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Low Vgth P-Channel MOSFET driving with Arduino

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wraper:

--- Quote from: elefurtronik on February 03, 2023, 10:26:34 pm ---
--- Quote from: mikerj on February 02, 2023, 11:20:37 pm ---
--- Quote from: elefurtronik on February 02, 2023, 11:04:55 pm ---When I disconnect the BJTs. Circuits are still running but I am still not sure whether this is caused because my MOSFETs having very low Vgth or else.

--- End quote ---

You still seem a little confused about Vgs(th).  If you disconnect the NPN transistor, then the resistor between gate and source ensures that Vgs=0, so the MOSFET should be fully turned off irrespective of having a low Vgs(th).  Unless that resistor isn't actually connected and your MOSFET gate is floating, or it's blown up...

--- End quote ---
OK. So when I disconnect the BJT, I pull up the gate and as Vs is larger than Vg, MOSFET must be off. However, it is not my case. Even though I disconnected the MOSFET, it was ON. I think my MOSFET Vgs(th) is very low. Maybe 10k is not sufficient. However, I don't know how to calculate the pull-up resistor for MOSFET.
In my case, I have a 6V source, and if I put pull-up resistor how Vg is 6V as there is a resistor there? And Gate and Drain are isolated, right?

--- End quote ---
Gate is isolated, it's like a small capacitor. So even if resistor was 1 Megaohm, MOSFET would still turn off, just with more delay. If you measure voltage between gate and source (across R2), it must be 0V. No current flows through the gate in steady state, some current only when you change voltage on the gate because it needs to charge/discharge its capacitance.

elefurtronik:
Hello everyone,
I found the problem but I still don't understand why this happens. As you can see two loads which are connected to MOSFETs. One is a solenoid valve and the other one is DC motor. When I connect the solenoid valve's and dc motors ground together, the solenoid valve doesn't work. However, when I connect the solenoid valve's GND leg separately to ground, the solenoid works. Could you help me on this issue, please?

inse:
How much current do your loads draw?
If you are testing the circuit on a breadboard, the voltage drop on the contacts and rails can cause unexpected behaviour.
Insufficient wire gauge does the same.

wasedadoc:

--- Quote from: elefurtronik on February 05, 2023, 01:46:23 am ---Hello everyone,
I found the problem but I still don't understand why this happens. As you can see two loads which are connected to MOSFETs. One is a solenoid valve and the other one is DC motor. When I connect the solenoid valve's and dc motors ground together, the solenoid valve doesn't work. However, when I connect the solenoid valve's GND leg separately to ground, the solenoid works. Could you help me on this issue, please?

--- End quote ---
All that does not align with your posts above that the FETs never turn off.

elefurtronik:

--- Quote from: inse on February 05, 2023, 06:23:42 am ---How much current do your loads draw?
If you are testing the circuit on a breadboard, the voltage drop on the contacts and rails can cause unexpected behaviour.
Insufficient wire gauge does the same.

--- End quote ---
Yes, I do on breadboard. I am using 25awg and pullin 600mA max. lets say.


--- Quote from: wasedadoc on February 05, 2023, 06:38:54 am ---
--- Quote from: elefurtronik on February 05, 2023, 01:46:23 am ---Hello everyone,
I found the problem but I still don't understand why this happens. As you can see two loads which are connected to MOSFETs. One is a solenoid valve and the other one is DC motor. When I connect the solenoid valve's and dc motors ground together, the solenoid valve doesn't work. However, when I connect the solenoid valve's GND leg separately to ground, the solenoid works. Could you help me on this issue, please?

--- End quote ---
All that does not align with your posts above that the FETs never turn off.

--- End quote ---
I actually changed bjts with n channel mosfets and solved this issue. However now I have another issue with the same circuit. So should I create another post for this. If yes, after if it is solved here, I can create a post and explain for people who might have the same issue.

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