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Topic: Replace relay with mosfet? (Read 2238 times) previous topic - next topic

steve_1988

#15
Jul 07, 2020, 08:35 am Last Edit: Jul 07, 2020, 10:28 am by steve_1988
This is the latest schematic and am going to try this tonight.



I'm wondering whether the emitter should connect to Arduino ground??


***Mini buck converter that im using***


Thanks

Steve


TomGeorge

#16
Jul 07, 2020, 02:32 pm Last Edit: Jul 07, 2020, 02:32 pm by TomGeorge
Hi,
You will need to connect the gnds of both batteries together with the Nano gnd.


Tom... :)
Everything runs on smoke, let the smoke out, it stops running....

steve_1988

Hi,
You will need to connect the gnds of both batteries together with the Nano gnd.


Tom... :)
Thanks Tom!  :)

After more research, I think my previous schematic has a problem... For P-channel Mosfet, it should always pull-up right? If yes, do i still need to have the pull-down at transistor base? will it cause "floating" issue??





TomGeorge

Hi,
You don't need the 10K at the base of the  BJT, is low impedance and won't float like a MOSFET gate will.
You can leave it in, 10K will not be a problem.
The 10K between the gate and source of the P-CH MOSFET is essential.

With P-CH MOSFET the gate has to go NEGATIVE with respect to the Source to make it conduct, that is what the transistor does to the MOSFET gate.

Do you have to switch the positive of the motor, that is High Side switch?
It would be easier and lower component count if you switched the gnd  side, or LOW SIDE switch with N-CH MOSFET.

Haven't you tried the circuit yet?

Tom... :)
Everything runs on smoke, let the smoke out, it stops running....

steve_1988

Hi,
You don't need the 10K at the base of the  BJT, is low impedance and won't float like a MOSFET gate will.
You can leave it in, 10K will not be a problem.
The 10K between the gate and source of the P-CH MOSFET is essential.

With P-CH MOSFET the gate has to go NEGATIVE with respect to the Source to make it conduct, that is what the transistor does to the MOSFET gate.

Do you have to switch the positive of the motor, that is High Side switch?
It would be easier and lower component count if you switched the gnd  side, or LOW SIDE switch with N-CH MOSFET.

Haven't you tried the circuit yet?

Tom... :)
Hi Tom

Ah...Ok understand. I haven't test the new circuit. Just want to make sure the new circuit is safe before I test as I only left 1 or 2 P-channel mosfet... :P





TomGeorge

Hi,
What is your 12V power supply?

Tom... :)
Everything runs on smoke, let the smoke out, it stops running....

steve_1988

Hi,
What is your 12V power supply?

Tom... :)
Hi!

I will be using a 9v battery to power the Nano, and a 12v battery for mosfet (to turn on/off the LED) to test the circuit below :)



I can still connect both 9v and 12v batteries ground together with nano ground right?


Thanks!

Steve

TomGeorge

I can still connect both 9v and 12v batteries ground together with nano ground right?
Yes..
Tom... :)
Everything runs on smoke, let the smoke out, it stops running....

larryd

What is the 1N4142 diode in your schematic ?




No technical PMs.
If you are asked a question, please respond with an answer.
If you are asked for more information, please supply it.
If you need clarification, ask for help.

steve_1988

Yes..
Tom... :)
Thanks Tom! Will share the result later :)

What is the 1N4142 diode in your schematic ?
Hi Larryd!

It's a 12v zener diode to limit the Vgs voltage.  :)

larryd

#25
Jul 09, 2020, 04:11 am Last Edit: Jul 09, 2020, 04:12 am by larryd
That circuit as is is going to destroy your BJT.

Change the 10k to ~8.6k

Add a ~1k between the BJT collector and the MOSFET gate.





No technical PMs.
If you are asked a question, please respond with an answer.
If you are asked for more information, please supply it.
If you need clarification, ask for help.

steve_1988

That circuit as is is going to destroy your BJT.

Change the 10k to ~8.6k

Add a ~1k between the BJT collector and the MOSFET gate.

Hi Larryd!

Do you mean change the pull-up resistor from 10k to 8.6k? if yes, Can I use 1k or 4.7k instead?

I have edited the schematic. :)






larryd

#27
Jul 09, 2020, 05:27 am Last Edit: Jul 09, 2020, 05:45 am by larryd









No technical PMs.
If you are asked a question, please respond with an answer.
If you are asked for more information, please supply it.
If you need clarification, ask for help.

steve_1988


Hi Larryd!

Thanks for the schematic! May I know why we need to add a capacitor at the load??





TomGeorge

Hi Larryd!

Thanks for the schematic! May I know why we need to add a capacitor at the load??
The motor has brushes and a commutator in it, when the motor runs they cause electrical noise.
The capacitor prevents the commutator noise from getting out into the rest of the circuit.
Usually 0.1uF cap is enough to do the job.
Tom... :)
Everything runs on smoke, let the smoke out, it stops running....

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