scr 12v battery charger circuit schematic

12V Battery Charger Circuit using SCR

This battery charger circuit differs from the norm in a number of ways, all of which make it difficult to understand. For this reason, I do not recommend it for the beginner.

Repairing /revamping a dead charger
What I started with was an inoperative 12amp battery charger. In hope of repairing it, I traced out the circuit, but did not like what I found—poor circuit design. So what I had to start with was an enclosure, ammeter, thermal overload interrupter, and center-tapped transformer all designed for battery charger application.

Since the maximum current delivered by the unit is a function of the transformer internal impedance, I recommend that the readers use the same type of transformer. If you are a good pack-rat (like me), you may already have a dead charger—or you can be on the lookout for one.

12V SCR Battery Charger Schematic

scr 12v battery charger circuit schematic

SCR (Thyristor) Rectifiers
First of all, the two SCRs (silicon controlled rectifiers or thyristors) are connected with their anodes (stud or tab) grounded—this makes for excellent thermal transfer because no insulating hardware is required (if it is permissible to connect the negative terminal of the charger directly to the steel enclosure). If you do not wish to ground this point, use insulating hardware to electrically isolate the SCRs. This makes the transformer center-tap the positive terminal. The reason for this circuit placement is the ease of driving the SCR gates via the positive battery voltage—it is very unconventional as I have never seen this trick done before.

SCRs are the ideal power device choice for a battery charger because they can both regulate battery charging voltage and prevent fault current when the battery is inadvertently connected reverse. I have actually connected mine reverse and thought that the charger was inoperative until I realized what I had done.

Power Device Selection
I used two 2N690 stud-mount SCRs that I had available. Any in the series will work (2N683 through 2N690)—only the voltage rating differs and anything greater than 100V is good for the application. Other more inexpensive TO-220 candidates are: STMicroelectronics TYN616, Teccor/Littlefuse S6015L (isolated package), NXP 151-500C, or ON Seimconductor 2N6403G. Avoid sensitive gate devices.

Circuit Common
Normally circuits use a negative common—that is just the way the world seems to work, but in this case, it was more convenient to make the positive rail the common point and all visualization must be made with this in mind. The only exception is D7 that was installed to prevent damage should the battery get connected reverse. For visualization, simply short out D7. The conventional ground symbol is used for the negative rail. This tends to tie your brain in knots…

Voltage Reference
A good battery charger tapers off when the battery voltage is above about 14V. For this to function, D6 is a 5.1V shunt zener regulator that puts out -5.1V relative to the positive rail. It is biased via R8.

Ramp Generator
C1 and R4 form a ramp generator that generates a negative going sawtooth voltage (relative to the positive rail). It is reset to the positive rail via Q1 and Q2 at line voltage zero crossing. At zero crossing, there is no voltage at the anodes of D3 & D4 (relative to the positive rail), Q1 is off, Q2 is on and C1 is shorted. At all other points in the AC line cycle, C1 is charging. My line frequency is 60HZ. For 50HZ, increase the value of R4 to 82K.

Error Amplifier
U1A is the error amplifier—it amplifies the difference between the -5.1V reference voltage and the feedback voltage at the arm of the V ADJ pot (R6). It is slowed down by the RC filter (R10 & C2), proportionately amplified by the ratio of R14 /R9, and integrated via C3. Perhaps you have heard of a PID (proportional, integral, derivative) control—this does just that, but neglects the derivative term as it is generally not required in most applications. If the error amplifier is not satisfied, it continues to integrate its output voltage until the feedback voltage equals the reference voltage. The function of the operational amplifier is to make the two input voltages equal.

The device selection here is the LF442 (or TL082) J-FET input operational amplifier. This is vital in this circuit because the common mode voltage range of the differential inputs must extend to the positive rail. Few op amps can do this (many have differential voltages that extend to the negative rail, but those will not work in this application).

Phase Comparator
U1B is the phase comparator. It compares the ramp voltage with the output of the error amplifier. It is also called the ramp-intercept technique. When the ramp generator voltage exceeds the error voltage signal (in the negative direction), the output of U1B switches negative and turns on Q3 thus providing gate current to the SCR that is forward biased. R13 is the gate current limiting resistor.

Flashing a Dead Battery
The battery provides the power to begin operation of the regulator circuit, so if the battery is fully discharged it may be necessary to “flash” the battery terminals with a good battery to bootstrap the regulator into operation.
I have toyed with the idea of installing a “Flash” pushbutton, but this adds more circuitry and I have not found it necessary.


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  • engkong joe

    I’am sorry ,can u send me PCB layout and all component ,coz I beginner

    Thx from indonesian

  • Bill Yacey

    That sets the maximum voltage it regulates to. For a 12V automotive wet cell battery, 14.4V is where you want to set it at.

    A digital meter that can record minimum / maximum voltage events is useful for setting this up. Adjust for a maximum of 14.4V across a fully charged battery.

  • Nicos Hadjisofokli

    Can you please explane the reason of R6 10kΩ adj, how it works in the system and in which value must be set?

  • Bill Yacey

    This circuit seems to work nicely, although I found the regulation varies slightly with line voltage, but probably not enough to worry about.

    A momentary push button switch can easily be added to flash a dead battery. Simply connect a N.O.switch across the collector and emitter of Q3.

  • luis

    hi ,your circuit its very usefull , it’s there any way to convert them into 36v or 48v system for make an golf cart charger?
    sorry for my english

    thxs from argentina

  • Krokkenoster

    I just wish I had this diagram 40 years ago.The circuit that I used then had two SCR’s connected virtually the same as the then common “Experimenter’s adjustable PSU. It was simple but it worked if Mr. Customer did not drain his battery down to dead. I used a full wave bridge and then the SCR in series. This arrangement was cheaper as the bridge was a lot cheaper than a second SCR (I could buy five 35amp 400 volt bridges for the price of one SCR)and also more reliable. What we need now is current limiting for that poor hard working transformer

  • gbengoos

    I like this design and I commend your effort. I need more info on the arrow on the Anode of the SCR ,R3 ,R7 and others. I has a thought of been negative grounding but the one on the Anode being negative direcly to the battery taminal confuse me. I want to design as my project.

    • Jim Keith

      Look at it this way–either the cathode(s) go to the battery positive terminal or the anode(s) to to the battery negative terminal. The current flows in the same direction.

  • Chibueze

    I like de stuff do you have any cct to give me and how to go about it i want to design a battery charger as my project

  • Stan

    I am wondering if you drew a pc board for this? If so can you post or email to me? I hate to duplicate work. Thanks Stan

  • tdraunciheal

    HI, I like this design, and meke it, I found your circuit searching for a lead-acid battery desulfators, you think what are usefoul for this, or not?


    • Jim Keith

      I have never looked into desulfators so I cannot say–also, I question how well desulfators work, but I am sure that they have some merit.

    • tdraunciheal

      It all began trying to “revive” a lead-acid battery, which is some sulfated, searching I found some circuits, the principle is generally the same, sending pulses of medium frequency, some kHz and big amplitude and current, these circuits are fed by the battery itself and apparently, ;-), remove the sulfate layer of the plates, that is how I did the idea of looking for something with thyristors and found your charger, which in any way will arm.