Using 2 Mosfet to Switch 2 Different voltage

ScottWang

Joined Aug 23, 2012
7,397
Scott the voltage on Gate is Okay ?
the datasheet tell the Gate-source voltage is around 12V.

And I think the diode at drain is reverse.
When the Q1 is turn on, the Vg of fet1 around 10V, that was used to protecting the Vg, the Vg can be input around +/- 12V, I changed to 10V.

And yes, the diodes I was drawn the wrong direction and I already modified.

12V24VSwitch_Pch-04_ScottWang.gif
 
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ScottWang

Joined Aug 23, 2012
7,397
I simulate on Proteus, I change R8 to 100R and the Vgs become -6V, what you think ? it is better or not ?
12V-(Q2_Vce)/100=(12V-0.2V)/100=118mA, so the Ic is too large for the Q2, the Imax of Q2_Ic <= 10mA is better.

If you like, maybe you can using a 2.2V zener to replace R8, and R8 moving to became Rc that it is between Q2_c and Vg, that is the same with R4, and the R8 change the value to 1.8K, it will set the current of zener between 5mA~10mA, it can be keep the Vf of zener to a stable status.
 

ScottWang

Joined Aug 23, 2012
7,397
I think it was correct before, clamping the Vgs to 2.2V with a zener is not going to allow the FET to turn on enough.
For a logical level 4.5V that it is enough to get into the saturation status.
If you afraid of that then you can using two 1n4148 to replace the 2.2V zener.
 

ScottWang

Joined Aug 23, 2012
7,397
Sensacell:
I don't know why I can't use the reply function even restart my computer is the same.

Why I want to use 2.2V zener, because I can't find the 2V zener on the the list of the online seller in Taiwan, and in the previous circuit that I used R8=22k, that is ok for the application, but the op still has a little worry, so I changed to 2.2V zener, and moved the R8 to the position as R4.
 

josip

Joined Mar 6, 2014
67
I think that this can be more simple, of course I can be wrong. :D

Let say that we have 5V logic (V2=5V) and there is need to have 5V or 10V (V1=10V) as output, then something like on my attached schematic should work... I guess. :rolleyes:
 

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ScottWang

Joined Aug 23, 2012
7,397
I think that this can be more simple, of course I can be wrong. :D

Let say that we have 5V logic (V2=5V) and there is need to have 5V or 10V (V1=10V) as output, then something like on my attached schematic should work... I guess. :rolleyes:
Do you want to throw the Q1-1 and /Q1-1 away?
 

ScottWang

Joined Aug 23, 2012
7,397
S2 should work as 5V (logic) input for (in my example) 5V / 10V selection.
If V1=24V then when bjt is turn on, the Vg=24V and then the mosfet will be blow up.

If V2=24V then the 24V will through X14, and through the body diode of mosfet, and to the V1(12V), what will be happen?
 

josip

Joined Mar 6, 2014
67
If V1=24V then when bjt is turn on, the Vg=24V and then the mosfet will be blow up.

If V2=24V then the 24V will through X14, and through the body diode of mosfet, and to the V1(12V), what will be happen?
Let say that I want to have 3V / 6V high current output, with 3V logic control. If S1 is down (logic input is 0V), there will be close to 3V at output. If S1 is up (logic input is 3V), there will be 6V at output. On schematic is used MUN5213 with integrated resistors.
 

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