I am building a variable power supply circuit and I am wanting to add a current limiter or a short circuit protector to its output. I feature that I'm attempting to add to the protector is an indicator of load resistance that falls below a certain threshold. To be more specific, the power supply is run from a 12VDC power source and has an LM350 voltage regulator. The output is narrowed between 3-5VDC. The load will vary between 1.8 to 2.5 ohms. The load on average will be 2.2 ohms. The load on the power supply will be 3 amps. It is to run between 1.7 - 2.4 amps normally. I am having trouble searching for a circuit to handle this without having to use HUGE transistors or resistors due to space constraints. It would have a green LED and a red LED. If the load on the circuit is within 1.8 to 2.5 ohms then the operation is normal and the green LED is lit. If the load falls below 1.8 ohms, the load is either cut or is used to have the red LED lit. I first felt that it would look similar to the circuit found at:
but finding an SK100 transistor apparently is not easy or I have to substitute it with a physically large transistor. I am currently using gEDA to design the circuit but can't seem to make heads or tail of NGSpice or GNUCap to test and adjust the circuit (or I simply don't know how to use them properly) and have only contemplate almost having to learn another programming language to use them. Some of the free online graphical simulators have given either vague or inconclusive results. One of the schematics that I have put together is the same PS with a different circuit protector. At this time, I'm sure it's calculations may be off. Can anyone point me in the right direction to help solve this or suggest a better protector circuit? Many thanks in advance.