Short-Circuit Detection: Simple LED Circuit for Model Trains

Description

This circuit is appropriate in any scenario demanding over-current safeguarding. Here, an instance originating from the model railway domain is presented. Experienced model railroad aficionados recognize that locating the source of a short-circuit presents considerable frustration. Within a small model railway featuring a single engine, detection is straightforward; however, on expansive layouts, all locomotives cease operation immediately upon a fault, necessitating a systematic examination of each to identify the responsible unit.

If the track is segmented, this remarkably uncomplicated circuit significantly streamlines operations. A poly-fuse is incorporated into one of the supply lines within each section. (A poly-fuse is also referred to as a multi-switch, polyfuse or polyswitch, depending on the manufacturer). This is a type of fuse that recovers its normal conductive state after the short has been resolved. The benefit is that only the affected section becomes isolated.

The remaining locomotives within the other sections continue their movement. The stationary engine is essentially the cause, but it's likely several engines remain motionless since not all would have been travelling simultaneously. To clearly indicate the problematic area, an LED indicator is placed across each poly-fuse. While any color LED can be used, we suggest low-current types producing substantial illumination at just a few milliamps.

Circuit diagram:

The value of the current-limiting resistor can be adjusted to achieve a suitable LED brightness. As long as the current remains modest, the resistance of the poly-fuse is also low, resulting in a minimal voltage drop. At elevated currents, the resistance increases, generating a voltage drop across the poly-fuse sufficient to activate the LED. Given that the direction of current flow is unknown (trains may move forwards or backwards and digital controls employ alternating current), two LEDs are connected in parallel with opposing polarities.

Poly-fuses are obtainable in numerous trip current values. Select a value slightly exceeding the maximum current consumption of a locomotive within a section. The table below outlines the specifications of several models from the MF-R series produced by Bourns. (Raychem is another prominent manufacturer of poly-switches.) Ihold represents the holding current, Itrip signifies the short-circuit current.

**Poly-fuse – MF-R Series (Bourns)**

This component is a time-delay fuse designed for overcurrent protection in low-voltage circuits. It’s available in a variety of trip current ratings, making it suitable for a wide range of applications. This particular model from Bourns offers fast and reliable overcurrent protection, minimizing the risk of damage to connected devices. The holding current (Ihold) is 1A, and the short-circuit current (Itrip) is 5A.