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Kindly suggest necessary change to be made for current liming circuit for buzzer.

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ If you draw your schematic more compactly, it will fill the page and be more readable. As is, I can't even read the part numbers on your components. Also, use the "flip" option instead of "rotate" to draw your transistors right-side up. \$\endgroup\$ – The Photon Sep 28 at 4:53
  • \$\begingroup\$ i have used BC547/847 for drive buzzer output and TCIL188 to current liming .I need changes near transistor ckt to control current, \$\endgroup\$ – Ajit N Sep 28 at 4:55
  • \$\begingroup\$ How about U1? I can't read that one either. And how is Q2 a current limiter? It's just hanging there doing nothing. Is there supposed to be something connected to its emitter? \$\endgroup\$ – The Photon Sep 28 at 5:01
  • \$\begingroup\$ U1-UM3561 sound generator IC.its multi tone sound generator. q2 is CIL188 and q1 is bc847/bc547. \$\endgroup\$ – Ajit N Sep 28 at 5:55
  • \$\begingroup\$ Now you have base and emitter of Q2 both grounded. It's not going to do anything hooked up like that. Drawing your schematic right side up would probably help you see these problems without having to ask us. \$\endgroup\$ – The Photon Sep 28 at 6:03
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First, let's fix your schematic.

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. OP's schematic redrawn.

  1. Add a fuse on the mains. It is required and should be shown on the schematic.
  2. Use the transformer symbol for XFMR1.
  3. Use GND symbols where possible to eliminate wiring.
  4. All GND symbols should point downwards (to ground).
  5. Use a switch symbol for the toggle. The TOGGLE SWITCH block doesn't convey the information clearly and we have no way of knowing what pins 1, 2 and 3 are. I have had to guess.
  6. The circuit logic should read left to right unless it makes sense to do it otherwise for reasons of clarity. Your power supply was OK but since U1 controls Q1 it should be on the left.
  7. Current flow should be from top to bottom. That means the speaker should be up at the 6 V supply and the NPN transistors should have their bases at the bottom.
  8. Capital 'V' for volt.
  9. The 7806 is missing a decoupling capacitor on the output. Check the datasheet but without one it may be prone to oscillation and unstable voltage particularly as the buzzer load will be presenting a varying load as it is turned on and off on each cycle.

Now that that's all fixed we can see clearly that Q3 can never turn on as its base is connected to its emitter. It can be removed from the circuit.

It is also clear that when Q1 is turned on that it applies 6 V to the buzzer. The simplest way to limit the current to the buzzer is to add a series resistor on either side.


schematic

simulate this circuit

Figure 2. A constant-current drive for the buzzer.

It's possible that you were trying to create a classic constant current driver as shown In Figure 2. In this case Q3 "steals the bias" from Q1 when the current through R4 causes the voltage across it to turn on Q3. For a typical NPN this will occur at about 0.7 V. With 68 Ω in there the current will be limited to \$ I = \frac {V}{R} = \frac {0.7}{70} = 10 \ \text {mA} \$ (doing a little rounding of the R value).

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