fuse saver

Description: This circuit is particularly beneficial for hobbyists utilizing a breadboard to experiment with ideas, especially those employing a simple homemade DC power supply composed of a transformer, rectifier, smoothing capacitor, and protective fuse, which lacks overcurrent protection. In this circuit, resistor R6 serves as the detecting element. Under normal conditions, its voltage drop is insufficient to activate transistor T1. The value of R6 can be adjusted to establish a different cut-off current as needed, based on Ohm's Law. In the event of a short circuit in the load, the voltage increases rapidly, causing T1 to conduct. This action activates the relay, switching its contacts to disconnect power from the external circuit and instead directly powering the relay coil, thus latching it in this state. The circuit remains in this condition until the primary power supply is turned off. Capacitors C1 and C2 retain enough charge (through diodes D3, D4, and D6, which prevent charge loss to the rest of the circuit regardless of its state) to keep T1 conducting and to power the relay during the switching process, while resistors R2 and R4 offer slow discharge paths. LEDs D1 (red) and D5 (green) indicate the circuit's operational state. Inductor L1 mitigates the inrush current when the circuit is powered on, preventing immediate cutoff. Diodes D2 and D7 provide standard back-emf protection across the relay coils. The circuit's input connects to the main transformer-rectifier-capacitor-fuse power supply via K1, while the output connects to the experimental load through K2. It is important to note that the input voltage must be a floating supply if Vout is grounded through the load, as Vin and Vout should not be interconnected. Consideration must also be given to several components, particularly the choice of relay Re1. For the prototype, this was sourced from Maplin, part number YX97F, featuring a coil resistance of 320 ohms, which, in conjunction with R1, forms the collector load for T1. Its nominal pull-in voltage range is between 9 V to 19 V, restricting the input power supply voltage to approximately 10 V to 30 V (DC only). R1 could be substituted with a wire link for operation at input voltages below 10 V, or its value could be increased as determined by either Ohm's Law or through trial and error for input voltages exceeding 30 V.

This circuit design integrates several essential components to ensure effective operation and safety during experimentation. The use of resistor R6 as a current sensing element is crucial; it allows for the adjustment of the cut-off current by changing its resistance value, enabling customization based on the specific needs of the application. The transistor T1 acts as a switch, responding to the voltage drop across R6. In the event of a fault condition, T1's conduction triggers the relay, which is a critical component for disconnecting the load and preventing damage to the power supply.

Capacitors C1 and C2 are selected to provide sufficient energy storage to maintain relay operation during the transition phase, ensuring that the relay remains engaged even when the primary power supply is interrupted momentarily. The inclusion of diodes D3, D4, and D6 is essential for maintaining charge integrity and preventing undesirable voltage drops that could affect circuit performance.

LED indicators D1 and D5 serve a dual purpose: they provide visual feedback regarding the operational state of the circuit, allowing users to quickly ascertain whether the circuit is in a normal or fault condition. The inductor L1 is strategically placed to manage inrush current, which is a common concern in power electronics, helping to prolong the lifespan of the components involved.

The relay Re1's specifications are critical to the overall functionality of the circuit. Its resistance and voltage ratings must match the design requirements to ensure reliable operation. The choice of relay impacts the overall efficiency and responsiveness of the circuit, making careful selection necessary for optimal performance.

Overall, this circuit serves as a robust solution for hobbyists seeking to protect their experimental setups while allowing flexibility in design and component selection. Proper understanding and implementation of the described components will lead to a reliable and effective power management solution in various electronic projects.This circuit will be particularly useful to those hobbyists who use a breadboard` to try out ideas and who also use a simple home-made` DC power supply consisting of a transformer, rectifier, smoothing capacitor and protective fuse, that is, one without over current protection! In this circuit, the detecting element is resistor R6. Under normal co nditions, its voltage drop is not high enough to switch on transistor T1. The value of R6 can be altered to give a different cut-off current, as determined by Ohm`s Law, if required. When a short circuit occurs in the load, the voltage rises rapidly and T1 starts to conduct. This draws in the relay, switching its contacts, which cuts off power to the external circuit, and instead powers the relay coil directly, latching it in this second state.

The circuit remains in this state until the primary power supply is switched off. Capacitors C1 and C2 hold enough charge (via D3, D4 and D6, which prevent the charge from being lost to the rest of the circuit, whichever state it is in) to keep T1 switched on and power the relay while it switches over, and R2 and R4 provide slow discharge paths. LEDs D1 (red) and D5 (green) indicate what state the circuit is in. Inductor L1 slows the inrush of current when the circuit is switched on, which would otherwise cut off the circuit immediately.

D2 and D7 provide the usual back-emf protection across the coils. In use, the input of the circuit is connected to the main transformer-rectifier-capacitor-fuse power supply via K1, and the output is connected to the (experimental) load via K2. Note that the input voltage must be a floating supply if Vout is grounded via the load, as Vin and Vout must not be connected together.

Some consideration needs to be given to a number of components. First, the choice of relay Re1. For the prototype, this was obtained from Maplin, part number YX97F. This is has a coil resistance of 320, which with R1 forms the collector load for T1. Its allowed pull-in voltage range is nominally 9 V to 19 V, which limits the input power supply voltage to between around 10 V to 30 V (DC only). R1 could be replaced by a wire link for operation at input voltages below 10 V, or increased in value, as determined by either the application of Ohm`s Law once more or trial and error, for an input voltage above 30 V.