# Transistors to switch 12V 2A

I need to switch some power to a device, so I was wondering, if this will work OK, or do I need to add some other stuff to protect it, or make it work correctly.

I ran some simulations, on CircuitLab, and according to them this should work, but I would like, the opinion of other professionals.

VCC = 12V (2A)

YELLOW_IN, WHITE_IN = 0-5V, 40mA (PWM from Arduino)

YELLOW, WHITE = VCC

I should also probably put a resistor between the Collector of Q1 and Base of T1.

This schematic is pretty straight forward, when you put 5V to YELLOW_IN it will act as a switch and then the YELLOW will be connected to VCC which is 12V.

Do you see any problems with this schematic or design?

As for this schematic, it acts the same as the other one, except for one more case, I need to be able to supply ~0.4V to WHITE when WHITE_IN is 0.4V, and when WHITE_IN is 5V, it will bridge VCC with WHITE.

But in that case what happens with the voltage/current coming from WHITE_IN.

Do I need to build that part of the circuit differently ?

Edit 1: I've modified the schematics according to the suggestions, is this sufficient now.

• as a side note, not having read the question much yet, please follow the guidelines for drawing better schematics, which don't make us turn our heads and fart to make sense of it.. – KyranF Dec 26 '14 at 17:30
• "I should also probably put a resistor between the Collector of Q1 and Base of T1." No, because R3 in your first schematic is already current-limiting into the base of T1, and R2 limits the current "out" of the base of T1. – KyranF Dec 26 '14 at 17:33
• Can you please explain what you are trying to do with D1 and R4 in the first diagrams, and in the second diagram explain what D3 is trying to do? And again, the D4 and R10 combination on the second diagram. They do not look logical/I don't see what you are trying to do there. – KyranF Dec 26 '14 at 17:34
• The diodes are there just in case there is current feed back, as the lines are attached to a small motor. There is no speed control. – ilijamt Dec 26 '14 at 17:37
• Shouldn't the collector be connected to GND with a resistor on a PNP transistor? – ilijamt Dec 26 '14 at 17:40

Your circuit looks OK, but it won't switch 2 Amps.

At 2A the TIP42 has typical DC current gain <=50 at 2A, so you need to feed at least 40mA into its Base. In your circuit Q3 cannot deliver this much, because R11 is limiting its Emitter current to about 9mA.

You should reduce the value of R11 to 100 Ohms or less. This will also increase Q3's Base current and R9 could drop several volts, so you should reduce the value of R9 to about 1k, and increase the value of C3 to about 10uF.

When switched on the 2N3904 will dissipate about 0.3W (7V x 0.04A) so it could get quite hot. You can reduce this heating by putting a resistor between the Collector of Q3 and Base of T3. A 100 Ohm resistor would drop 4V at 40mA, taking half the heat away from Q3.

• Would you recommend I replace 2N3904 it with a different transistor then, and if so with which one? Is there a different approach to do this? – ilijamt Dec 27 '14 at 12:15
• I was planning to add a heatsink to the TIP42, or should I replace this one with a different one. – ilijamt Dec 27 '14 at 12:17
• Should I replace the TIP42 with a MOSFET then? What if I want to switch up to 5A – ilijamt Dec 27 '14 at 12:55
• The TIP42 should work with the changes I suggested, but a suitably rated MOSFET (eg. IRF4905) would be more efficient. – Bruce Abbott Dec 27 '14 at 18:46
• What about the 2N3904, should I maybe replace it with 2N2222 ? – ilijamt Dec 27 '14 at 20:30

This is simple, simulates nicely, and should work just fine.

R1 is the 2 ampere load, 10 millihenries is a WAG as to the motor's inductance, and I just semi-randomly picked the FDS6576 out of the LTspice library; almost any P channel MOSFET will work there. Likewise, there's nothing holy about the 2N2222.

Here's a link to the LTspice circuit list if you want to play with the circuit.

• I ran simulation on the top circuit too, it simulated nicely, and there wasn't any issues. Do you see any issues with the edited circuits ? – ilijamt Dec 28 '14 at 0:06
• @ilijamt I don't see the need for [on their schematics] C1, D1, R2, or R4, and R3 should be in the 100 ohm ballpark. – EM Fields Dec 28 '14 at 20:03
• what about the second circuit, any recommendations on that ? – ilijamt Dec 29 '14 at 23:57
• @ilijamt: Gimme a break, OK? I post a silk purse for consideration and you ask me how to turn your sows' ears into equivalents. To what end? – EM Fields Dec 30 '14 at 0:58
• @ilijamt: How can you not see a difference and then define the difference you don't see? – EM Fields Jan 1 '15 at 1:43