With a MOSFET you don't need a resistor at all and there will be less heat produced.

 

Just make sure the MOSFET is logic level, fully on at 5V.

A darlington is not good at 5V. It will lose nearly 2V.

Bob

 

I would recommend a P-channel MOSFET for the job.
This would be good: https://www.digikey.com/product-detail/en/NDP6020P/NDP6020P-ND/1055922

Would a TIP31C work or a L7905CV (as I have them) or would I still need another transistor.

 

As it has an emitter-base voltage of 5v.

Would a TIP31C work or a L7905CV (as I have them) or would I still need another transistor.

 

The 7905 is a negative voltage regulator and the TIP31C is NPN so neither of those is going to help you.

 

The 7905 is a negative voltage regulator and the TIP31C is NPN so neither of those is going to help you.

Thanks. Can I just re arrange my circuit so it's on the negative side?

 

A TIP31 is an NPN, your original circuit has a PNP BD140 which is the opposite.
A TIP32 is a PNP which can replace a BD140.
The maximum allowed reversed base-emitter voltage is 5V which will never occur in your circuit because the base-emitter voltage is never reversed. The datasheet says maximum emitter-base voltage not maximum base-emitter voltage.
An L7905 is a negative voltage regulator, not a transistor. It does nothing when powered from only -5V since its minimum input is -7V.

Why are you using a transistor as a resistor heater instead of using a resistor?
Why are you making a simple heater?

Each transistor has a different amount of gain. The datasheet says its minimum, typical and maximum gain at various currents. Your circuit has nothing to control the amount of output current that might be 1.5A with a low gain transistor or be 7.5A with a high gain transistor. For a collector current of 1.5A then the datasheet for a minimum gain BD140 (hFE= about 30) shows that its base current must be at least 1.5A/30= about 50mA.
An Arduino cannot produce anywhere near 50mA.

Your extremely simple circuit is just a heater that would heat with 1.5A x 5V= 7.5W if you add an additional circuit to limit the current to 1.5A. With 50mA at the base then some transistors will produce a load current of 1.5A and others more, maybe as high as 7.5A or any current in between. Also the current changes as the transistor heats up. To control the output current at 1.5A then you must add a current regulator circuit.

EDIT: This post is wrong since the transistor has a separate load. See my next post for my correction.

 

To control the output current at 1.5A then you must add a current regulator circuit.

I think the load is taking a maximum of 1.5A

 

Thanks. Can I just re arrange my circuit so it's on the negative side?

Yes, you could and the TIP31C is rated for 3A so that's OK, but its gain is low so you would still need a second transistor to boost the arduino output current.

 

Also, you could use a relay.
And running transistors above their max current is not a good idea. They often "protect" fuses as the transistor will blow quicker than a fuse.

 

Thanks for everyone's help. I don't want to be using 2 transistors and frankly I really don't get transistors as people are saying that just need another transistor and then others are saying that I've basically made a heater. I think I will use a relay but thanks for everyone's help again.

 

I think I will use a relay but thanks for everyone's help again.

I think you will need a small transistor to drive the relay from the arduino.

 

I think you will need a small transistor to drive the relay from the arduino.

You can get boards with transistors already on them. Thanks for all your help.

 

You can get boards with transistors already on them. Thanks for all your help.

Indeed you can and that makes a simple solution to the problem.

 

Sorry, I did not see your load since your schematic was sideways and did not fit on my computer's screen.
You are using the transistor as a switch, on and off controlled by an Arduino.
When a transistor is turned on as a switch it is said to be saturated. When it is saturated then its datasheet says how much saturation voltage loss occurs and how much the base current should be. Most transistors including the BD140 and TIP32 datasheets say the transistor is saturated when the base current is 1/10th the collector current, hFE gain is not used when a transistor is saturated.

With a collector current of 1.5A and a base current of 150mA, the saturation voltage loss of a BD140 is shown as a maximum of 1V. Then your 5V load gets only 4V and the BD140 heats with 1.5W which is higher than the maximum allowed heating of 1.25W without having a heatsink. Some BD140 transistors will haver a little less saturation voltage loss and less heating.
Your Arduino cannot produce 150mA to drive one power transistor so a logic level Mosfet should be used as a switch instead.