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Tuesday, March 31, 2009

5V power supply with overvoltage protection.


For circuits using TTL ICs the supply voltage is a great concern and a slight increase in supply from the rated 5V may damage the IC. Using fuses alone does not solve the problem because a fuse may take several milliseconds to blow off and that’s enough time for the IC to get damaged.

In this circuit a crowbar scheme is used in which a triac short circuits the power supply and burns the fuse. The burning time of the fuse is not a concern because the power supply is already shorted by the triac and the output voltage will be zero. When the output voltage exceeds 5.6 volts the zener diode D2 conducts and switches ON the triac T1.Now T1 acts as a closed switch, shorting the circuit. The output voltage drops to zero and fuse gets burned off. Since the switching of triac takes place within few micro seconds there will be no damage to the TTL ICs or any other such voltage sensitive components in the load circuit.

Circuit diagram with Parts list.


  • Assemble the circuit on a general purpose PCB.
  • If 1A Bridge is not available, make one using four 1N4007 diodes.
  • The trip voltage can be varied by varying the values of D2 and R2.
  • All capacitors must be rated at least 25V.
  • The transformer T1 can be a 230 V AC primary, 12v secondary, 2A step-down transformer.

Lead acid battery charger circuit


Here is a lead acid battery charger circuit using IC LM317.The IC here provides the correct charging voltage for the battery.A battery must be charged with 1/10 its Ah value.This charging circuit is designed based on this fact.The charging current for the battery is controlled by Q1 ,R1,R4 and R5. Potentiometer R5 can be used to set the charging current.As the battery gets charged the the current through R1 increases .This changes the conduction of Q1.Since collector of Q1 is connected to adjust pin of IC LM317 the voltage at the output of of LM 317 increases.When battery is fully charged charger circuit reduces the charging current and this mode is called trickle charging mode.

Circuit Diagram with Parts List.

Notes .

  • Connect a battery to the circuit in series with a ammeter.Now adjust R5 to get the required charging current. Charging current = (1/10)*Ah value of battery.
  • Input to the IC must be minimum 15V to get 12 V for charging the battery .Take a look at the data sheet of LM 317 for better understanding.
  • Fix LM317 with a heat sink.

LED torch using MAX660


This is a simple LED torch circuit based on IC MAX660 from MAXIM semiconductors. The MAX 660 is a CMOS type monolithic type voltage converter IC. The IC can easily drive three extra bright white LEDs.The LED's are connected in parallel to the output pin 8 of the IC. The circuit has good battery life. The switch S1 can be a push to ON switch.

Circuit diagram with Parts list.


  • Assemble the circuit on a general purpose PCB.
  • The IC must be mounted on a holder.
  • The circuit can be powered from two torch cells connected in series.
  • The capacitors C1 and C2 must be Tantalum type.
  • The diodes D1 to D3 must be of 1N4148.

Bidirectional H-Bridge DC-Motor Motion Controller

In applications requiring absolute accuracy in the speed control of dc servo motors, there’s no substitute for the traditional tachometer-based feedback loop. But for somewhat less demanding situations, adequate accuracy often can be achieved without the complication and expense of a tach. This can be done by taking advantage of the built-in electromechanical constants of the motor itself.

For example, the fact that every permanent-magnet dc motor exhibits a stable relationship between rpm and armature back-EMF implies that a reasonable job of constant-speed operation can be accomplished merely by driving the motor from a well-regulated voltage supply.

Even better speed regulation, sometimes rivaling tachometer feedback, can be achieved by adding a armature current, to the motor drive voltage. If this term is trimmed to accurately cancel armature resistance equal to motor-rated-voltage/lockedrotor-stall-current, the motor rpm will remain nearly constant over a wide range of loads. Although armature resistance cancellation via positive current feedback is hardly a new idea, the circuit described here gives it a novel twist by combining this trick with a motion-reversing H-bridge circuit topology.

The circuit works as follows: A speed-set point control voltage is produced by multi-turn precision potentiometer R3, acting in concert with VR1’s 1.25-V reference voltage. The resulting 0-0.75 V is scaled by a factor of 16 by op-amp A1 to produce a 0-12 V no-load M1 target armature voltage.

Speed-stabilizing, current-proportional positive feedback comes from current-sensing R1, is attenuated by R2, and summed by A1 with the speed-set point voltage. Optimum adjustment of R2 can produce almost perfect cancellation of M1’s parasitic resistance, resulting in a very “stiff” torque-versus-rpm characteristic. Motor speed-control performance will therefore be nearly independent of mechanical loading up to the voltage limit of the drive circuit.

The regulation of the motor drive in response to the composite control signal output by A1 (speed-setpoint plus current-feedback) is the job of either differential amplifier A3 or A4. It depends on the desired motor drive polarity and consequent direction of rotation as indicated by the state of direction-control flip-flop A2. For positive (clockwise) rotation, A2’s output is low and A4 is in control. This occurs because A3’s low output turns on Q3, which pulls Q5’s gate high, grounding the negative M1 connection via R1. Meanwhile, the same Q3 voltage applied to A3’s positive input causes A3 to rail Q6’s gate positive, holding the p-channel FET off. This prevents the possibility of “shoot-through” conduction between Q5 and Q6. A4 then can accurately sense, via the R5-R6-R7-R8-R9 differential network, the voltage applied to M1 and regulate it via power MOSFET Q4.

Thus, M1 is forced to run at the speed set by R2 until optical retro sensor E2/Q2 senses the arrival of the mechanical load at its clockwise limit. Light reflected into Q2 results in conduction, which overcomes the detection threshold set by feedback pot R4.This pulls A2’s positive input high and toggles the state of the direction control flip-flop. The resulting positive excursion of A2’s output turns on Q7 and forward-biases D1, disabling the A4/Q4 control loop.

Meanwhile, Q3 turns off, releasing Q5 and A3. This results in reversal the motor drive polarity and the initiation of speed-regulated counterclockwise motion. The motor will now continue to run counterclockwise until retro-sensor E1/Q1 senses the arrival of the mechanical load at the counterclockwise limit. Bridge polarity will consequently toggle again, causing the motor to reverse again and so forth ad infinitum (or at least until power is removed!).


Alarm Sounds When Refrigerator Door Remains Open Too Long

Not properly closing a refrigerator door will no doubt invite huge electricity bills. This gadget is an alert device that beeps if you leave the refrigerator door open for more than 20 seconds. When the door opens, the lamp illuminates and the IC (a 4060B counter/oscillator) starts counting down. After a preset delay of 20 seconds, the piezoelectric buzzer beeps intermittently for 20 seconds and then stops for the same amount of time. This cycle repeats until the refrigerator door closes.

Producing a small dc voltage from the ac mains to run an electronic control requires a step-down transformer or a capacitor dropper circuit. This design uses an innovative and easy solution. When someone opens the refrigerator door, the lamp receives power via the diodes in the bridge rectifier, D1-D4, and through the Zener diode, ZD1. The voltage drop across the Zener diode is smoothed by the filter capacitor, C1. This voltage is sufficient to run the rest of the circuitry.

To install the circuit, cut the existing wire as shown in the figure and connect the circuit (shadowed) at points A and B. The circuit can be conveniently placed in the compressor compartment where there’s ample space. With the door closed, the lamp is off and no power goes to the timer circuit.

The circuit runs directly from the mains. So care must be exercised and a little knowledge of refrigeration wiring will ease the job.


Automatic Battery Charger

The following automatic battery-charger design is created with a circuit that could qualify as the simplest window comparator ever built around a single transistor. It starts charging when the battery voltage drops beyond a preset value, and it stops when an upper preset voltage is attained.

With the help of a precise variable voltage supply, the upper and lower voltage levels were set. The normally connected (NC) lead of the relay isn't joined to the 15-V dc supply, which blocks this voltage from passing to the battery leads. This will accurately set the upper and lower levels. But the charging supply of 15 V dc was connected to the circuit.

First, the variable supply is fixed at 13.3 V dc—the voltage of a fully charged battery—and linked to the battery point of the circuit. The slider of VR1 is turned to the extreme end on the side that's attached to the positive terminal of the battery. VR2's slider should be turned toward the end that's connected to VR1. The transistor turns on, shunting VR1. Then the slider of VR1 is turned toward the other extreme, which is the end connected to VR2.

The test supply voltage is now set to 11.8 V dc, which is the voltage of a drained battery. VR2 is then adjusted so that it just turns off the transistor again. The test voltage is raised to 13.3 V dc again, and VR1 is adjusted so that the transistor turns on. With the upper and lower levels set, the NC point is connected to the circuit (15-V dc charging voltage). Now the battery charger is set and ready to go.