A charger based on a thyristor is a classic solution for restoring the capacity of a car battery, which combines simplicity of design, reliability and the ability to accurately regulate the current. Unlike modern pulse charging, thyristor circuits are less sensitive to network interference, and their repair and modification are accessible even to novice radio amateurs. The main advantage of such a memory is smooth adjustment of output current without surges, which is critical for lead-acid batteries.

In this article we will look at three proven schemes on thyristors KU202N, T122-25 and their analogues, we will explain the principles of operation of phase-pulse control, and also show how to avoid typical errors during assembly. We will pay special attention transformer calculation and protection against polarity reversal - these are the nuances that are often missed in simplified instructions. If you are looking for a reliable solution for a garage or workshop, a thyristor charger is the best choice.

Operating principle of a thyristor charger

The thyristor in the memory circuit plays the role controlled rectifier, which passes current only at certain points in time, thereby regulating the average value of the output voltage. Unlike diode bridges, where rectification occurs constantly, the thyristor β€œcuts off” part of the alternating current sinusoid, changing the opening angle (phase). The later the thyristor opens in each half-cycle, the lower the output power.

Key elements of the scheme:

  • πŸ”„ Transformer β€” reduces the mains voltage to a safe 12–24 V (depending on the type of battery).
  • ⚑ Thyristor (for example, KU202N) - controls the charge current, opening under the influence of impulses from the control system.
  • πŸ“‰ Phase control circuit β€” generates pulses to open the thyristor at the right moment (most often based on DB3 dinistor or transistor switch).
  • πŸ”‹ Ammeter and voltmeter β€” to control charge parameters (mandatory for safety!).

The advantage of thyristor memory devices over transistor ones is high overload capacity and short circuit resistance. For example, thyristor T160-50 can withstand current up to 50 A, which allows you to charge batteries with a capacity of up to 200 Ah without the risk of overheating. However, correct operation requires the correct selection of control circuit elements - more on this in the next section.

πŸ“Š What type of charger are you using?
Thyristor
Transistor
Pulse
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Top 3 thyristor memory circuits: from simple to advanced

We have selected three schemes of varying complexity that cover most tasks - from charging motorcycle batteries to restoring deeply discharged truck batteries. All schemes have been tested in practice and can be repeated with minimal modifications.

1. The simplest circuit on KU202N (for beginners)

This circuit is suitable for charging batteries with a capacity of up to 60 Ah. It does not require scarce parts and can be assembled in 1–2 hours. Main components:

  • πŸ”Œ Transformer TS-180 or similar (220V β†’ 18V, 2–3 A).
  • πŸ”§ Thyristor KU202N (can be replaced by T122-25).
  • πŸ’‘ Dinistor DB3 for generating impulses.
  • πŸ“ Resistors: 1 kOhm (tuning), 100 Ohm, 10 kOhm.
  • πŸ”‹ Capacitors: 0.1 Β΅F, 1 Β΅F (16V).

Features of the scheme:

  • βœ… Minimum number of elements.
  • βœ… Smooth current adjustment from 0.5 to 5 A.
  • ⚠️ There is no protection against polarity reversal (needs improvement!).
  • ⚠️ Requires precise adjustment of the 1 kOhm resistor to correctly open the thyristor.
Detailed description of the circuit operation

When voltage is applied to the transformer, alternating current flows to the anode of the thyristor. Dinistor DB3 opens when the voltage on the 0.1 Β΅F capacitor reaches the threshold value (~30V), forming a pulse to the control electrode of the thyristor. The opening moment (phase) is regulated by a 1 kOhm resistor, which changes the average charge current.

To increase the output current, you can use a higher power transformer (for example, TS-320), but you will need to replace the thyristor with a more powerful one (T160-50) and add a cooling radiator.

2. Circuit with protection against polarity reversal and short circuit

This version is suitable for permanent use in the garage. In addition to the basic scheme added:

  • πŸ›‘οΈ Protection relay (e.g. RES-55A) to switch off if the terminals are connected incorrectly.
  • πŸ”₯ 10 A fuse in the primary circuit of the transformer.
  • πŸ“Š LED indicators: β€œCharge”, β€œPolarity reversal”, β€œShort circuit”.

Critical point: the protection relay must operate at a reverse polarity voltage of more than 1 V. To do this, a zener diode is added to the relay control circuit D814D (stabilization voltage 12–14 V). The thyristor control circuit remains the same, but now it is safe even for inexperienced users.

β˜‘οΈ Components for the circuit with protection

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3. Advanced circuit with temperature compensation and desulfation

To restore old batteries with plate sulfation, a circuit with pulse charging mode. It includes:

  • 🌑️ Thermistor NTC to correct the current depending on the battery temperature.
  • ⚑ Pulse generator on NE555 for periodic shutdown of the charge (desulfation effect).
  • πŸ“ˆ Two-stage adjustment: coarse (switch) + smooth (potentiometer).

This scheme is more difficult to set up, but allows you to restore up to 80% of the capacity of β€œdead” batteries. The main thing is to choose the right pulse frequency (optimally 1–5 Hz) and the charge/pause time ratio (for example, 2 seconds charge / 1 second pause).

Parameter Simple scheme Circuit with protection Advanced circuit
Max. charge current, A 5 10 15 (pulse mode)
Reverse polarity protection ❌ No βœ… Yes (relay) βœ… Yes + thermal protection
Assembly complexity Low Average High
Possibility of desulfation ❌ No ❌ No βœ… Yes

Transformer calculation and thyristor selection

The correct selection of the transformer depends on Charger efficiency and its reliability. Main parameters for calculation:

  • πŸ”Œ Secondary voltage: Should be 20-30% higher than the rated voltage of the battery. For a 12V battery, 15–18 V is optimal.
  • ⚑ Secondary current: equal to the maximum charge current (for example, for a 10A charger - at least 10–12 A).
  • πŸ”„ Transformer power: P = U Γ— I Γ— 1.3 (with a margin of 30%). For 12V/10A: 15V Γ— 10A Γ— 1.3 = 195 W.

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

  1. Measure the resistance of the secondary winding - it should be in the range of 0.1–1 Ohm (for 10–20 A windings).
  2. Connect the transformer to the network through a 60 W incandescent lamp (for short circuit protection) and measure the voltage on the secondary winding.
  3. If the voltage is lower than the calculated one, add additional turns of wire with a cross-section of at least 2 mmΒ².

Thyristor selection depends on the maximum current and reverse voltage:

  • πŸ”‹ For currents up to 10 A: KU202N, T122-25, BT151.
  • πŸš› For currents 10–50 A: T160-50, T250-50 (heatsink required!).
  • ⚑ The thyristor reverse voltage must be no less than 1.5 Γ— Umax secondary winding (for example, for 18 V - at least 27 V).
πŸ’‘

If you don’t have a suitable transformer at hand, you can use two series-connected transformers from microwave ovens (for example, MOT - Microwave Oven Transformer). However, they need to be modified: remove the shunts and wind a new secondary winding with 2–3 mmΒ² wire.

Step-by-step assembly of the charger

For assembly, we will take the second circuit (with protection against polarity reversal) as the most balanced. You will need:

  • πŸ”§ Soldering iron (40–60 W), solder, flux.
  • πŸ”¨ Drill for attaching elements to the body.
  • πŸ“ Multimeter for checking circuits.
  • πŸ› οΈ Case (you can use the power supply from your computer).

Step 1. Preparing the transformer

If the transformer is homemade, wind the secondary winding with a wire with a diameter of 1.5–2 mm. Calculate the number of turns using the formula:

N = (Uw Γ— 10⁡) / (S Γ— f Γ— B)

where:

  • UW β€” voltage of the secondary winding (15–18 V),
  • S β€” cross-sectional area of the core (cmΒ²),
  • f β€” network frequency (50 Hz),
  • B β€” magnetic induction (for iron ~1.2 T).

Step 2. Installation of the power section

Install a thyristor on the radiator KU202N (use thermal paste!). Connect it to the transformer via a diode bridge (if using a bridge rectification circuit) or directly (for half-wave rectification). Attention: with half-wave rectification, the ripple current will be higher - this can lead to overheating of the battery!

πŸ’‘

To reduce ripple, add a 2200–4700 Β΅F Γ— 25V capacitor in parallel with the charger output. This will smooth out the current and extend the life of the battery.

Step 3. Assembling the control circuit

Observe the polarity of the dinistor connection DB3 and resistors. To configure:

  1. Connect a 12V/21W incandescent lamp to the charger output instead of the battery.
  2. Rotate the 1 kOhm trimmer while observing the brightness of the lamp.
  3. Achieve a smooth change in brightness from flashing to constant light.

Step 4. Install protection

Connect the relay RES-55A so that its contacts open the circuit when polarity is reversed. You can check the operation of the protection by swapping the terminals at the output of the charger - the relay should operate and turn off the load.

⚠️ Attention: Do not connect the battery to the charger without a load (for example, a lamp) when you turn it on for the first time! This can lead to an inrush current and failure of the thyristor. Always test the circuit at idle with ballast load.

Common mistakes and how to avoid them

Even experienced radio amateurs make mistakes when assembling thyristor chargers. Here are the most common:

  1. Wrong choice of transformer:

    Using a transformer with insufficient power leads to overheating and voltage drop under load. Symptoms: The charger works normally without load, but when the battery is connected, the current drops to zero.

    Solution: Replace the transformer with a more powerful one or add an additional winding.

  2. No radiator on the thyristor:

    Thyristors series KU202 they heat up even at currents of 5–7 A. Without a radiator, they fail within 10–15 minutes of operation.

    Solution: Use a radiator of at least 50 cmΒ² with thermal paste.

  3. Unstable operation of the control circuit:

    If the thyristor opens chaotically or does not open at all, the problem is in the dinistor circuit DB3 or capacitors.

    Solution: Check the values of resistors and capacitors, replace the dinistor if a breakdown is suspected.

  4. Ignoring reverse polarity protection:

    Connecting the battery in reverse polarity leads to instant failure of the diodes and thyristor.

    Solution: Add a protection relay or diode to the control circuit (for example, 1N4007).

Hidden problem: many diagrams on the Internet do not take into account battery internal resistance, due to which the actual charge current may differ from the calculated one by 20–30%. To avoid this, calibrate the ammeter under load (for example, using a 12V/55W car lamp).

⚠️ Attention: If your charger β€œboils” the battery (intensive gas emission) at a current below 1/10 of the battery capacity (for example, 6 A for 60 Ah), then the output voltage is too high. Reduce the number of turns of the transformer secondary winding or add a zener diode to the thyristor control circuit.

Setting up and testing the charger

After assembling the charger, you need to check its operation in several stages:

  1. Checking idle speed:

    Connect the storage device to the network no load and measure the output voltage. It should be within 15–18 V (for a 12V battery). If the voltage is above 20 V, immediately turn off the circuit - a breakdown of the thyristor is possible!

  2. Load test:

    Connect a 12V/21W lamp or a 10 W resistor (resistance 2–5 Ohms) to the charger output. Set the current to 1–2 A and check the stability of operation for 10–15 minutes. Monitor the heating of the thyristor and transformer.

  3. Ammeter calibration:

    Compare the readings of the built-in ammeter with an external multimeter (in current measurement mode). If the discrepancy is more than 10%, adjust the ammeter shunt or replace it.

  4. Security check:

    Deliberately change the polarity at the output of the charger - the relay should operate and turn off the load. If there is no protection, do not use the scheme without modification!

To fine-tune the charge current:

  • Use ballast resistor (for example, a nichrome spiral) to simulate a battery.
  • Adjust the potentiometer so that the current varies smoothly from 0.5 A to the maximum value.
  • Check the operation of the circuit at minimum and maximum current - ripple should not exceed 10% of the average value (measured with an oscilloscope or multimeter in AC mode).

Test on a real battery:

Connect the charger to the discharged battery (voltage not lower than 10.5 V!). Start with a current of 1-2 A and observe:

  • 🌑️ Temperature of the battery case (should not exceed 40Β°C).
  • πŸ’‘ Voltage at the terminals (should increase smoothly to 14.4 V).
  • ⚑ Charge current (should stabilize after 1–2 hours).
πŸ’‘

If the charging current does not decrease after 4–5 hours, and the battery voltage exceeds 15 V, immediately disconnect the charger! This is a sign of a battery malfunction (short-circuited cells) or incorrect circuit settings.

Modernization and improvements

The basic thyristor memory circuit can be improved by adding useful functions:

1. Automatic shutdown when fully charged

To do this, use a comparator on LM358 or a voltage relay that opens the circuit when it reaches 14.4 V. The circuit is simple:

  • πŸ” The comparator compares the voltage on the battery with the reference one (14.4 V, set by a zener diode TL431).
  • πŸ”Œ When the threshold is exceeded, a relay is triggered, turning off the thyristor.

2. Two-stage charging mode

To speed up charging, add a switch that changes the current:

  • πŸ”‹ "Fast" mode: current 5–10 A (to restore 80% capacity).
  • 🐒 β€œRecharge” mode: current 0.5–1 A (for a full charge without overheating).

3. Remote control

For convenience, you can place the controls (potentiometer, switch) on a separate panel with a cable 1–2 meters long. This will allow you to place the charger in a ventilated place, and the control panel next to the battery.

4. Overheat protection

Install a temperature sensor (for example, KTY83) to the thyristor radiator. At temperatures above 70Β°C, the control circuit must block the opening of the thyristor.

Useful modification for winter use: add to diagram NTC thermistor (for example, 10kΞ©), which will adjust the charge current depending on the battery temperature. At -10Β°C the current should automatically be reduced by 30–40% to avoid sulfation.

FAQ: Frequently asked questions about thyristor memory devices

❓ Is it possible to use a thyristor charger for Li-ion batteries?

No, thyristor circuits are not suitable for lithium batteries for two reasons:

  1. Li-ion required stabilized voltage (usually 4.2 V per bank), and thyristor chargers produce a pulsating voltage.
  2. For Li-ion you need balancer (BMS), which equalizes the charge of individual elements - a thyristor circuit does not provide this.

Using such a charger for Li-ion may lead to overcharge, fire or explosion!

❓ Why doesn’t my charger produce maximum current?

The reasons may be as follows:

  • πŸ”Œ Weak transformer - check the voltage on the secondary winding under load.
  • πŸ”§ Insufficient opening angle of the thyristor - adjust the resistor in the control circuit.
  • πŸ”₯ Thyristor overheating - add a radiator or check the thermal conductive paste.
  • πŸ’‘ Dinistor breakdown DB3 - replace it.

Also make sure that the cross-section of the wires in the power circuit is at least 2.5 mmΒ² - thin wires create additional resistance.

❓ How to calculate battery charging time?

Charging time (in hours) is approximately equal to:

T = (Battery capacity [Ah] Γ— Coefficient) / Charge current [A]

Where coefficient depends on the degree of discharge:

  • 1.1 - for a weakly discharged battery (voltage 12.0–12.3 V),
  • 1.2 - for medium discharge (11.5–12.0 V),
  • 1.4 - for deep discharge (below 11.5 V).

Example: Battery 60 Ah, discharged to 11.8 V, charge current 6 A.

T = (60 Γ— 1.2) / 6 = 12 hours.

In practice, the time may differ by Β±20% due to self-discharge and charge nonlinearity.

❓ Is it possible to charge a gel battery with a thyristor charger?

Yes, but with reservations:

  • βœ… Charge current should not exceed 1/10 of the capacity (for example, 4.5 A for 45 Ah).
  • βœ… Voltage should be strictly 14.1–14.4 V (gel batteries are sensitive to overvoltage!).
  • ⚠️ Prohibited use pulse modes (as in the desulfation scheme) - this destroys the gel.

It is recommended to add to the diagram Zener diode 14.4 V into the thyristor control circuit to limit the maximum voltage.

❓ Why does the transformer get very hot when charging?

Reasons for transformer overheating:

  1. Insufficient power - if the transformer is rated for 100 W, and you are trying to get a current of 10 A (120 W), it will heat up.
  2. Poor ventilation β€” the transformer must be located in a housing with holes for air circulation.
  3. Short-circuited turns β€” check the resistance of the windings with a multimeter (should be 0.1–1 Ohm for the secondary winding).
  4. High line voltage - if your network has 240 V instead of 220 V, the transformer will heat up more. Add varistor or voltage stabilizer.

Solution: if the transformer heats up to 60Β°C or higher, replace it with a more powerful one or add active cooling (12 V cooler).