Transistors differ from each other in a number of parameters: structure, maximum dissipated power, open current and open voltage, etc. Only a properly selected transistor will work for a long time in the circuit in which it is installed.
Instructions
Step 1
The load of the transistor is connected between the power bus and the collector of the device. If the voltage on this rail is positive, use an n-p-n transistor, and if negative, use a p-n-p structure. Note that the control signal to the base must have the same polarity as the supply voltage.
Step 2
If the transistor will operate in analog mode, halve the supply voltage and multiply by half the maximum load current. This will be the power dissipated by the device in the most unfavorable conditions - when it is open exactly halfway. If it works in key mode, the maximum power dissipated by it will be much less. To find out, multiply the voltage drop across the transistor in the fully open state (usually only tenths of a volt) by the rated load current. Based on the maximum power dissipation, decide if your appliance needs a heatsink.
Step 3
Take the maximum current drawn by the load as the maximum on-state current, and the power supply voltage as the maximum off-state voltage. These parameters of the transistor must exceed the values in the circuit, at least one and a half times.
Step 4
Select the current transfer ratio depending on the ratio between control current and load current. For example, if this indicator is 50, then the load current can exceed the control current by at least 50 times. Based on this, select the value of the resistor in the base circuit.
Step 5
If the load is inductive, connect a diode in parallel with the opposite polarity of the power supply.
Step 6
Find a transistor in the reference book, all characteristics of which exceed the selected ones with some margin. Do not allow the device to operate at two or more extreme parameter values. Having installed the transistor of your choice in the device, let it work for several hours, then turn off, discharge the capacitors in it and measure the temperature of the transistor. It should not exceed 50 degrees.
