
932
Wow -- ain't technology GRAND!
Yes it is grand.
It is not good practice to connect the output of digital logic directly to the base of a transistor.
This type of transistor can be used as a invertor or several in parallel can make "gates".
1600363569532.png
It costs more to place and solder a resistor than the cost of the resistor.
A "digital" transistor has a much lower failure rate than a transistor and two resistors.
This makes a good LLT to LED transistor. There are also transistors for driving relays that include a catch diode.

I used PNP+NPN transistor arrays (with built in resistors) that greatly reduced board complexity.
1600364569775.png
 

Ian0

Joined Aug 7, 2020
490
N-channel MOSFETs are invariably cheaper than P-channel with the same current rating. A driver will cost 40p. So it probably depends on how big your motor is!
 
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