Thyristor chargers (CHDs) remain one of the most reliable and repairable solutions for restoring the capacity of car batteries. Despite the spread of switching power supplies, thyristor circuits stand out for their simplicity, resistance to overloads and the ability to accurately adjust the current. In this article we will analyze 3 working schemes (on KU202N, T122-25 and BT151), we will explain the principle of their operation and show how to avoid common mistakes during assembly.

Feature of thyristor memory - phase current control, which allows you to smoothly regulate the output voltage without complex microcircuits. This makes them ideal for desulphating old batteries where slow, low current charging is required. However, incorrect calculation of elements or installation errors can lead to thyristor breakdown due to reverse voltage or transformer overheating. Next is a detailed analysis of the circuits with explanations for beginners and experienced radio amateurs.

Operating principle of a thyristor charger

The thyristor in the memory circuit plays the role electronic key, which opens at a certain moment in the half-cycle of the mains voltage. The later the opening occurs, the less average current flows to the battery. This process is controlled phase control system (usually based on dinistor or optocoupler), which analyzes the voltage on the battery and adjusts the opening angle of the thyristor.

Key stages of work:

  • 🔌 Voltage conversion: mains 220V is reduced by a transformer to 15–20V (for a 12V battery).
  • ⚡ Straightening: A diode bridge converts alternating current into pulsating direct current.
  • 🔄 Adjustment: the thyristor “bites off” part of the half-wave, reducing the average current.
  • 📊 Control: The feedback circuit monitors the voltage on the battery and adjusts the opening angle.

The main advantage of this scheme is no high frequency interference, which are typical for pulse memories. This is especially important for older batteries with sulfated plates, where sudden surges of current can exacerbate degradation. However, thyristor circuits require proper selection of a transformer: its power should be 20–30% higher than the maximum charging current (for example, for a 10A charger you need a 120–150 W transformer).

⚠️ Attention: Thyristor chargers are not suitable for lithium batteries! Lithium-ion batteries require strict voltage control (typically 4.2V per cell) and multi-stage charging, which is impossible to implement with simple thyristor circuits. Use them only for lead-acid batteries (WET, AGM, GEL).

Scheme No. 1: Classic memory on the KU202N thyristor

This circuit is one of the most popular among car enthusiasts due to the availability of components and stable operation. It is designed for current up to 10A and is suitable for batteries with a capacity of 40–100 Ah. The basis is a thyristor KU202N (analogue 2N1595), which is controlled by a dinistor DB3 via RC circuit.

Key elements of the scheme:

  • 🔹 Transformer: TS-180 or similar with an output of 18–20V/10A.
  • 🔹 Diode bridge: KD203A, KD2999 or 20–30A assembly.
  • 🔹 Thyristor: KU202N (maximum current 10A, reverse voltage 200V).
  • 🔹 Control circuit: DB3 dinistor, resistors and capacitors for adjusting the opening angle.

Customization Features:

  1. Connect to the output of the charger control lamp (12V/21W) instead of battery.
  2. Rotating variable resistor R5 (10 kOhm), achieve a smooth change in the brightness of the lamp from flashing to full glow.
  3. Connect the battery and check the charging current with an ammeter. Optimal value - 10% of battery capacity (for example, 6A for 60 Ah).
Schematic element Denomination/Model Purpose Notes
Transformer TS-180-2 Reduction 220V → 18V Can be replaced with TPP-312 with rewinding of the secondary winding
Thyristor KU202N Current adjustment Install on the radiator! Max. case temperature 80°C
Dinistor DB3 Triggering the thyristor Analogues: DB4, HT-32
Resistor R5 10 kOhm (variable) Charging current adjustment It is better to use a multi-turn potentiometer

Correct polarity of the diode bridge | Reliability of fastening the thyristor to the radiator | Absence of short circuits in the control circuit | Presence of a fuse at the input (2–3A)

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Typical assembly errors:

  1. Thyristor overheating due to the lack of a radiator or poor heat dissipation. The minimum radiator area is 50 cm².
  2. Unstable current at low loads. Solution: increase the capacitor capacity C1 up to 0.47–1 µF.
  3. Diode bridge breakdown when the load is interrupted. Add snubber chain (R+C) in parallel with the thyristor for surge protection.

Scheme No. 2: Powerful memory on the T122-25 thyristor (up to 20A)

This circuit is designed for charging truck batteries (24V) or simultaneously two 12V batteries connected in series. Thyristor T122-25 withstands current up to 25A, and the transformer must have a power of at least 300 W (for example, TSZI-300 with 2×24V output).

Differences from the previous scheme:

  • 🔹 Full-wave control (2 thyristors or triac are used).
  • 🔹Additional zener diode (for example, D814D) to protect the control circuit.
  • 🔹 Ability to work in desulfation (pulse current with pauses).

For assembly you will need:

Transformer: TSZI-300 (2×24V, 12.5A)

Thyristors: T122-25 (2 pcs.) or triac TS122-25


Diode bridge: KVRS5010 (50A)


Resistors: R1=1kOhm, R2=4.7kOhm (variable), R3=100 Ohm


Capacitors: C1=0.1 µF, C2=10 µF/50V


Dinistor: DB3

Desulfation mode implemented by adding a time relay (for example, on NE555), which periodically turns off the load for 1–2 seconds. This helps break down the lead sulfate crystals on the battery plates. Relay connection diagram:

Relay connection diagram for pulse mode

The relay is connected in parallel to the thyristor control circuit through a transistor KT815. The base of the transistor is connected to the output of the timer NE555, tuned to a frequency of 0.5–1 Hz (period 1–2 seconds). The relay closing time (pause) is regulated by a variable resistor in the timer circuit.

⚠️ Attention: When charging a 24V battery it's impossible use two 12V batteries connected in series if they have different capacities or degrees of wear! The difference in internal resistance will lead to uneven distribution of current and overcharging of one of the batteries. Always charge batteries separately or use special balancing circuits.
📊 What type of battery do you charge most often?
Lead Acid (WET)
AGM
GEL
Calcium (Ca/Ca)
Lithium (LiFePO4)

Scheme No. 3: Compact memory on the BT151 thyristor (for low currents)

This circuit is suitable for charging motorcycle batteries (6V/12V) or maintaining a charge for car batteries in winter (current up to 3A). Thyristor BT151 (analogue 2N6504) is compact and does not require a massive heatsink, which allows you to assemble the charger in a case from a computer power supply.

Advantages of the scheme:

  • 🔹 Minimum number of components (can be assembled by hanging installation).
  • 🔹 Low interference (suitable for sensitive electronics in the car).
  • 🔹 Possibility of power supply from a 110V network (when replacing the transformer).

Installation features:

  1. Use a transformer with output voltage 14–16V (for example, from an old laptop adapter).
  2. To protect against reverse voltage, install Schottky diode (for example, 1N5822) parallel to the thyristor.
  3. To indicate charging, add an LED with a 1 kOhm resistor in parallel with the load.

Calculation of elements for a 12V battery:

  • 📌 Resistor R1 (in the control circuit): 1–2.2 kOhm (limit the current to the dynistor).
  • 📌 Capacitor C1: 0.047–0.1 µF (adjusts the current range).
  • 📌 Input fuse: 1A (to protect against short circuit in the primary winding).
💡

To check the functionality of the circuit without a battery, use a 12V/21W car lamp. If the brightness of the lamp changes smoothly when rotating the potentiometer, the circuit is working correctly.

Calculation of a transformer for a thyristor charger

The correct choice of transformer is the key to stable operation of the charger. Main parameters for calculation:

  1. Secondary voltage: should be 20-30% higher than the battery voltage (for example, for a 12V battery - 15-16V).
  2. Secondary current: not less than the maximum charging current (for example, for a 10A charger - 10–12A).
  3. Power: P = U × I × 1.3 (where 1.3 is the reserve for losses). For 12V/10A: 15V × 10A × 1.3 = 195 W.

If you are using a ready-made transformer (for example, from old equipment), check its parameters:

  • 🔧 Measure the resistance of the windings: the primary usually has a resistance of 10–100 Ohms, the secondary - 0.1–2 Ohms.
  • 🔧 Connect the transformer to the network through a 60 W incandescent lamp (in phase break). If the lamp does not light, the windings are not closed.
  • 🔧 Measure the output voltage under load (for example, using a 12V/21W lamp). A voltage drop of more than 10% indicates insufficient power.

Transformer rewinding may be required if the finished version does not meet the parameters. To do this:

  1. Calculate the number of turns per volt: N = 50 / S, where S — cross-sectional area of the core (cm²).
  2. For the secondary winding, use a wire with a cross-section of at least 0.7–1 mm² for every ampere of current.
  3. After rewinding, check the insulation between the windings with a megohmmeter (resistance should be >10 MOhm).
💡

Using a transformer with insufficient power will lead to its overheating and a decrease in the efficiency of the charger. The optimal power reserve is 30–50% of the calculated value.

Typical faults and their elimination

Even a properly assembled thyristor charger can fail due to voltage surges, improper operation, or aging components. Let's look at the most common problems:

Symptom Possible reason Remedy
The charger does not turn on, the fuse burns out Short circuit in the primary winding of the transformer or breakdown of the thyristor Ring the windings, check the thyristor with a multimeter (in diode mode)
Charging current is not adjustable The dinistor is faulty or there is an open circuit in the control circuit Replace DB3, check resistors and capacitors in the starting circuit
The charger is heating up, but the current is low Insufficient transformer power or poor contact in the diode bridge Increase the power of the transformer or replace the diode bridge
Sparking inside the housing Breakdown of capacitors or poor insulation of windings Replace capacitors, check insulation with a megohmmeter

Thyristor diagnostics:

  1. Unsolder the thyristor from the circuit.
  2. Connect the multimeter in diode test mode: it should show a break between the anode and cathode.
  3. Briefly connect the control electrode to the anode - the thyristor should “open” (the resistance between the anode and cathode will drop to zero).
  4. If the thyristor does not open or shows a short circuit, replace it.
⚠️ Attention: When replacing a thyristor with an analogue one, check it maximum reverse voltage (Vdrm)! For example, KU202N has Vdrm = 200V, and BT151 — 600V. Using a thyristor with less Vdrmthan the amplitude voltage of the secondary winding (for example, 24V × √2 ≈ 34V), will lead to its instant breakdown!

Safety precautions when working with thyristor chargers

Thyristor chargers operate with life-threatening voltages (220V and above). Following safety rules will help avoid electric shock and fire:

  • ⚡ Housing insulation: all metal parts (radiators, transformer) must be reliably grounded or isolated from the network.
  • ⚡ Fuses: Install the fuse on 1–2A to the primary circuit and to 10–15A - to the secondary.
  • ⚡ Wires: Use cables with a cross section of at least 1.5 mm² for the primary winding and 4 mm² for secondary.
  • ⚡ Ventilation: provide free air access to the radiators (the thyristor temperature should not exceed 70°C).

Rules for connecting to the battery:

  1. Always connect first Charger for battery, and then plug it into the network.
  2. Do not touch the battery terminals while the charger is running - there may be dangerous voltage on them!
  3. Use serial connection of an ammeter to control the current (parallel connection will lead to a short circuit!).
  4. Do not leave the charger unattended for more than 2–3 hours (risk of overcharging and explosion of the battery).

What to do in case of fire:

  • 🔥 Immediately disconnect the charger from the network.
  • 🔥 Use carbon dioxide or powder fire extinguisher (water conducts current!).
  • 🔥 If a transformer catches fire, do not try to extinguish it while it is connected to the network!
💡

For additional protection, install varistor (for example, S14K275) parallel to the primary winding of the transformer. It will absorb power surges and extend the life of your charger.

FAQ: Frequently asked questions about thyristor chargers

Is it possible to use a thyristor charger for gel (GEL) batteries?

Yes, but with reservations. Gel batteries require strict voltage control (usually no more than 14.4V for 12V battery). In a thyristor memory it is necessary:

  1. Add voltage stabilizer at the output (for example, at LM317).
  2. Use trip relay, which breaks the circuit when it reaches 14.4V.
  3. Charge with current no more than 5% of capacity (for example, 2.5A for 50 Ah).

Without these modifications, the risk of overcharging and destruction of the gel is very high!

Why does my charger get very hot, but the charging current is low?

This is a typical sign insufficient transformer power or bad contact in the power circuit. Check:

  • 🔹 Transformer temperature - if it is hot, its power is low.
  • 🔹 Condition of the diode bridge: test the diodes with a multimeter (one way - 0.5–0.7 V, the other - open).
  • 🔹 Quality of soldering of the thyristor and diodes - overheat suspicious contacts with a soldering iron.

If the transformer is weak, it can be modified by adding a second winding or replacing the core with a larger one (for example, from TS-270).

How to modify the circuit to charge 24V batteries?

For a 24V battery you need:

  1. Use a transformer with an output 28–30V (for example, two 15V secondary windings connected in series).
  2. Replace the thyristor with a model with Vdrm ≥ 50V (for example, T122-50).
  3. Increase the capacitor ratings in the control circuit by 1.5–2 times (for example, C1=0.22 µF).
  4. Add balancing resistor (10–20 Ohm/10 W) in parallel with each 12V battery section (if you charge two 12V batteries in series).

Also make sure that the diode bridge is rated for reverse voltage not less than 50V (for example, KD2999).

What is the difference between a thyristor memory and a pulse memory?

Main differences:

Parameter Thyristor charger Pulse memory
Efficiency 70–80% 85–95%
Interference level Low High (filters required)
Circuit complexity Simple (analog elements) Complex (PWM controller, MOSFET)
Maintainability High Low (difficult to diagnose)
Cost Low Medium/High

Thyristor chargers are better suited for restoration of old batteries and work in conditions of unstable network voltage. Pulse - for fast charging and compact devices.

Is it possible to charge the battery without removing it from the car?

Yes, but subject to the rules:

  • 🔹 Disable negative terminal Battery before connecting the charger (to avoid voltage surges in the on-board network).
  • 🔹 Do not connect the charger while the engine is running - this may damage the generator.
  • 🔹 Monitor the voltage: if it exceeds 14.8V, disconnect the charger (risk of damage to the vehicle electronics).
  • 🔹 Use surge protector for protection against impulse noise.

If the car has sensitive electronics (for example, modern ECUs), it is better to remove the battery and charge it separately.