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.


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    \$\begingroup\$ 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) \$\endgroup\$
    – KyranF
    Mar 25, 2015 at 17:57
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    \$\begingroup\$ 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? \$\endgroup\$
    – KyranF
    Mar 25, 2015 at 17:58
  • \$\begingroup\$ @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. \$\endgroup\$ Mar 26, 2015 at 5:12
  • \$\begingroup\$ @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? \$\endgroup\$ Mar 26, 2015 at 5:12
  • \$\begingroup\$ 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. \$\endgroup\$
    – KyranF
    Mar 26, 2015 at 15:28

1 Answer 1


enter image description here Here is a redrawn circuit with a Lm321 IC used as the current limit detector. The output from the detector could also go to the gate of a SCR, with the Anode going to pin 1 of your regulator and the cathode tied to ground, killing the current regulator output. I designed and used this circuit in a cnc stepper motor driver circuit.

Use 0.1 ohm resistor at 3 Watts and 2.2k (1/4 to 1/2) watt. A 2N5060 is a sensitive gate low current SCR. The output Voltage may not go to zero volts but it will be close enough to Stop most current flow. You are right their is a minimum current requirement to keeping an SCR Latched. A simple test put an SCR on pin 1 Anode and Cathode to ground then trigger the gate. If it holds it will work. The gate will trigger just by touching it with your finger. Yes the SCR will stay latched until the 12 volts goes to Zero, or a momentary NC switch is installed between the Cathode and Ground. 2N5060 Low Holding Current = 5 mA Maximum Current Require to keep latched.

A transistor circuit will work if it is stabilized with a capacitor, values I am no sure of, That would have to be experimented with, otherwise the voltage will oscillate between high and low.

enter image description here

  • \$\begingroup\$ That's a great start! One question about using the SCR to short the LM350 pin to ground. The adjustment pin of the LM350 according to the datasheets states that the typical current draw to 50uA and 100uA max. Most of the SCR's I am seeing afford a great number of AMPS for their operation limits. Is it safe to guess that the current draw by shorting the adjust pin to ground depends on VOLTSout - VOLTSin difference? I'm not sure I have understood the data correctly But, once the SCR's gate is triggered, it will stay latched until the circuit is reset or powered off, correct? \$\endgroup\$ Mar 29, 2015 at 4:44
  • \$\begingroup\$ Are there SCR's that handle very small current if the adjust pin current is small and may come in a TO-92 instead of a TO-220? I'm certain that a PNP / NPN pair can be used to subtitute if needed. Can a transistor (selection depends on the current / volts traversing the adjust pin to ground) be used instead to prevent the persistent latching once the comparator's high output goes low? \$\endgroup\$ Mar 29, 2015 at 14:34
  • \$\begingroup\$ What are the ratings of the .1 and 2.2k ohm resisters next to the 2.2 ohm load resister? I have an inclination that the .1 ohm may need to be quite large. \$\endgroup\$ Mar 30, 2015 at 21:43
  • \$\begingroup\$ Thanks for clarifying the resistors. I breadboarded a mock-up of the comparator circuit using a 741 op amp until an LM321 arrives. Once I calculate the output from the LM321 (from the 4.7k ohm feedback resistor/500 variable on the inverting input and the 2.2k ohm / .22 ohm resistors on the non inverting input, I will be able to calculate how much (or how much to reduce) the bias from the LM321 output to 2N5060 SCR. Datasheets are a huge help. I will do some more tweaking later to get near exact measurements. \$\endgroup\$ Apr 4, 2015 at 11:16
  • \$\begingroup\$ For some reason the edit I made did not take, Their should be a capacitor of 1uf going to ground on each of the op amps + and - inputs. Both could be from 5 to 100 volts DC. Positive + side to op amp inputs. \$\endgroup\$
    – user66377
    Apr 7, 2015 at 1:00

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