Circuit Schematic learn

The above pictured schematic diagram is just a standard constant current model with a added current limiter, consisting of Q1, R1, and R4. The moment too much current is flowing biases Q1 and drops the output voltage. The output voltage is: 1.2 x (P1+R2+R3)/R3 volt. Current limiting kicks in when the current is about 0.6/R1 amp. For a 6-volt battery which requires fast-charging, the charge voltage is 3 x 2.45 = 7.35 V. (3 cells at 2.45v per cell). So the total value for R2 + P1 is then about 585 ohm. For a 12 V battery the value for R2 + P1 is then about 1290 ohm. For this power supply to work efficiently, the input voltage has to be a minimum of 3V higher than the output voltage. P1 is a standard trimmer potentiometer of sufficient watt for your application. The LM317 must be cooled on a sufficient (large) coolrib. Q1 (BC140) can be replaced with a NTE128 or the older ECG128 (same company). Except as a charger, this circuit can also be used as a regular power supply.

Parts List:

R1 = 0.56 Ohm, 5W, WW
R2 = 470 Ohm C2 = 220nF
R3 = 120 Ohm
R4 = 100 Ohm
C1 = 1000uF/63V
Q1 = BC140
Q2 = LM317, Adj. Volt Reg.
C3 = 220nF (On large coolrib!)
P1 = 220 Ohm

Some workbenches can’t help ending up looking like a rats nest of cables and equipment, so its always an advantage if a piece of mains equipment can be removed from somewhere to free up an extra mains socket. Here we are using the ubiquitous PC as a battery charger. An unused serial interface port can supply enough current to charge (or trickle charge) low-capacity Nickel Cadmium (NiCd) batteries. You could for example, use the batteries in a radio and charge them during use.

The three serial port connections TxD, DTR, and RTS, when not in use, are at –10 V and can supply a current of around 10 to 20mA (they are short-circuit protected). The circuit shown supplies a charging current of approximately 30mA. If it is necessary to alter the polarity of the charging circuit then it is a simple job to reverse the diodes and using software, switch the port signals +10 V. Those interested could also write a software routine to automatically recharge the batteries.

Source : www.extremecircuits.net

A car battery deteriorates in use and its life seldom exceeds four years. When new, its voltage may drop to only 2V while cranking the engine. As the battery ages, its internal impedance increases and so the voltage drop while cranking also increases, until ultimately the drop is high enough to prevent the engine from starting. This gradual increase in voltage drop while cranking can be used as an early warning of looming battery failure and so this circuit triggers an alarm when the battery voltage drops to 8V during cranking. IC1 is a precision 2.5V device used as the reference for two comparators based on IC2, an LM358 dual op amp.


IC2a monitors the voltage from trimpot VR1 and normally its output at pin 1 will be low while the output of IC2b will be high and LED1 will be green. When pin 2 of IC2a falls below pin 3, its output at pin 1 will go high to drive the red section of LED1 to indicate a fault. At the same time, IC2b inverts the signal from pin 1 and its output at pin 7 goes low and turns off the green section of LED1 to indicate a fault. Since the battery voltage drop occurs momentarily while cranking, a more permanent indication of the fault is provided by flashing LED2. When IC2a’s output goes high momentarily, the SCR is latched and LED2 flashes and can only be deactivated by pressing push-button S1.

This circuit charges two NiCad cells with a constant current and features dual charging rates, voltage cutoff and an audible alarm. The circuit is powered by a 12VAC centre-tapped mains transformer, together with two rectifier diodes (D1 & D2) and a 1000mF filter capacitor. A 7806 3-terminal regulator is used to generate a 6V rail for the remainder of the circuit. Transistor Q1 and LED1 constitute a basic constant-current source. The forward voltage of the red LED (about 1.5V) minus Q1’s base-emitter voltage (about 0.6V) appears across the 6.8W or 15W emitter resistors, depending on the position of S1. With a 15W resistance in the emitter circuit, the charging current is about 60mA, whereas with 6.8W it is about 130mA.

This is sufficient to charge 600mAH "AA" cells in 14 hours and five hours, respectively. An LM393 voltage comparator (IC1) is used for the voltage cutoff function. Its inverting input is set to 2.9V (nominal) via trimpot VR1, while the non-inverting input senses battery voltage. This means that while the cells are being charged, the output transistor (in the LM393) is switched on, also switching on Q1 and enabling the current source. Once the cells are charged to approximately 80% or more of capacity, their terminal voltages will exceed 1.45V, so the voltage at the non-inverting input (pin 3) of IC1 will exceed the reference voltage on the inverting input (pin 2).


This causes IC1’s output to switch off, in turn switching Q1 off and disabling the current source. To prevent rapid switching action around the voltage cutoff point, a 100nF capacitor provides feedback between the output and inverting input of the comparator. Four NAND gates are used to build two simple oscillators of different frequencies. When cascaded together, the result is a pulsed tone from the piezo transducer to indicate charge completion.

Editors note:
Absolute terminal voltage is not always a reliable indicator of Nicad battery charge state. Importantly, batteries should never be charged for longer than the manufacturer’s specified period.

Except for use as a normal Battery Charger, this circuit is perfect to constant-charge a 12-Volt Lead-Acid Battery, like the one in your flight box, and keep it in optimum charged condition. This circuit is not recommended for GEL-TYPE batteries since it draws to much current. The above circuit is a precision voltage source, and contains a temperature sensor with a negative temperature coλficient. Meaning, whenever the surrounding or battery temperature increases the voltage will automatically decrease. Temperature coλficient for this circuit is -8mV per °Celcius. A normal transistor (Q1) is used as a temperature sensor. This Battery Charger is centered around the LM350 integrated, 3-amp, adjustable stabilizer IC. Output voltage can be adjusted with P1 between 13.5 and 14.5 volt.
T2 was added to prevent battery discharge via R1 if no power present. P1 can adjust the output voltage between 13.5 and 14.5 volts. R4s value can be adjusted to accommodate a bit larger or smaller window. D1 is a large power-diode, 100V PRV @ 3 amp. Bigger is best but I dont recommend going smaller. The LM350s adjust pin will try to keep the voltage drop between its pin and the output pin at a constant value of 1.25V. So there is a constant current flow through R1. Q1 act here as a temperature sensor with the help of components P1/R3/R4 who more or less control the base of Q1. Since the emitter/base connection of Q1, just like any other semiconductor, contains a temperature coλficient of -2mV/°C, the output voltage will also show a negative temperature coλficient.

That one is only a factor of 4 larger, because of the variation of the emitter/basis of Q1 multiplied by the division factor of P1/R3/R4. Which results in approximately -8mV/°C. To prevent that sensor Q1 is warmed up by its own current draw, I recommend adding a cooling rib of sorts. (If you wish to compensate for the battery-temperature itself, then Q1 should be mounted as close on the battery as possible) The red led (D2) indicates the presence of input power.Depending on what type of transistor you use for Q1, the pads on the circuit board may not fit exactly (in case of the BD140).

A circuit for monitoring the status of the battery and generator is undoubtedly a good idea for motorcyclists, as for other motorists. However, not every biker is willing to drill the necessary holes in the cockpit for the usual LED lamps, or to screw on an analogue accessory instrument. The circuit shown here manages to do its job with a single 5-mm LED, which can indicate a total of six different conditions of the onboard electrical system. This is done using a dual LED that can be operated in pulsed or continuous mode (even in daylight). Built on a small piece of prototyping board and fitted in a mini-enclosure, the complete circuit can be tucked inside the headlamp housing or hidden underneath the tank.

The heart of the circuit is IC2, a dual comparator. The comparator circuit is built without using any feedback resistors, with the indication being stabilised by capacitors C4 and C5 instead of hysteresis. Small 10-µF tantalum capacitors work well here; 220-µF ‘standard’ electrolytic capacitors are only necessary with poorly regulated generators. Voltage regulator IC1 provides the reference voltage for IC2 via voltage divider R2/R3. The onboard voltage is compared with the reference voltage via voltage dividers R4 /R5 and R6/R7, which are connected to the inverting and non-inverting comparator sections, respectively.

Using separate dividers allows the threshold levels to be easily modified by adjusting the values of the lower resistors. IC2a drives the anode of the red diode of LED D4 via pull-up resistor R10. The anode of the green diode is driven by IC2b and R11. T2 pulls R11 to ground, thereby diverting the operating current of the green diode of the LED, if the voltage of the electrical system exceeds a threshold level of 15 V (provided by Zener diode D3). The paralleled gate outputs on pins 10 and 11 of IC3 perform a similar task. However, these gates have internal current limiting, so they can only divert a portion of the current from the red diode of the LED.

The amount of current diverted depends on the battery voltage. The two gates are driven by an oscillator built around IC3a, which is enabled via voltage divider R14/R15 and transistor T1 when the battery voltage is sufficiently high. Depending on the state of IC3a, the red diode of the LED blinks or pulses. The circuit is connected to the electrical system via fuse F1 and a low-pass filter formed by L1 and C1. If you cannot obtain a low-resistance choke, a 1-Ω resistor can be used instead. In this case, the values of C3, C4 and C5 should be increased some-what, in order to help stabilise the indication. D1 protects the circuit against negative voltage spikes, as well as offering protection against reverse-polarity connection. Due to its low current consumption (less than 30 mA), the circuit could be connected directly to the battery, but it is better to power it from the switched positive voltage.

Lead acid batteries often fail prematurely due to over-charging, under-charging, deep discharging and low electrolyte level. All of these can lead to sulphation of the plates which leads to high internal resistance and eventual failure. Normally, this process is regarded as irreversible but this circuit is claimed to reverse the process by applying high voltage pulses to break down the lead sulphate compounds. The circuit is essentially a high-voltage pulse generator which is powered directly from the battery in question. If the battery is badly sulphated, it will be necessary to connect it to a low power charger as well, say 2A. We have strong doubts about whether battery sulphation can be effectively reversed but we are publishing this circuit because the subject is of particular interest.


This circuit has been submitted to us from a number of sources so we do not know who is the original designer. More information can be found at http://shaka.com/~kalepa/desulf. The 555 timer is connected as an astable oscillator with its output frequency set by R1, R2 and C2. Its output pulses drive the gate of Mosfet Q1 which turns on to charge inductors L1 and L2. At the end of each pulse, Q1 turns off and the inductors develop a high-voltage high-current pulse which is applied across the battery via fast recovery diode D1 and the 100µF capacitor. The 555 is protected from the high voltage pulses via its isolated supply, by virtue of the 15V zener diode ZD1, the 47µF capacitor and the 330Oresistor R3.

Many blind and deaf-blind people use portable electronic devices to assist their everyday lives but it is difficult for them to test the batteries used in that equipment. Talking voltmeters are available for the blind but there is no commercially available equivalent usable by deaf-blind persons. This device enables blind, deaf-blind and sighted people to test batteries. It will test AAA, AA, C and D cells, as well as 9V "transistor" batteries. All rechargeable and non-rechargeable cell types are supported. The circuit needs no calibration. To use the tester, turn potentio-meter VR1 fully counter-clockwise and then connect the battery to be tested to the appropriate set of test terminals. If the battery has any usable charge, the pager motor in the tester will immediately vibrate.

VR1 is then slowly rotated in a clockwise direction just far enough to stop the vibration. The position of VR1 then indicates the loaded voltage of the battery on a scale of 1-1.5V (if the battery is connected to the 1.5V test terminals) or 6-9V (if the battery is connected to the 9V test terminals). A regulated +5.1V rail is generated from the battery under test with the aid of zener diode ZD1. For 9V tests, a 150O resistor limits the zener current, while diode D2 protects the circuit from reverse polarity battery connection. For 1.5V tests, a blocking oscillator formed by Q1, Q2 and L1 steps up the battery voltage before it is applied to the regulator. This configuration works reliably with inputs down to below 0.9V. The output of the oscillator is rectified by D1 and smoothed by the 33µF capacitor.

The circuit has to survive reverse connection of the battery under test. This creates a problem, because the LM393 cannot withstand a voltage more negative than -0.3V at its inputs. Diodes D1 and D2 indirectly protect the non-inverting inputs from negative voltages but series diodes cannot be used to protect the inverting inputs because of the unpredictable voltage drop they introduce. The solution used is to shunt negative voltages at the 1.5V test terminals with diode D3 in conjunction a 1kO resistor (R1). D3 limits the voltage at its cathode to about -0.7V, while resistors R2-R4 divide this by three to give no less than -0.23V at the inverting input (pin 2) of IC1a. When the battery is connected the right way around, D3 is reverse-biased and R1-R4 form a voltage divider that applies a quarter of the battery voltage to IC1a’s inverting input.

Similarly, D4 and R5-R10 protect the inverting input (pin 6) of IC1b from reverse-connected batteries at the 9V test terminals. However, in this case only 1/24th of the battery voltage appears at IC1b’s inverting input. Battery voltages in the range 1-1.5V at the 1.5V test terminals will therefore produce 0.25-0.375V at the inverting input of IC1a, while battery voltages in the range 6-9V at the 9V test terminals will produce 0.25-0.375V at the inverting input of IC1b. Potentiometer VR1 forms part of a voltage divider used to generate a comparison voltage that is variable over the same 0.25-0.375V range. This is applied to the non-inverting inputs of both IC1a and IC1b. When the sampled battery voltage exceeds this comparison voltage, the respective comparator output swings low, switching on Q3/Q4 to energise the pager motor.

The 68O resistor in the collector circuit of Q4 ensures that higher battery voltages do not overdrive the motor. When testing an earlier version of this circuit with batteries that have high internal impedance, it was found that when VR1 was advanced to the indicating point, the pager motor slowed down rather than switched off. This occurred due to a rebound in battery voltage at motor switch-off, which in turn caused the circuit to immediately switch the motor back on again. To counteract this effect, a small amount of positive feedback is applied around the comparators when the motor switches off. The feedback is disabled while the motor is running so that the indicating point of VR1 is not affected. This works as follows: when the motor is running, Q5 is conducting and D5 is reverse biased, so the comparison voltage at the non-inverting inputs of the comparators is not affected.

If the motor stops running, Q5 switches off and the 2.7MO resistor pulls the comparison voltage higher via D5 to ensure that the resulting battery voltage rebound does not restart the motor. Finally, diode D7 prevents reverse breakdown of Q4 in case of reverse battery connection at the 9V terminals. There is no need for a similar diode in the 1.5V part of the circuit because 1.5V is well below the reverse breakdown voltage of Q3. The prototype used "Magtrix" magnetic connectors on short flexible leads as the 1.5V test terminals. These allow the connection of AAA, AA, C and D cells but are arranged so that they cannot be brought closely together enough to connect 9V types. Unfortunately, magnetic connectors cannot be used for the 9V test terminals because some brands of 9V batteries have non-magnetic terminals. A conventional 9V battery snap can be used instead. For blind people, the knob on VR1 should be pointer-shaped (eg, DSE P-7102) so that the degree of rotation can be easily assessed by touch.