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The circuit schematic is shown in Figure 1. The entire circuit has only five components, which is simple and compact. The current limiting circuit is used in a DC circuit to automatically cut off the power when the load current exceeds a certain value, so this circuit can also be called an electronic fuse.

Figure 1

Circuit principle:
When the switch K is pressed (normally open type micro switch), the unidirectional thyristor SCR is turned on, and the direct current voltage is output to the load through the low resistance high power resistor R1. R1 acts as a current sensing element in the circuit.

When the load current increases beyond the specified allowable value, the voltage across the resistor R1 is greater than 0.7V, and the high-power transistor BG is turned on to a saturated state. At this time, the voltage between the collector C and the base B of the triode drops below the sustain voltage of the thyristor SCR, the thyristor is turned off, the power supply circuit is also turned off, and no voltage is output.

Component selection:
When the power supply voltage = 24V, current limit value = 2A, BG uses 3DD15A gold seal triode, and unidirectional thyristor SCR uses 4A/400V. Both the triode and the thyristor need to be selected based on the actual application voltage, load power, and current limit.

The resistance of the current detecting resistor R1 is selected according to the current limiting value, and the calculation method is R1=0.7/I (I is the current limiting value). The power calculation method of R1 is R1=0.7×I (I is a current limit value). When the current limit value is set to 2A, the resistance of R1 is 0.35Ω, and the power of R1 should be greater than 1.4W.

How the triode works

A triode is a semiconductor electronic device having three pins. The transistors are a collector (c), a base (b), and an emitter (e). The electron triodes are a screen, a gate, and a cathode, respectively. It can amplify weak signals into electrical signals with large amplitudes, also known as bipolar transistors and crystal transistors.