[SOLVED] How to get this fet to disconnect voltage source

Status
Not open for further replies.
Z

Zak28

Advanced Member level 2
Joined
Aug 19, 2016
Messages
579
Helped
6
Reputation
12
Reaction score
6
Trophy points
18
Activity points
4,681
How to get this fet to disconect voltage source

Why doesn't this simple circuit disconnect the voltage (battery) at the zener voltage? Falstad simulator https://www.falstad.com/circuit/circuitjs.html?cct=$+1+0.000005+23.47059216675035+27+5+43%0Ac+384+-48+384+112+0+1+0.001%0Ag+384+112+384+128+0%0Af+240+112+304+112+32+1.5+0.02%0Az+128+176+128+144+1+0.805904783+2.6%0Ag+128+176+128+192+0%0Aa+160+112+240+112+8+15+0+1000000+0+0+100000%0Aw+160+128+128+128+0%0Aw+128+128+128+144+0%0Ar+128+128+48+128+0+1000%0AR+48+128+16+128+0+0+40+15+0+0+0.5%0Ag+304+128+304+144+0%0Aw+304+80+304+96+0%0Av+304+80+304+48+0+0+40+12+0+0+0.5%0Aw+304+-48+384+-48+0%0Aw+160+96+128+96+0%0Aw+128+96+128+-48+0%0Aw+128+-48+304+-48+0%0Ar+304+-48+304+48+0+0.3%0Ao+0+64+0+4097+5+51.2+0+2+0+3%0Ao+0+64+0+4098+1.25+0.1+1+2+0+3%0Ao+2+64+0+4098+5+0.1+2+2+2+3%0A
 

Re: How to get this fet to disconect voltage source

Wrong MOSFET polarity, bulk diode is forward biased. Flip drain and source, allow negative comparator output to switch off the MOSFET.
 
Reactions: Zak28

    Z

    Zak28

    Points: 2
    Helpful Answer Positive Rating
Re: How to get this fet to disconect voltage source

FvM said:
Wrong MOSFET polarity, bulk diode is forward biased. Flip drain and source, allow negative comparator output to switch off the MOSFET.
Click to expand...

It doesn't seem like flipping or mirroring components has been implemented in falstads' simulator
 

Re: How to get this fet to disconect voltage source

Falstad's simulator:

Right-click on a component to see pop-up menu with 'Edit' option. (Macintosh, press control-mouseclick)

Mosfet edit window:

 

Re: How to get this fet to disconect voltage source

BradtheRad said:
Falstad's simulator:

Right-click on a component to see pop-up menu with 'Edit' option. (Macintosh, press control-mouseclick)

Mosfet edit window:

View attachment 149322
Click to expand...

That did it however it seems the MOSFET is trickle charging the cap past zener reference. I didn't know falstad simulated leakage. Glad you clarified the repositioning issue. Is the trickle charging a simulator only issue?
https://www.falstad.com/circuit/circuitjs.html?cct=$+1+0.000005+23.47059216675035+27+5+43%0Ac+384+-48+384+112+0+1+3.192175129765366%0Ag+384+112+384+128+0%0Af+240+112+304+112+40+2+0.2%0Az+128+176+128+144+1+0.805904783+1.5%0Ag+128+176+128+192+0%0Aa+160+112+240+112+8+15+0+1000000+3.192175129765366+1.5256418402919818+100000%0Aw+160+128+128+128+0%0Aw+128+128+128+144+0%0Ar+128+128+48+128+0+1000%0AR+48+128+16+128+0+0+40+15+0+0+0.5%0Ag+304+128+304+144+0%0Aw+304+80+304+96+0%0Av+304+80+304+48+0+0+40+12+0+0+0.5%0Aw+304+-48+384+-48+0%0Aw+160+96+128+96+0%0Aw+128+96+128+-48+0%0Aw+128+-48+304+-48+0%0Ar+304+-48+304+48+0+0.05%0Ao+0+64+0+4097+5+51.2+0+2+0+3%0Ao+0+64+0+4098+5+0.1+1+2+0+3%0Ao+2+64+0+4099+20+25.6+2+2+2+3%0A
 
Last edited:

Re: How to get this fet to disconect voltage source

The mosfet model in Falstad's sometimes does the unexpected, since I cannot be sure how much it will conduct as a result of a certain bias voltage.

I tend to use the transistor models, because the relationship is clearer between c-e conductivity and its bias current.
 

Status
Not open for further replies.

Similar threads

Menu
Log in
Register
Cookies are required to use this site. You must accept them to continue using the site. Learn more…