On/Off switch MOSFET based

[jozef007]

Hi,
as per following schematic I tested On/OFF switch. I was able to turn it on and somehow I understand "on" functionality but regarding "off" function never worked for me and I would like if someone can explain it and there is not a problem in schematic overall. I also measured voltage at certain points:
1 - after connected to power
2 - after first pressing button
3 - after second pressing button
I used original schematic but I used different MOSFETs
Range of Vgs for MOSFETs: Q1, Q3 VBF2355 ((-1.4) - (-2.5)), Q2 P140NF75 (2-4V).

Thx

 

[AnalogKid]

1. All three FET symbols are incorrect, making this a difficult circuit to analyze.

2. When the button 1s held down the first time, the voltage on C1 increases to approx. Vbatt / 2. But when the button is released, the C1 voltage should increase to 7.4 V - ish in around 0.3 seconds.

Part of the problem might be that your choices for Q1 and Q2 are way way bigger than in the original circuit. This means that they have a huge gate capacitance, and this can affect how they interact with C1.

ak

 

[crutschow]

Why is R3 scribbled in?

 

[jozef007]

Why is R3 scribbled in?

I got schematic from Art of electronics (third edition), R3 was missing there, only value was there. I assumed, they missed it.

 

[jozef007]

1. All three FET symbols are incorrect, making this a difficult circuit to analyze.

2. When the button 1s held down the first time, the voltage on C1 increases to approx. Vbatt / 2. But when the button is released, the C1 voltage should increase to 7.4 V - ish in around 0.3 seconds.

Part of the problem might be that your choices for Q1 and Q2 are way way bigger than in the original circuit. This means that they have a huge gate capacitance, and this can affect how they interact with C1.

ak

Hi Ak,
I order MOSFET with lower capacitance to test it. Q1 and Q3 are P-MOSFET, Q2 is N. Can you clarify on "incorrect symbols"? Thx

 

[AnalogKid]

https://en.wikipedia.org/wiki/MOSFET#Circuit_symbols

ak

 

[jozef007]

are you testing that on program or on real ground

I tested it on real ground.

 

[jozef007]

I updated schematic: