Thyristor power supply: device, circuit and application in auto electronics

Thyristor power supplies are specialized devices that are actively used in automotive electronics to regulate voltage, protect circuits, and control powerful loads. Unlike traditional transistor circuits, thyristors can handle high currents with minimal losses, making them great for battery charging systems, lighting controls, or even homemade garage welders.

If you have ever faced the need to smoothly adjust the brightness of headlights, stabilize the voltage in the on-board network, or assemble a charger for AGM batteries, then a thyristor power supply could be the optimal solution. However, working with thyristors requires an understanding of their principles - from control features to surge protection. In this article, we will look at how such blocks are designed, where they are used in cars, and how to assemble a simple circuit with your own hands without the risk of burning the car’s electronics.

What is a thyristor and how does it work in a power supply?

A thyristor is a semiconductor device with three (or more) terminals, which can be in two states: open (passes current) and closed (does not miss). Unlike a transistor, a thyristor remains open even after the control signal is removed, until the current through it drops below a certain level (holding current). This property makes it ideal for phase regulation — the basic principle of operation of thyristor power supplies.

There are two types of thyristors most often used in automotive circuits:

• 🔹 Single operation (SCR) — open with a pulse to the control electrode and close only when the current is interrupted (for example, in DC circuits with forced zeroing).
• 🔹 Symmetrical (TRIAC) — they pass current in both directions, which is convenient for working with alternating voltage (for example, in dimmers 12V LED).

In the power supply, the thyristor plays the role of an electronic switch that “cuts” the AC sine wave, changing the average voltage across the load. For example, if you need to reduce the voltage from 14.4V (from generator) to 13.8V (for safe charging of the battery), the thyristor will open with a delay in each half-cycle, actually “cutting off” part of the wave.

⚠️ Attention: Thyristors are sensitive to the rate of current rise (di/dt) and voltage (dv/dt). Without protection (eg RC chains on the control electrode) they may falsely open due to interference in the vehicle’s on-board network, which will lead to a short circuit.

Where are thyristor power supplies used in cars?

Thyristor circuits in auto electronics are used where required smooth power control or overload protection. Here are the most common cases:

• 🔋 Battery chargers - thyristors allow you to accurately maintain the voltage level 13.8–14.4V regardless of network fluctuations or battery discharge level.
• 💡 Headlight/backlight dimmer controls — smooth adjustment of LED or halogen lamps without flickering (unlike PWM controllers).
• 🔥 Heater control - for example, in seat heaters or fuel pre-heaters (Webasto, Eberspächer).
• ⚡ Circuit protection — thyristor crowdbar circuits instantly bypass the load in case of overvoltage (relevant for cars with an unstable generator).

One of the key advantages of thyristors in cars is high reliability at extreme temperatures. For example, in a radiator cooling fan control unit, a thyristor can operate for years at +105°C without degradation, whereas transistor switches require additional cooling.

Scheme of a simple thyristor power supply for a car

Let's consider the classic scheme regulated thyristor power supply KU202N (analogue BT151), which allows you to smoothly change the output voltage from 0V up to 14V at the entrance 12–15V from the on-board network. This scheme is suitable for:

+12V -----[Load]-----
|
Rload
|
Thyristor (A-K)
|
UE ---[R1]--|
|
[VR1]--[C1]--
|
GND

int triacPin = 9; // Arduino pin connected to optotriac
void setup() {
pinMode(triacPin, OUTPUT);
}
void loop() {
for (int i = 0; i < 255; i++) { // Smooth increase in brightness
analogWrite(triacPin, i);
delay(10);
}
}