Please, help me chose the right mosfet!

Using Arduino General Electronics
1

Hello all,
I am quite lost when it comes to choose the mosfet i think i need.
So i currently have a arduino that controls a little perisaltic pump (X5) to water my plants.
That works well except that i use a relay to turn the pump on/off.
I would like to change the relay for a mosfet that would probably be smaller and generate less noise that (i belive) impact my moisture sensor.

I just do not understand how to choose the mosfet i need...

The pump is a very basic DC 5v motor

Any help apreciated.

2

Here are some Logic level MOSFETs that can be used.

Confirm the amperage you need for the motor; place a kickback diode across the motor.

Logic Level MOSFETs 20 07 17.pdf (1.32 MB)

3

Probably not, so while there's nothing wrong with using a mosfet, I'm afraid you'll still have to go ahead and do some troubleshooting.

As to the mosfet question; see response by @LarryD - there are probably few hundred types that would qualify.

4

Choose the transistor based on the stall current of the motor. The stall current can be several times the running current. The stall current should be listed in the motor data sheet. If you have no data sheet it is possible to estimate the stall current, measure the motor winding resistance. Take several measurements rotating the motor a bit between readings. Use the lowest reading in the calculation. The estimated stall current is the motor supply voltage divided by the measured resistance.

You need a logic level MOSFET. That is a MOSFET that can be fully turned on by a logic level signal (5V for a 5V Arduino, 3.3V for a 3.3V Arduino). You can tell a logic level MOSFET by the Vgs specification in the Rds(on) specification. The Vgs should be logic level signal voltage or lower.

Do you want surface mount or through hole package?

6

Thanks for all the answer,
I use 5v Arduino
i have no data sheet. Took my voltmeter and i got between 2.2ohm and 1.9 ohm on the motor. so the stall current would be 3V ?
Also they all have sevral RDS on the file LarryD provided how to choose the 5v one?
On paper IRLML2402 looks good, it has RDS on 4.5V and can take 1.2A...

7

No. Current is volts divided by resistance. So 5V / 1.9 = 2.63A. I would choose a MOSFET that has a continuous current rating of 5A or more.

There will be a specified current specified at a Vgs voltage and the resultant Rds(on). The Vgs needs to be logic level (5V in this case) or lower and the current higher than the stall current.

No. Its continuous current is too low. You did not answer the question of do you want surface mount or through hole?

8

If you want a small SMD package, SOT23, an IRLML2502 ( ≈ 4A) would work.

The TO220s will work as a THT component.

BTW
You can get SMD to DIP/SIP converter PCBs.

image

9

Quick answer

Through Hole would be better for me.

10

No, probably 10A or more - the current rating is not useful in a MOSFET as it applies to the thermal limit (liquid-cooled situation). You never want to run the device anything like that hot.

What you do is calculate the dissipation from the on-resistance and load current, and decide whether that's workable for your situation (perhaps you don't want to have to use a heatsink).

For instance a 10 milliohm on-resistance FET with 5A dissipates 5 * 5 * 0.01 = 0.25W which is OK without a heatsink.
An 80 milliohm FET with 8A would be 5W, required a medium-sized heatsink.

etc etc.

11

Thanks. I will remember that.

12

Thanks for the help here
I had a deep search at my workshop and found 2 candidates
IRL88743 and TIP 120
I can't uderstand the data sheet for the TIP but for IRL88743 i found
Rds 5v
Vds 100

So that sounds perfect, right? Even a bit overkill...
Why is there no RDs for the TIP?

13

Because it's not a MOSFET. It's a Darlington transistor. You could use it as well, but it is conceptually different from a MOSFET; a Darlington is basically two NPN transistors, one piggy-backed to the other. It can work in your setup, but a MOSFET is slightly more straightforward.

14

The TIP120 is a darlington, not a MOSFET. But the IRL88743 doesn't exist so you've clearly made a mistake with the part number.

15

It's just an obvious typo for the IRLB8743 which is a quasi-logic-level MOSFET. :grin:

16

The IRLB8743 Rds is 3.2 milliohm, Vds = 30V, so its not clear at all which device you were looking at.

17

You are right, i made a typo, fed it to google that sent me to a different MOSFET (IRL540)...
Now which of the two is best suited? IRLB8743 , TIP120 ?
I'll give them a try monday ...

18

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