This pair is my favorite when driving small solenoids or motors. SOT-32 and 12 Watts on Heatsink. This means any small solenoid, relay or DC motor can be controlled with this. I would use it upto 24V DC and max of 1A for applications of full on and full off.
“The NPN types are the BD135 and BD139, and the complementary PNP types are the BD136 and BD140.” – – – BD139 – NPN, 80V and 1.5A, 40 beta min.
“They are designed for audio amplifiers and drivers utilizing complementary or quasi-complementary circuits.” – – – BD140 – PNP, 80V and 1.5A, 40 beta min.
This device is meant for small amplifiers but can be used for small device controls. Ensure you use freewheeling diodes to protect devices from high kickback voltage. These punches from Solenoids or Conductors should be bypassed. This inductive kickback phenomena is used to generate the sparks for your motorbike or car to start with the spark plug.
In an applications like a regulator or variable motor control, you have to keep in mind the current thru and voltage across the transistor which results in power dissipation. Let us say it is driving 0.5A thru a motor in speed control and the Voltage across the device is 30V then the dissipation could be 30 x 0.5 = 15 Watts. That would generate a lot of heat and damage the device.
Without Heatsink The Power Dissipated is less and with a Good Heatsink the Power Dissipated can be much more. Using any device at high junction temperatures can cause failures. Use devices much within the absolute max ratings.
That is the reason we use switched regulators or switched controllers. So if we chop the DC and do PWM you have only the Switching losses and also a bit of DC loss. Switched controllers are also green, they save power for the same work done, but generate EMI-RFI and care is required in sensitive uV circuits. The transistors for switching are also special as they have to be fast with less capacitance, hence the mosfets are preferred in those applications.