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12V SCR Battery Charger

2016-05-10 22:33  
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12V SCR Battery Charger
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 chargerWhat 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 Battery Charger Schematic SCR (Thyristor) RectifiersFirst 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 SelectionI 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 CommonNormally 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 ReferenceA 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 GeneratorC1 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 AmplifierU1A Source: electroschematics.com