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Electronics => Beginners => Topic started by: namero999 on October 22, 2019, 03:35:36 pm

Title: First time with mosfets, am I doing it right? (wireless irrigation system)
Post by: namero999 on October 22, 2019, 03:35:36 pm

Hello all (and sorry, english is not my native language).

I'm working on a personal project, an automatic watering system. It has the following features:
- esp8266
- analog and i2c ports for sensors (such as temp and soil moisture)
- port for water pump (250mA @ 5V)

In front of the whole thing, I'm putting a soft latching on/off switch, as described in this eev video

https://www.youtube.com/watch?v=Foc9R0dC2iI (https://www.youtube.com/watch?v=Foc9R0dC2iI)

I attach the schematic, where I divided the circuit in logical components (the relevant one being Power and Pump, where the 2 mosfets are circled in red).

[attachimg=1]

My questions are:
- is it correct to have a mosfet piloting the main power and one piloting only the pump?
- how can I calculate the current draw of the soft latching on/off circuit? I'm afraid I can't see how much current will pass through it when the pump (or pumps!) will be on, therefore I'm not sure what components I should choose to implement it.

Thanks, generally speaking if you have any feedback on the schematic I'll appreciate it, I'm a beginner on these topics.

Best regards.

Title: Re: First time with mosfets, am I doing it right? (wireless irrigation system)
Post by: Arjunan M R on October 22, 2019, 04:33:28 pm

Which MOSFET are you using for Q4?

Title: Re: First time with mosfets, am I doing it right? (wireless irrigation system)
Post by: namero999 on October 22, 2019, 04:36:17 pm

I still have to decide but I think this is my candidate https://www.mouser.de/datasheet/2/308/NTR1P02T1-D-100317.pdf (https://www.mouser.de/datasheet/2/308/NTR1P02T1-D-100317.pdf)

Title: Re: First time with mosfets, am I doing it right? (wireless irrigation system)
Post by: Arjunan M R on October 22, 2019, 04:39:43 pm

Also, how much current will the pump draw? The maximum continuous drain current of Q1 is 750mA.

Title: Re: First time with mosfets, am I doing it right? (wireless irrigation system)
Post by: namero999 on October 22, 2019, 04:40:48 pm

The pump draws 250mA @ 5V and the reason why I chose the 750mA is because I might opt to have multiple pumps (2 or 3) attached to it.

Title: Re: First time with mosfets, am I doing it right? (wireless irrigation system)
Post by: Arjunan M R on October 22, 2019, 04:48:05 pm

If you are going to control 3 pumps using the MOSFET,  you might go with another one. You are probably going to heat up the MOSFET if you draw its max current. I recommend only using a max of 2.

Title: Re: First time with mosfets, am I doing it right? (wireless irrigation system)
Post by: namero999 on October 22, 2019, 04:51:05 pm

Thanks, indeed I am way more inclined towards having max 2 pumps (maybe in a second revision of the project).

When it comes to Q4 instead (so, the "main" mosfets), generally speaking, is that a design that makes sense? One "power" block with the soft latching on/off scheme, and then the rest with a second mosfet for the pump(s)?

Title: Re: First time with mosfets, am I doing it right? (wireless irrigation system)
Post by: NivagSwerdna on October 22, 2019, 04:51:31 pm

Looks good to me.  You could use NDS332P for Q4 if you haven't picked a candidate yet.  Have fun  :)