THE Breadboard Project Protector circuit diagram shown in Fig.1, is a simple cutout device that should be particularly useful for the experimenter who regularly builds circuits on breadboards but who does not have the luxury of a power supply with built-in current limiting. The device is used as an add-on and wired between a “home-made” power supply, i.e. a simple transformer-rectifier-smoothing capacitor arrangement, and the breadboard.

The “Protector” will cut power to a circuit that is trying to draw too much current. This can, of course, occur quite regularly when building on a breadboard due to the very nature of the process. Even when great care is being taken, component leads tend to come into contact unintentionally. And who has never accidentally connected the positive supply to zero volts?

Cutout Current

The value of current at which the cutout trips is determined by a single resistor that can be chosen to suit the particular requirement. With the design shown, the cutout current value is around 700mA. A green l.e.d. (D1) lights when all is well. Once the cutout has operated, the device latches in its cutout state, and a red l.e.d. (D2) lights to indicate that the cut-out has operated. The “Protector” is reset via pushbutton switch S1.

The use of this device thus protects both the power supply (or prevents large numbers of fuses being wasted if it is properly fused) and also the components on the breadboard. At its heart is a bistable multivibrator based on transistors TR1 and TR3. The l.e.d.s in the emitter paths of the transistors indicates which of the two states the bistable is in, one corresponding to an OK state, and one corresponding to the situation where Overcurrent has been detected. In the OK state, transistor TR3 is turned on so allowing current to flow from the positive supply line, through the pnp Darlington transistor TR4 and to the breadboard circuit under test. It also flows through resistor R8.

The bistable is triggered into indicating the overcurrent state when transistor TR2 is switched on. This occurs when the voltage across resistor R8 exceeds around 1.4V. The voltage across R8 is, of course, proportional to the current passing through R8, which is the same current passing through the breadboard circuit. When triggered into the over-current state, transistor TR3 is turned off, and with it, so too is TR4, removing power from the breadboard.

The value of R8 can be varied to provide different values of current that will trigger the cut-off state. However, it must have asufficiently high power rating to match the conditions under which it is used – the usual 12  R rule can be applied here.Once the bistable is in its overcurrent state, it will remain so until Reset switch S1 is pressed, at which point the bistable latchesinto the OK state. (This assumes that the cause of the excessive current consumption has been removed in the meantime.)

Note that a heatsink may be needed for the Darlington transistor TR4 if the current being drawn by the breadboard being protected (under normal conditions) is significant for other than short periods. The TIP127 transistor used for TR4 is rated at maximum 5A, but breadboard contacts are not normally rated at more than 1A. Apart from R8, all other resistors are 0.25W carbon film; Reset switch S1 only passes a few milliamps; TR1 to TR3 are general-purpose npn types and the l.e.d.s are similarly standard.

Article reproduced by permission of Wimborne Publishing. www.epemag.com