# current limiter / short circuit indicator for a variable power supply

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:

Power supply short circuit protection

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.

• What if I told you that a transistor to act as a high side current limiter/protection can be a very small SOT23 P-channel FET? For the given current (<=3A) Commented Mar 25, 2015 at 17:57
• What if I told you that a 0.1 or even 0.01 Ohm current shunt resistor and a very small SOT23-5 Op-amp could be used as a comparator to drive a red or green LED to indicate load state? Commented Mar 25, 2015 at 17:58
• @KyranF I'm certain that I'd believe you on that matter. The circuit was designed based on older electronics and I'm sure that the principles are the same but component designs have advanced a great deal. I'm Just uncertain where to start. At this point, the posted circuit is not the perfect design. Commented Mar 26, 2015 at 5:12
• @KyranF During my search, I had noticed that some designs used N-FETs or P-FETs to handle the load but wasn't certain on how to fit in the visual indicators. I was under the assumption that the load was to be cut from the supply and the state of the circuit would remain that way until the load which fell below threshold was corrected. Now, I see fold back limiters in some of the designs but without indicators. Perhaps I'm not searching the correct criteria? Commented Mar 26, 2015 at 5:12
• Well PFETs are sort of easier to design as high side power switches, because NFETs need a charge-pump or voltage doubler to get their gate above the source voltage (can be difficult at high voltages). You would need a complex circuit with load shunt resistors in there as well, to maintain knowledge of the load once you "turn it off" during a fault condition. Commented Mar 26, 2015 at 15:28