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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  • Bill Yacey

    If the intended use is to charge a discharged battery close to 100% to eventually crank an engine, this charger works well. After all, a vehicles alternator keeps the battery charged for all intents and purposes at around 14.1 to 14.4V without any gassing or a need for trickle charging.

    If the desired use is for maintaining a standby battery indefinitely, the trickle charge mode would be a good idea..

  • Chicken

    Nice work, but don’t think being regulation stiff is the most important issue. Admirable really but it’s more important IMO to implement a dual stage so once voltage reaches gas point then switch to a trickle mode somewhere under 13.8 volts. Say for example, 13.2v max 12.5 min Temperature compensation is a consideration during charging as well, to me means thermistor compensation, low temps require higher voltage.

  • Bill Yacey

    Just a quick tip:

    Yesterday I took my charger out because I had a low battery in a vehicle that has been sitting since summer. The charger was working fine when I initially connected it, but later when I checked, it quit charging while outside in the cold. When I brought it into the shop it was working normally again.

    It turns out that the surplus 50A SCRs I used need a little more gate current to fire them; more so when cold. The 220 ohm resistor (R13) needed to be decreased slightly. I put a 180 ohm in and it seems to be working fine now, even in sub-zero temperatures.

    • Jim Keith

      Thanks for the feedback–yes, SCR gate drive requirements increase as temperature decreases.

  • Max

    someone from your friends already set up the project?
    someone has the board layout and could provide for me to try to mount.
    thank you

  • Bill Yacey

    I adjusted mine to shut off at 14.4V when the battery is at 20 degrees Celsius room temperature. Any higher than that and you risk boiling and gassing the battery. Lower than that 14.4V may not fully charge the battery and promote sulfating of the plates.

  • Nicos Hadjisofokli

    Problem solved it was just a silly bad lead. One more thing please is that how to measure and set the charging voltage. With the battery connected and adjusting the potentiometer would take the right measure?

  • Nicos Hadjisofokli

    Hello.I have built this circuit twice with no any results. I followed up all the circuit diagram and checked many times for mistakes but there none. I used btw69 for scr tl082cn for u1 and u2 and 2n2907 pnp transistors. Transformer and battery used are in good condition. Any ideas for what may is wrong?

    • Bill Yacey

      First thing is to check the SCRs for the proper polarity. If you temporarily connect a jumper between collector and emitter of Q3, it should pass full charge current to the battery; this will verify if you have the SCRs connected properly.

      Did you make sure the transistor leads are oriented properly? diodes and rectifier polarity the right way?

      If you email me, I may be able to help you out:

  • Milton

    Só faltou o detalhe do transformador:
    Qual é a tensão de saída do secundário e a corrente (I)

  • 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.

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