Could anyone help tell me what type/size components I need with this circuit diagram?

I’m going to be using a pump as the motor rated at DC: 3.5-9v 1-3W and my power supply will be DC 8.4V 1.5A.
What capacitor, diode, transistor and resistor do I need? Pin 9 will be connected to an Arduino nano.
Also, if I wanted to connect two pump/motors to the same circuit what would I need?
The pumps will only be on for around 20 seconds at a time every few hours.
I’m guessing the transistor would need to be a NPN BJT rated at least 8.4V and 6W but haven’t got a clue about the rest as I’m still just learning.

Cheers

Mark

 

Welcome to AAC!

From the information in the "DC:" line, you can determine that the coil resistance is 27 ohms
\( \small P = \frac{V^2}{R} => R = \frac{V^2}{P} = \frac{9*9}{3} = 27 ohms\)

When the transistor is saturated, it will be conducting about
\( \small I = \frac{V}{R} = \frac{8.4V}{27 ohms} = 0.31A \)

For a transistor switch, the rule of thumb is to drive the base with 0.1X of the collector current, so you need 31mA. Assuming the Arduino is operating at 3.3V, and can source enough current, you need about 100 ohms. In the likely event that the Arduino can't source 31mA, you need to add another stage for current gain (e.g. use a Darlington transistor). With a logic level NMOSFET, you wouldn't have that problem.

When the transistor is saturated, it's power dissipation will be fairly low:
\( \small P_t = I_c * V_{CE}_{sat} = 0.31A * 0.1V = 31mW \)

C1 seems superfluous...

 

Cheers for such detailed help Dennis.
The Arduino nano is operating at 5V andf the pins can supply max 40mA, recommended 20mA
I've decided to use 5v supply as the pumps are too noisey at 8.4v
I'm guessing that the transistor would be saturated at 5V/27ohms = 0.18A
So running two pumps in parallel would need a 0.36A NMOSFET

Would this one be ok?
N-Channel MOSFET 5.6A 100V Rds=0.54
https://www.bitsbox.co.uk/index.php...ucts_id=1019&zenid=o6gf1u45n5r6d6msci852okcu4

 

It'll work for your application. For higher current a different device would be better. The turn voltage could be as high as 4V and Rds will be greater than 0.54 ohms. Probably still okay because you'll only be drawing 360mA.

 

Would this be better suited?

N-channel MOSFET Transistor, 2.4 A, 20 V, 3-Pin SOT-23
http://uk.rs-online.com/web/p/products/7082591/

 

I think so. Are you able to solder SOT-23 parts? They're quite small and rework is difficult without a hot air tool.

 

I think so. Are you able to solder SOT-23 parts? They're quite small and rework is difficult without a hot air tool.

Nope I've never tried soldering SOT-23 parts. I'm new to all this and so far my soldering experience extends to soldering the header pins to the arduino nano. Guess there's only one way to learn!

 

I would use a mosfet like IRF540..

 

The post #8 circuit shows a 1N4148 diode, which would be unsuitable (too low a current rating) for the present application. The 1N4004 shown in post #1 would be better. Bear in mind that the stall/start-up current of the pump motor will be several times its normal (rated) running current.

 

I would use a mosfet like IRF540..

View attachment 118662

Nice one, would this one be ok?
http://uk.rs-online.com/web/p/mosfet-transistors/7085143/

 

Overkill again. Since your Arduino is operating at 5V, you could use a 2N7000 for each of the pumps. You'd need an extra resistor and snubber diode, but total parts will cost less than a single power MOSFET.

You don't need R2 in the schematic in post #8. It would reduce your drive voltage by 10% and is unnecessary.

 

Yes Mark, tge mosfet will do, you wont need a gate resistor as they are high resistance, however you can try it with.

 

Would this one be ok?
N-Channel MOSFET 5.6A 100V Rds=0.54
https://www.bitsbox.co.uk/index.php...ucts_id=1019&zenid=o6gf1u45n5r6d6msci852okcu4

take a look at figure 1/2 and if 5v is applied to the gate, how much current can it flow and at what Vds?

in general, you will need a logic level mosfet - they start to conduct at 2 - 3v and are fully open at 5v. older mosfets, like th eirf you liked to, need much high voltages, like 7 - 9v, to be fully open.

 

Nice one, would this one be ok?
http://uk.rs-online.com/web/p/mosfet-transistors/7085143/

***No one*** from the IRF5xx family will work, when driven by an arduino.

 

***No one*** from the IRF5xx family will work, when driven by an arduino.

Care to clarify why? At a Vgs of 5V, the mentioned device would be sufficient for switching 360mA.

 

take a look at figure 1/2 and if 5v is applied to the gate, how much current can it flow and at what Vds?

in general, you will need a logic level mosfet - they start to conduct at 2 - 3v and are fully open at 5v. older mosfets, like th eirf you liked to, need much high voltages, like 7 - 9v, to be fully open.

How about this one, will this work with the Arduino?
http://uk.rs-online.com/web/p/mosfet-transistors/8075863/

 

Unless you plan to drive pumps that require more than 180mA, why don't you look at MOSFETs that will handle 200-500mA?

Like 2X 2N7000?

 

How about this one, will this work with the Arduino?
http://uk.rs-online.com/web/p/mosfet-transistors/8075863/

you want to learn how to read the datasheet yourself. just follow what I laid out earlier and it is simple to see if a given mosfet can or cannot do it.

 

you want to learn how to read the datasheet yourself. just follow what I laid out earlier and it is simple to see if a given mosfet can or cannot do it.

Well I'v just spent the last few hours reading up on mosfets and interpreting their data sheets and so have just ordered a pack of of these.
http://uk.rs-online.com/web/p/mosfet-transistors/6715367/
Hope I got it right

 

Hope I got it right

"hope" usually is a good plan B,

in cases like this, you want to know what's the worst case load. motors typically draws the most current at low rpm -> when they are starting. so I would first measure the dc resistance of the motor, calculate its start current, double that for some headroom in the future.

Once that is in place, I would check the datasheet to make sure that the mosfet can switch at least that much current given the driver / signal source you have.

from there do some loss / thermal calculation to see if you need to heatsink the mosfet.

Rds / Id don't come into play until and unless you have established that the mosfet can be safely switched.

alternatively, you can always hope for the best,