A simple thyristor charger: circuit, assembly and configuration

Introduction: Why do you need a thyristor charger?

Car batteries require regular maintenance, especially in the cold season. Conventional chargers are often either too expensive or do not provide the required flexibility in current regulation. Thyristor circuits solve this problem: they are easy to assemble, reliable, and allow smooth adjustment of charging parameters.

The main advantage of a thyristor charger is the ability to work with different types of batteries (from 6V up to 24V) without complicated switches. Thyristors such as KU202 or BT151, withstand high currents and voltages, which makes them ideal for homemade designs. In addition, this circuit protects the battery from overcharging and short circuits if the threshold values ​​are set correctly.

If you have ever held a soldering iron in your hands and know how to read electrical diagrams, assembling such a device will not be difficult. In this article we will analyze two working schemes (on one thyristor and on triac), we'll tell you about the selection of components and give step-by-step instructions for setting up. We’ll also warn you about typical errors that can damage both the charger and the battery itself.

Operating principle of a thyristor charger

The thyristor in the charger acts as an electronic key that regulates the flow of current to the battery. Unlike transistors, thyristors are capable of passing large currents (up to 10-50 A) and withstand high reverse voltages (up to 400-1000 V). This makes them indispensable for high-power chargers.

Basic operating principle:

• 🔌 Network 220V goes to a step-down transformer, where the voltage is reduced to 12-24V (depending on battery type).
• 🔄 Thyristor opens at certain times (controlled by a circuit based dinistor or transistor), passing current pulses.
• ⚡ Pulse current charges the battery, and the duration of the pulses regulates the current strength (from 1A up to 10A and above).
• 📉 Stabilization occurs due to feedback: when the voltage on the battery reaches a given level, the thyristor closes.

The key difference from diode rectifiers is The thyristor allows you to smoothly regulate the current without power loss on the resistors. This is especially important for lead-acid batteries, which are sensitive to overcharging.

⚠️ Attention: If the thyristor does not close completely when the load is turned off, this leads to current leakage and overheating of the transformer. Always test the circuit at idle before connecting the battery!

Thyristor charger circuits: selection and comparison

There are several proven thyristor charger circuits. We will look at the two most reliable and easy to repeat:

1. Circuit based on one thyristor (KU202)

This circuit is suitable for charging batteries 12V current up to 10A. It includes:

• 🔌 Step-down transformer 220V/12-15V (power no less 150 W).
• 🔧 Thyristor KU202N (or similar T122-25).
• 📌 Diode bridge (for example, KD202 or 1N5408) for straightening.
• 🔄 Adjustment resistor (potentiometer) to adjust the current.

Benefits:

• ✅ Easy to assemble (minimum number of parts).
• ✅ Low cost (all components can be found in old radios or purchased for 200-300 rub.).
• ✅ Reliability (if assembled correctly, it works for years).

2. Triac circuit (bidirectional thyristor)

This circuit is more complicated, but allows you to charge batteries 6V, 12V and 24V with automatic shutdown when fully charged. Used:

• 🔌 Transformer with taps on 6V, 12V, 24V.
• 🔧 Triac BT138 or MAC97.
• 📊 Microcircuit LM358 to compare voltages (feedback).
• 🔋 Relay to automatically turn off when reaching 14.4V (for 12V batteries).

Benefits:

• ✅ Automatic shutdown when fully charged.
• ✅ Ability to work with different types of batteries.
• ✅ Higher efficiency (less heating losses).

Parameter	Thyristor circuit (KU202)	Triac circuit (BT138)
Maximum current	Up to 10A	Up to 15A
Battery voltage	Only 12V	6V, 12V, 24V
Automatic shutdown	No	Yes
Assembly complexity	Low	Average
Component cost	150-300 rub.	400-800 rub.

Step-by-step instructions for assembling the charger for KU202

This pattern is ideal for beginners. You will need:

• 🔌 Transformer TS-180 or similar (output voltage 12-15V, current 10A).
• 🔧 Thyristor KU202N (can be replaced by T122-25).
• 📌 Diode bridge KD202 or four diodes 1N5408.
• 🔄 Potentiometer 10 kOhm (to regulate current).
• 🔋 Resistors: 1 kOhm, 100 Ohm, 10 Ohm/5W (current limiting).
• 📊 Capacitors: 0.1 µF, 100uF/25V.
• 🔌 Fuse on 10A.

Assembly takes place in several stages:

1. Preparing the transformer.

   Check the output voltage of the transformer without load - it should be 13-15V. If the voltage is higher, add a stabilizing resistor or use a transformer with suitable taps.

2. Diode bridge assembly.

   Connect four diodes in a bridge circuit (cathode to anode). If you are using a ready-made bridge KD202, skip this step.

3. Thyristor installation.

   Thyristor KU202N mounted on a radiator (required!), since it heats up during operation. The anode of the thyristor is connected to the positive terminal of the diode bridge, the cathode is connected to the positive wire of the battery.

4. Control circuit.

   Assemble a control circuit based on a potentiometer and resistors. The control electrode of the thyristor is connected through a resistor 100 Ohm to the midpoint of the potentiometer.

5. Battery connection.

   The negative wire of the battery is connected to the negative of the diode bridge. Positive - through an ammeter (optional) to the cathode of the thyristor.

⚠️ Attention: If the transformer starts to hum or get hot when turned on, immediately turn off the circuit! This is a sign of a short circuit or incorrect connection of the thyristor.

Setting up and testing the charger

After assembly, you need to configure the device so that it charges the battery correctly. To do this:

1. Checking the open circuit voltage.

   Connect a voltmeter to the output terminals (without battery). By rotating the potentiometer, achieve a smooth change in voltage from 0V up to 15V.

2. Battery connection.

   Connect the charger to the discharged battery (voltage not lower than 10.5V). Set the charging current to 1/10 of capacity (for example, for 60Ah — 6A).

3. Current and voltage control.

   During charging, the voltage on the battery will increase, and the current will gradually decrease. If the current does not drop within 2-3 hours, check the circuit for leaks.

4. Checking automatic shutdown (for a triac circuit).

   When the battery voltage reaches 14.4V, the device should turn off. If this does not happen, adjust the response threshold with a resistor in the feedback circuit.

Typical problems and their solutions:

• 🔥 The transformer is heating up. → Check for short circuit in the diode bridge or thyristor.
• ⚡ The charging current is jumping. → Replace the capacitor 0.1 µF or check the soldering of the thyristor control electrode.
• 🔋 The battery is not charging. → Make sure the output voltage is higher 12.6V and the connection polarity is correct.

Safety when working with thyristor circuits

Thyristor chargers operate with hazardous voltages (220V), therefore compliance with safety precautions is mandatory. Basic rules:

• 🔌 Insulation of all live parts. Use heat shrink tubing or electrical tape for exposed connections.
• 🛠️ Reverse polarity protection. Install the diode in the battery circuit (anode to positive, cathode to charger).
• 🔥 Fuses. Be sure to use a fuse on 10A in the primary circuit of the transformer.
• 📌 Ground contact. If the charger case is metal, ground it.

What to do it's impossible:

• 🚫 Connect the battery to the charger with the transformer connected to the network.
• 🚫 Leaving the charger unattended for a long time (risk of overheating).
• 🚫 Use wires with a smaller cross-section 2.5 mm² (they may overheat).

⚠️ Attention: If you smell burning or see smoke from the transformer, immediately unplug the device! Continued operation may result in a fire.

What to do if the thyristor breaks?

If the thyristor fails (short circuit between anode and cathode), it must be replaced. Before installing a new thyristor, check the control circuit: often a breakdown occurs due to a faulty potentiometer or capacitor in the base.

Upgrading the charger: useful improvements

Basic thyristor circuit KU202 can be improved in several ways:

1. Adding an ammeter and voltmeter

To monitor the charging process, it is convenient to install two devices:

• 📊 Ammeter (on 10A) - in series to the battery circuit.
• 📊 Voltmeter (on 20V) - parallel to the battery terminals.

Can use digital modules based on TM1637 or analog dial gauges.

2. Automatic shutdown when fully charged

For the circuit on KU202 you can add a simple relay that will open the circuit when it reaches 14.4V. You will need:

• 🔧 Relay on 12V (for example, SRD-12VDC-SL-C).
• 📊 Comparator on LM358 or transistor trigger circuit.
• 🔄 Trimmer resistor for setting the threshold.

3. Short circuit protection

Add a fast-blow fuse to the circuit or self-repairing polymer fuse (for example, MF-R010). This will protect the thyristor and transformer in case of accidental short circuit of the terminals.

4. Voltage switch (6V/12V/24V)

If your transformer has multiple taps, you can add a switch to handle different battery types. The main thing is not to forget to recalculate the charging current!

Finalization	Difficulty	Cost (RUB)	Benefits
Ammeter + voltmeter	Low	200-400	Charging process control
Automatic shutdown	Average	300-600	Overcharge protection
Short circuit protection	Low	50-150	Preventing breakdowns
Voltage switch	High	100-300	Versatility

FAQ: Frequently asked questions about thyristor chargers

❓ Is it possible to use a thyristor from an old washing machine?

Yes, but you need to make sure that it is designed for a current of at least 10A and voltage not less 400V. Popular models: KU202, T122, BT138. Before use, check the thyristor with a multimeter in diode test mode (it should pass current only in one direction when a control signal is applied).

❓ Why does the charger heat up but not charge the battery?

There are several reasons:

1. The thyristor is faulty (breakdown or break).
2. Open circuit in control circuit (check resistors and potentiometer).
3. The voltage from the transformer is too low (less than 12V).
4. The contacts on the battery terminals are oxidized.

To diagnose, disconnect the battery and measure the voltage at the charger output. If it is lower 12.6V, the problem is in the circuit. If there is voltage, but no current flows, check the polarity of the connection.

❓ How to calculate the power of a transformer?

Transformer power (P) must be at least:

P = U × I × 1.3, where:

• U — charging voltage (14.4V for 12V battery),
• I - maximum charging current (for example, 6A for battery 60Ah),
• 1.3 — power reserve (takes into account losses in diodes and thyristor).

Example: for current 6A need a power transformer 14.4 × 6 × 1.3 ≈ 112 W. It is optimal to take a transformer to 150-180 W.

❓ Is it possible to charge lithium batteries with such a device?

No! The thyristor chargers described in this article are intended only for lead acid batteries (WET, AGM, GEL). Lithium batteries require:

• Accurate voltage control (usually 4.2V on the jar).
• Special controllers (BMS).
• Level current limits 0.5C-1C.

Using a thyristor charger for lithium batteries may cause them to fire!

❓ How long does it take to fully charge the battery?

Charging time depends on:

• Battery capacity (Ah).
• Charging current (A).
• Degrees of discharge.

Formula: T = (Capacity × Ratio) / Charging Current, where:

• Coefficient = 1.2 (takes into account charging efficiency).

Example: battery 60Ah, 50% discharged, charging current 6A:

T = (60 × 0.5 × 1.2) / 6 = 6 hours.

In practice, the time may differ due to self-discharge and non-linearity of the charging process.