# The Simpliest Short Circuit Protector With Current Limiter

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## Introduction: The Simpliest Short Circuit Protector With Current Limiter

Yesterday, I was post an instructable about DC current limiter. Today, I'll post about short circuit protector with current limiter.

I'm about to make a heavy duty lab bench power supply for my project. It's can provide up to 108W and it is really, really bad if I accidently short it out. The circuit must be simple, effective, fast respond to the short and reliable. The circuit also must work at low output voltage.

At first, I use a relay to cut off the current the it shorted. It's simple and effective but slow respond and only operate at pretty high voltage. Later on, I think up a perfect circuit

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## Step 1: Understand the Circuit

Right at first, I know that I need 2 things in this circuit: it must have reset button (to cut off the current until the short is fixed) and to do it i will need a logic gate

To easy to understand, I'll call the (out) positive is A, negative is B, High means 2,7v to 5v, Low means 0,7 or lower

The logic gate need to cut off the current when it short but also act normal when no short happen. It's mean when A reach Vcc (High) and B is ground (Low), the output is High. When shorted, A=B=Low, the output also be Low. From this basic state, I choose OR logic gate IC for this circuit (see in image). In future, I'll make a power supply with multi output voltage so I'll need multi OR gate IC. Then I choose 74HC32 IC

How can the current stay off after shorted? Without reset button, the current stay shorted until no current pass through or it is un-shorted. But how and where to place this button? Obviously a simple button can't help anything at all. I need another IC, the PC817 or PIC817 opto-coupler

The opto-coupler is used to isolate 2 different voltage stage, use one to control another and this is what I want. Using OR gate, reset button and opto-coupler, I made this circuit

## Step 2: How It's Operating?

At the begining, the power is turned on but no current pass through because the LED in the opto-coupler haven't lighted up

When reset button is pressed down, the cureent start to flow from the battery to output voltage. This voltage make 5v regulator work, create 5v source for OR gate IC to operate, also feed 5v to input terminal (High), make output sate is High. High output voltage make the opto-coupler start to conduct, current can pass from collector to emitter. The transistor is added to drive higher current. After release button, the current can pass through opto-isolator to output voltage. Note: the image in previous step using voltage divider to feed input terminal; you can use voltage divider for feedback voltage if voltage source is 10v or lower

When current accidently shorted, the output voltage is zero, the voltage regulator moake output is Low, feed into logic IC and make output is Low. Low output make opto-isolator stop conducting, cureent stop flowing from source voltage. The logic IC respond to the short in 6 ns. It will save your life and the circuit that you applying current

After the short is cleared, press the reset button to reset the circuit.

## Step 3: Some Word for You

The logic IC need 5v to operate (5v is max) so i recommend using a voltage regulator. You still able to use voltage divider to feed its input terminal. This circuit should be built inside the power supply. You can add a toggle switch as below circuit to drive the output at maximum current (use a 220ohm to avoid burn out the LED inside)

My instructable is ended. Hope it's helpful for you (I think it really helpful). Comment what you think bellow.

This instructable was inspired by Julian Illet's video about his invention: DCOI logic.

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## 8 Discussions

hi
this projects test to proteuse 8.7
1-okay
2-set 8 dc motor for tested current
but volt low
input volt dc=35
output=2....7 volt
motor ampter 8 motor=11 a
thanks
flightsazan@gamil.com

Only after you did so, then here is my answer for your question:
- RV2 is a component name created by simulator software. it's a normal potentiometer
- point your mouse over individual pic and you'll found explaination for all component (you are newbie so I understand why you ask)
- battery= power supply
- i choose V=35v because it is absolute value for LM7805. you can set it high as much as you want, just need to protect the IC
- check all pic. some later one is for detail explaination
- the "complete circuit" one
- q2 can be either NPN or PNP. you'll need some basic knowledge to modify
I hope it help

I need a simple short circuit protector to save my power source.
I'm trying to understand your circuit but I have so many questions and so confused I am afraid to build it.
RV2- What is an RV? A special type potentiometer?
Which switch is the reset switch? I see 3 switches. What do the other 2 switches do?
Where do I connect the load?
This shows battery powered but where do I connect power supply?
How much voltage goes in?
Many schematics.
I don't see references in text, but some differences.
Which one do I build from?
Q2 is NPN can I use PNP?
A bom would help.
Thank you for this instructable and any instructions you can give.

would it be possible for you to show me how to make a short finder for my truck? I have a short in it somewhere and I can't afford to take it in to have it fixed. Would it be hard to make one that I can use by myself because I have no one to help me and I am disabled so it's going to be hard to do to begin with. Thanks for sharing help either way.

Do you have a multimeter? If you do turn on resistance function or continuty function. If you don't have any multimeter, use any (low voltage) battery with an indicator (light, buzzer, etc...) From my experience, shorted part usually has overheat, burn mark, bad smell and sometimes burning glow. You could find the shorted area from it

And the list of components?

You could see the hint in the instruction. They are 74HC32 IC, PC817, a push button switch and a power transistor