Battery Charger Circuit using Triac

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.

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.

When mains power is applied the circuit does not switch ON by itself, and remains in a standby position.
indicated push button is positioned for initiating the charging
process, therefore as soon as this switch is pressed the triac is
momentarily 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.
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. Te 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.

25 Replies to “Battery Charger Circuit using Triac”

  1. Unknown

    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 any assistance would be greatly apprieciated thank you

    1. Swagatam Post author

      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.

    1. Swagatam Post author

      Hi Edwin,

      feed the exact over-charge voltage from an external voltage source across the 100uF capacitor.
      then adjust and tweak the pin#3 preset such that the LED just stops glowing…
      Do not connect any battery while doing this…
      that's all, the circuit is set now and will perform as proposed.

    1. edwin siy

      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

    2. Swagatam Post author

      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

    3. edwin siy

      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?

    4. Swagatam Post author

      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

    5. edwin siy

      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.

    6. edwin siy

      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.

    7. Swagatam Post author

      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.

    8. edwin siy

      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.


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