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 (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.
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…
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.
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.
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).
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.