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Triac Based Battery Charger Circuit

Triac Based Battery Charger Circuit

The post explains a simple a battery charger circuit using a triac auto shut-off facility. The circuit can be used for charging any high current, high AH types battery with a full-charge auto cut-off feature. The idea was requested by Mr. Rakesh Parmar.

Using Triac Instead of Relay

In one of the earlier posts we learned a high current battery charger circuit based on a relay total shut off concept, which used a relay to initiate the charging process by switching ON the mains to the transformer and then shutting off the mains as soon as the full charge level was reached for the

In the proposed triac based battery charger circuit the operational principle is exactly similar except the incorporation of a triac instead of a relay.

Circuit Diagram

battery charger using TRIAC



When mains power is applied the circuit does not switch ON by itself, and remains in a standby position.

The indicated push button is positioned for initiating the charging process, therefore as soon as this switch is pressed the triac ismomentarily shorted allowing the transformer to access the mains power
for that instant.

The above action also instantaneously allows the circuit to get powered for that particular period of time.

How it Works

Assuming the battery to be in the discharged position, the above initialization causes a voltage to appear at pin#2 of the opamp at a level lower than the referenced pin#3 of the IC.

This in turn causes pin#6 of the opamp to go high, activating the triac and also latching the transformer in the powered position.

The entire circuit now gets latched and powered even after the switched is released, providing the required charging parameters to the battery. The red LED illuminates confirming the charging initialization of the battery.

As the battery gets charged, pin#2 potential gradually begins rising, until when finally it goes above the reference level of pin#3, which immediately prompts the output of the IC to go low. The moment this happens the triac gate trigger gets cut-off, breaking the latching action, and the entire circuit gets switched OFF.

The circuit returns to its previous standby position, until the next time when the switch is pushed again
for a new caging cycle.

If you liked this battery charger circuit using triac, please do share it with the others.


About the Author

I am an electronic engineer (dipIETE ), hobbyist, inventor, schematic/PCB designer, manufacturer. I am also the founder of the website: https://www.homemade-circuits.com/, where I love sharing my innovative circuit ideas and tutorials. If you have any circuit related query, you may interact through comments, I'll be most happy to help!

30 thoughts on “Triac Based Battery Charger Circuit”

  1. greetings, your material is first line, very good, I want to know the cause, why the load starts automatically without using the button, I think it is because of the positions of the triac pins mt1 and mt2, I predict that when inverted, the circuit is turned on without using the button, otherwise we must consider Edwin’s comment where it is corrected, and the second option is given, since in one of the terminals it remains continuous with the triac chassis, I would like to know if the chassis of the triac is continuous with earth or goes with the other, if it is the second option because I assume that the triac remains active, since I made the mistake of using the button and the operational one is damaged.

    • Hello John, the diagram is already corrected, and in this corrected version there’s no chance of an accidental triac switch ON, unless the opamp output generates a spurious voltage leakage.

      The ground symbol does not indicate chassis connection, it simply indicates the negative supply line of the circuit. By no means this line must be connected to the body or chassis of the enclosure, which could otherwise create a catastrophic situation and short circuit.

      To avoid any chance of the triac misfiring you can connect a 10uF capacitor across its gate and the ground line.

    • Hi Swagatam,as I'm reviewing the circuit,I came accross the BT136,can you pls tell me which pin goes to the push button switch,I mean going up?Is it MT1 or MT2?Thank you

    • with printed side facing you, the first pin from left will go to push button, center to ground, and the right lead to the 2K2 resistor

    • transformer can be 12V/5amp,6amp, 7amp or 8amps

      use bridge rectifier made by using four 15 amp rectifier diodes

      filter capacitor must be rated at 3300uF or above at 25V

    • Thank you,I'll be using my 12v/6A transformer which is available right now and using a 25A bridge rectifier,actually my transformer is a 12-0-12 ct,I also have plenty of silicon rectified diode of 10A,which is better for my 6A transformer,Is it the 25A bridge or 2x 10A silicon for CT?

    • With regards to the BT136,printed side facing me,left is the MT1,center will be MT2 and the right is the gate,Am I right sir?

    • yes 12V 6 amp trafo will do, and you can use the 10amp dioeds for the bridge.

      the triac pinouts that you have mentioned are correct…

      but the connections mentioned by me in the previous comment is incorrect….

      here's the correct format:

      the left (MT!) is supposerd to go to the ground, the center (MT2) to the push button….and the right side pin which is the gate will go to the trigger input

    • Okay thanks for the correction,by the way should I use a heatsink for the BT136? If so,should it be large or just a regular size with several fins and also the diode going to the positive terminal of the battery,will I use the same rating as shown in the circuit or should I replace it with 10A?Thank you.

    • Hello sir Swagatam,I only need a appropriate casing for my charger and everything will be ready for testing but I like to incorporate a Ammeter for this charger,I bought a rated 50A ammeter on ebay and only to find out it doesn't include a shunt,the description of the ammeter said its 75mV 50A and need a shunt to operate,my question is that if I connect this ammeter,should I need a 50A shunt since I'm only consuming 6A from the transformer?I like to be sure I will not damage the ammeter if ever theres no shunt connected.Thank you and hope to hear your advise soon.

    • Hello Edwin,

      you may have to fix with some trial and error,

      or by using Ohms law you can calculate how much resistance (shunt) may produce a potential difference of 75mV across it at 6A and then use this shunt for the meter.

    • I search the web and saw some tutorial using a solid copper wire to make a substitute for current shunt,maybe this will be a temporary solution but should calculate it very well to produce a decent shunt,other option is to buy a ready available shunt.Thank you again.

  2. Hi sir Swagatam,can you explain how to setup/setting for this circuit? I like to know how to adjust the setting for cutoff, thank you

  3. hello im trying to find out the input voltage, if its 240 or a lesser voltage as the end has been cut off my lead.its a ryobi intelliport 14.4 nicad and 18.0 volt lithium one plus charger. i just want to put another lead on it , buggered without it .cheers wayne contact at senyaw69@hotmail.com any assistance would be greatly apprieciated thank you

    • Hi, all AC/DC SMPS adapter units are rated to work with a minimum 100V to a max 285V, so there's nothing critical about the input supply according to me.

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