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CURRENT TRIP

This overload module is very cleverly designed.
The relay starts to “chatter” when the “set current” has been reached and a short-circuit will only allow the “set current” to flow. This lets the operator clear the fault and protect the power supply.

The CURRENT TRIP module is fitted between the Power Supply and Controller (throttle).
It does no “trip” or shut off the power but limits the current to the track and produces chatter from the relay to indicate the "set current" has been reached.

The module costs $12.00 usd and is available from Talking Electronics.  Email Colin Mitchell for more details.

Here is the circuit:


 

To see how the circuit works, you just look at 2 components. These are two resistors in series:

When you have two very low value resistances in series, a high current will flow when a supply voltage is present.
 

When a current flows, a voltage will be developed across each resistor.
The value of voltage is not important but the important point to note is the 0v rail has a voltage on it, on the left part of the rail, as show in the diagram above.

When a short circuit occurs, point A on the diagram has a voltage on it.
This is called the "Detecting Point" and we connect the base of a transistor to this point.
When the voltage at Point A rises to 0.6v, the transistor is activated.
A 2k2 resistor between the Detecting Point and the base allows the voltage to increase higher than 0.7v without damaging the transistor. The value of this resistor is not important. It can be from 100R to 4k7 or higher.

The only thing that matters is the voltage generated across the 1R2 resistor. This is the voltage that "turns the transistor ON."  In other words, it saturates it. It changes from OFF to ON.
This is what we call a "strong action" or "strong reaction" and it controls the next stage in the circuit.
You can see the left part of 0v rail rises to a higher voltage and this triggers the circuit.
The two components in the "detecting circuit" have a very low value of resistance and this is called a LOW IMPEDANCE STAGE.
This means a HIGH CURRENT must flow for the detection to occur.
The switch simply introduces more DETECTING RESISTORS that reduce the resistance of the detection so that a higher current is needed for the detection to occur.
Three resistors in parallel create 0.4R and this means the minimum current will be slightly more than 1 amp.