# World's Simplest DC Current Limiter

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## Introduction: World's Simplest DC Current Limiter

Greeting. My hobby is electrical, I'm an amateur electrician and also a DIYer. Today I will make a DC current limiter.

Recently, I designing a bench lab power supply and I need a current limiter to avoid overload. I also want a current limiter to play with other things like motor, etc...

As a lazy people, I always looking for easy, simple, and cheap solution. I had been search for some idea on the internet but couldn't found ones (surprisingly not a single one on Instructables). Original idea is using LM317 IC as current limiter but it's not work as I expected. Other things are complicated. After that, I came up with my idea and it really simple: using a MOSFET.

Enough chit chat, let's back to work.

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## Step 1: Gather Things You Need

Everythings you need are:

- A piece of perf board (I marked the real size of perf board which needed) - 0.23\$ for that full size perfboard

- A ruler

- A knife

- A MOSFET (I use IC 06N03LA which I recycle from an old computer) - 0.18\$

- Soldering wire and a solder (I forgot to take a photo)

- A pair of crocodile clip (Red and black) - 0.05\$

- A potentiometer. I choose 1k ohm but any potentiometer will work too. I recommend using a multi-turn potentiometer for better controlling current -0.05\$

- A 9v battery clip and a 9v battery (You can use the voltage souce to limit itself if the current limiter in power supply) - 0.41\$

- 2 wires with diffirent color

- An enclosure that fits the project if you have

- And some basic knowledge

Total cost : 0.87\$ < 1\$ :D

## Step 2: Learn Some Basic

To understand how this instrument work, you have to understand how MOSFET work. For N channel MOSFET, if we apply a voltage between gate (positive voltage) and drain/source pin (negavetive voltage), the current will start to flow from drain to source. Normally, to control the current down to absolute zero, the voltage will be applied between gate and source. Very simple, effective and long life for battery (it takes only 7mA to work, about a LED). Transistor also work similar like this but I don't use transistor because the transistor work if we apply current to it. This current cause lots of heat produced by potentiomter and it drain out your battery very quickly. Mosfet also designed with some others advantage, including allow higher current to pass (06N03LA IC allow up to 50A continuosly).

## Step 3: Start to Build

Note: read carefully step by step. Read all step beforce attemp to build

- First, cut the perf board to its size 2.5x5 cm

- Place the potentiometer in a good position and bend its legs

- Put the battery clip's wire into leg 1 and 3 of potentiometer's holes then solder it. Watch the color of the wire. We want higher voltage to the right so positive wire to pin 3 of the potentiometer

- Place the IC into posision and solder the tab first. Pin 1 and pin 3 must stay on a copper mark after solder. Tips: apply heat to the tab for 2 seconds that quickly add solder to the part between perf board and IC's tab. Avoid damaging the IC. Blow some cool air to the IC right after soldering

- Connect gate to pin 2 of the potentiometer, source to pin 1. Use 2 different color wires: black for negative, white for positive

- Solder 2 wires to alligator clips. The length of wire shouldn't longer than 20 cm

- Solder 2 ends of that to source and drain pin. Red alligator clip connect with drain, black with source

## Step 4: Final Step: Testing

Connect the current limiter in series between the component that you wish to limit its current and the voltage source. If red clip is connected to posive terminal, the black clip must connected to Vcc of the component. Check before apply some voltage. The photo show I dimming a 3W LED with a current limter. This instrument can be used in many application like power supply, dimmer circuit, ... I recommend you make one to have on your workbench

This is my first instructable and I hope it's helpful for you. Share what you think in comment, add to collection, build this and show it to me. I like to see them all. I'll be back with my next project: build the simplest short circuit protector with current limiter. Thanks for reading.

Cheer!

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## 37 Discussions

That is very cool. I've pondered whether you can use mosfet to control current in this way, but I'm not that skilled so I never put much work into it. This looks very handy.

One question, how would you make this work without the 9v battery, just using the power from the input? I will have another read when I get home from work to see if I can make sense then.

Thanks for posting this

Yes. You can use source voltage as control voltage too. Just connect pin 3 of the potentiometer to pin drain. The control voltage must be at least 3v

Very nice solution! But don't you have in this way constant usage of power by resistor? Let me be more precise. I want to build simple solution to limit current that my bike lamp use from battery but I want to have max efficiency so the lamp work as max as it can if you understand me. So, if I use resistor like you draw above, am I going to have constant power(in this case capacity from battery) waste by resistor? thank you for your help.

This won't save you power. You need to duty cycle the light. Look into those types of circuits. Of course, your light will dim in proportion to the power you save.

You are right. He should using PWM or buck converter in that case

If you have a set supply and load, you can just use V=IR or I=V/R to place a single resistor in series with the lamp.

It can only applied to resistor-like load. LED isn't follow Ohm's law so it would perfom differently on different voltage. The circuit works like a dimmer by control flowing voltage

A trap for young players: EVERYTHING follows Ohms law. Thats why it is a law of electronics. LEDs may not provide a resistive load themselves, but a series resistor will still limit current. If you have a V and you have an R, then you MUST have a known I. This is the reason you will often see a current limiting resistor on the positive pin of an led inside of arduino or other diy projects.

You are right. Of course V=IR. It works on stable condition like a LED powered by a battery. But he asked me about series of LED working on various conditions (12v battery, unfiltered dynamo which can raised up to somewhat 25v, and both) with no wasted heat. A single resistor isn't good enough

Ok I do understand what you mean. Firstly, you will have to figure out how much voltage your lamp need. Then you have to know they are connected in parallel or series. If difference between battery and the lamp is lower than 2v, I suggest you use this circuit. If the difference larger than 2v, I suggest using a dc-dc buck converter which its efficiency is around 90%. And if the battery voltage is swing too much, this circuit still does better than a buck converter

Thank you. You can add this instructable into your list. Now I'm building my lab bench power supply. I'll release it tomorrow. You can add it too if you want

I also searching for this for too long. So long that I have to find my own solution. But

I am thankful for all of those who don't do anything at all. It’s because of them I’m doing it myself :D

I've never used anything like this before. Thanks for sharing your knowledge.

Thank you. I came up with this idea accidently. You might need one if you do some type of project so go and build one :P

The main problem with this circuit I have, is that with increasing current through mosfet cause more heat and when mosfet heats up it's resistance drop, so the current increase even more. (sorry for bad English, I'm not a native speaker)

Would this work for a 12 v power supply to charge a lead acid battery? At approximately 800 mA?

What is then the voltage across the led? Is it constantly 9volts ?

the mosfet, once properly adjusted, drops whatever voltage the LED doesn't need.