Switching DC Loads With Transistors

Selecting A Transistor For Switching DC Loads

In this section I am looking at transistors suitable for switching big loads: the transistor acts like a physical switch and is operated either in fully open or fully closed mode.

Transistors can only switch DC loads. They cannot switch AC. If you want to switch AC, use a relais, a solid-state DA relais, or a thyristor.

Selecting Suitable Transistor

To select a suitable transistor for switching purposes, these are the key aspects to check:

Positive vs. Negative Voltage

N-type transistors are more common because they are more readily available, and also cheaper to produce. They need to be negative at their emitter (which is typically connected to GND), so they are controlled by a positive voltage.

If you must control the transistor by a negative voltage, choose a P-type transistor.

Whether you require a N- or P-type transistor depends entirely on your circuit design.

Available Voltage

What is the maximum voltage your circuit can apply to the transistor control pin?

MOSFET transistors are controlled by voltage: if you cannot supply enough voltage to the MOSFET control pin, then it may never close the switch, or more likely: not fully close: you are now accidentally operating the transistor like an amplifier. It now has a very high on-resistance, producing excessive heat and wasting energy.

In low voltage circuits (i.e. ESP32 with 3.3V), you need to be extra careful when selecting a transistor: make sure it can fully turn on with the voltage you can provide.


What kind of load do you want the transistor to switch?

  • Current: what is the maximum current you want to switch (just like physical switches)? If you have a choice of suitable transistor types, choose the one with the highest allowable current.
  • Voltage: in DIY projects, voltage is typically secondary as most circuits use the same single low supply voltage. If however you design a circuit with multiple supply voltages, i.e. using a microcontroller at 3.3V and switching an LED strip at 48V, then make sure the transistor is rated for the voltage you want to switch, i.e. 48V or more.


If efficiency is a priority, i.e. because the device is battery-operated, take a look at the on-resistance (what is the loss you have when the load is turned on?), and the off-resistance (how much electrical power is still flowing even though the transistor is in off state)?

Ideal Physical Switch

A physical switch uses air as insulator: when it is turned on, two contacts touch and provide a connection with very low resistance. When the switch is turned off, the two contacts are moved away from each other, and the air inbetween insulates them so that they have a very high resistance: no current flows.

Not-So-Ideal Transistor Switch

A transistor is a semiconductor: electrons are controlled by the control pin, and these electrons in turn control the conductance of the transistor.

  • Transistor Gets Hot: Depending on transistor type, the on-resistance can be significant. When switching small loads, this loss can typically be ignored. However, with higher currents this loss can quickly turn into an unwanted heat source. The most common practical reason for transistors that get too hot is a MOSFET transistor that is controlled by insufficient voltage: the chosen MOSFET requires a control voltage (much) higher than the voltage that was actually supplied.

  • Battery Dies Prematurely: Transistors are never completely off: some electrons still conduct energy, even in off state. The leakage current is typically very small but can add up over time. Battery-operated devices that may need to run for weeks or years can easily be depleted prematurely when selecting the wrong transistor type with a relatively high off-resistance.

When powering a device through a power adapter, worrying about tiny leakage currents makes neither sense nor a difference.


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(content created Apr 27, 2024)