Introduction: DIY Constant Current Load

Picture of DIY Constant Current Load

In this small project I will show you how to make a simple adjustable constant current load. Such a gadget is useful if you want to measure the capacity of chinese Li-Ion batteries. Or you can test how stable your power supply is with a certain load. Let's get started !

Step 1: Watch the Video!

The video gives you all the information you need to build a constant current load. But I will present you some extra help in the following steps

Step 2: Order Your Parts!

Picture of Order Your Parts!

Here is the small list of parts that you will need:


1x Vero board:

1x 1Ω / 5W resistor:

1x LM358:

2x PCB terminals:

1x IRLZ44N N-channel MOSFET:

1x 500k potentiometer:

1x Vero board:

1x 1Ω / 5W resistor:

1x LM358:

2x PCB terminals:

1x IRLZ44N N-channel MOSFET:

1x 500k potentiometer:

1x Vero board:

1x 1Ω / 5W resistor:

1x LM358:

2x PCB terminals:

1x IRLZ44N N-channel MOSFET: -

1x 500k potentiometer:

Step 3: Build the Circuit!

Picture of Build the Circuit!

Here you can find the schematic for the build and the board design that I created. Make sure to interrupt the copper traces underneath the LM358.

Step 4: Success!

Picture of Success!

Now you should be able to build your own constant current load.
Feel free to check out my Youtube channel for more awesome projects:

You can also follow me on Facebook, Twitter and Google+ for news about upcoming projects and behind the scenes information:


JeremyB206 (author)2018-01-19

Hey guys,

Wondering if someone could help me. I got the circuit working, but the project I'm working on at school requires the discharge current to be at minimum 4A. Does anyone know a way I could alter the circuit to allow greater discharge currents? I tried using a 12V source and reducing the resistor value, but it only makes small changes, and seems like I am capping at 2.5A.


khavoc (author)2018-01-15

I was thinking about trying to use the second comparater to measure the voltage of the battery without a voltage divider and then use that to somehow trigger the device to stop. I built it and love it, and have used it a lot, I put USB capabilities on it for super easy battery bank testing, but also use it for cells, but since you have to watch cells it would be nice if I could basically make it shut off when the battery reaches the desired voltage. Dunno, pretty new but maybe I could us the second one to test voltage and as soon as it drops to the desired end voltage, it could trigger a flip flop to shut off the circuit.

Any ideas would be awesome.

Manoj BR (author)2016-12-24

I made it.
I used IRFZ44 with 12v.
it works good enough.
but the max current is 2.5amps.
Thanks a lot.

ElenaV13 (author)Manoj BR2017-01-09

How can I increase the discharging current?

Iqbal Samin (author)ElenaV132017-01-09

You can increase the discharging current by increasing the supply voltage and using a MOSFET with a lower Gate-threshold voltage (Vgs).

But remember, more discharge current = more heat = more danger! BE CAREFUL!!

ElenaV13 made it! (author)Iqbal Samin2017-01-09

I found this circuit on an article. It is said it can be used to have a continuous discharging current of 70 A! Is it possible in your opinion?


JeremyB206 (author)ElenaV132018-01-12

do you have a link to where you found this?

Arduineer (author)ElenaV132017-01-11

No way don't touch it.

Iqbal Samin (author)ElenaV132017-01-09

It might be possible, but, I don't think you should discharge a battery at 70A ?

ElenaV13 (author)Iqbal Samin2017-01-09

It depends on the capacity of the battery you are dealing with

Harie Amjari (author)Iqbal Samin2017-03-25

A,B,C Stand for -
B = BE

AnvitV (author)2017-11-21


I have tried to recreate this exact circuit, but have always failed. What do we attach at "load", like a resistor or what?

SašoD1 (author)AnvitV2017-12-25

The "load" is the battery that you want to discharge.

Keiichii FranciscoT (author)2017-03-07

hi sir great scott...may i have a question? since the op amp LM358 have 2 comparator...can i use them both??

2pcs. IRFZ44N, 2pcs. 500k potencio meter, 2pcs 1ohm resistor but 1pcs LM358....

thanks for your time to reply...

I had to power with 10v to get the IRFZ44 to work 5v would not turn it on enough.

khavoc (author)Scuba_22017-12-11

The IRFZ44N is not a logic level, so yeah, you would need more voltage, the IRLZ is the logic level version, but as long as you can supply the turn on voltage I don't see why either would not work. There are also other mosfets that are logic level that should work.

cmdnymous1 (author)2017-08-24

Hi Guys,
I want to build this Project but wich mosfet do i have to buy ? I found those 2 at the local electronics shop IRLZ44PBF 1 N-Kanal 150 W TO-220AB or IRLZ44NPBF 1 HEXFET 110 W TO-220
Thanks for your help.

Scuba_2 (author)2017-07-28

Today I ran some tests.

2A 12v for 40 minutes with no fan and pcb sitting on bench so no air flow underneath.

The heat sink had reaches 107 deg C which I think is the safe limit for my setup.

After standing pcb off of the bench and applying a small fan , the heat sink dropped to 40deg C

So at this point I turned the power up to 4A @ 12V. The heat sink is about 54deg and the resistor is around 200 deg C ( the coating is good to 350 deg C)

All looks great.

Schnaugnfaisl (author)2017-07-21

Hi, what is the max. discharge Ampere?
How can I pimp it?

Scuba_2 (author)Schnaugnfaisl2017-07-27

see my post

Scuba_2 (author)Schnaugnfaisl2017-07-24

I have just ordered the parts, I calculate with a 3c/w heat sink I can take 30w.

So I think 2.5A @ 12v.

Hoping to build this in the next 3 days , so will find out if I'm completely wrong or not !

I will post the results good or bad!

Scuba_2 made it! (author)2017-07-27

Wow it works!

Firstly if you use an IRFZ44N it will not work at 5v ,use 10v to power the project .

( you need 10v on the gate of the MOSFET to make it open with low resistances)

I built this using a 10W 1 ohm resistor and a 3 degree per watt heat sink.

I am running it happily at a load of 2A at 12v (24W)

Scuba_2 (author)2017-07-24

Hi I'm going to try and build this in a few days time.

I hope it will be good for 30w

2.5A @ 12v

I will be using a 3C/W heat sink to get rid of the extra power dissipated by the MOSFET.

I am new to this so will post the results.

I found these which may help.

Info on constant current load

Heat sink calculator

Chloe_Lemaire (author)2017-05-27

Can it be use as a constant current source for a 10w LED?

kamhagh (author)Chloe_Lemaire2017-05-31

I don't see why not! just replace the resisto with your led! but I have to remind you I'm a noobie just like you c:

Chloe_Lemaire (author)kamhagh2017-05-31

Well the thing is I found this circuit used as a constant current discharge for a battery thus I'm not really familiar with it. Also what do you mean by replacing the resistor with an LED? :3 Can i just simply put a 10w LED as a load?

kamhagh (author)Chloe_Lemaire2017-06-01

Yeah. I actually tried it on a breadboard ^_^ it does work if you replace the Resistor with your LED (or whatever you want to power with a constant current).
not sure though, because I didn't have a logic level mosfet and used 12v for the op amp!

the load+ mosfet works as a voltage divider afaik!

Chloe_Lemaire (author)kamhagh2017-06-01

So what you mean is that I cam simply change the 1ohm 5w Resistor with a 10w LED and power up the circuit with +12v through the op amp, after that what do I do with the load?
~I'm way too noob for this circuit sorry :3

kamhagh (author)Chloe_Lemaire2017-06-01

Well you have to use a sense resistor which is just a resistor with a small value to detect current.

I would rather use the other circuit that uses a transistor. Just Google simple diy led driver. It's on instructables too! Look at that one!

Chloe_Lemaire (author)kamhagh2017-06-01

I really appreciate your replies and I think I really understand how to make Constant Current circuit. Hence, I'll do my best to create one for my desired project. ~Thanks ^^

kamhagh (author)Chloe_Lemaire2017-06-01

Woops sorry! Wrong link xD try this one:

LEDs are a pin to drive ;/

kamhagh (author)Chloe_Lemaire2017-06-01

This is a bit better and has a good explanation! I think you'll understand how this works afterwards too. ^_^

vanmarleguus (author)2017-05-17

Can I use a irf44n instead of the irlz44n?

kamhagh (author)vanmarleguus2017-05-31

It has to be logic level. the Lm358 only outputs 5v.

the datasheets says it allows 10A at 5v so I guess you're ok! Not sure though

KirkS33 (author)2017-05-11

I built this with slightly different parts. I used a 1R 10W resistor, no biggie, a JRC324D op-amp, and a random n-channel MOSFET I had in my parts bin that had a low gate threshold voltage. I also used a regular 500k pot, instead of a trimmer pot. It took some fiddling, and attaching a large chunk of aluminum to the MOSFET with thermal compound, but I finally got it sorted stable at 500mA. It fluctuated a bit from .45A to .55A, but it mostly stayed around .5A. My parallel set of 3 18650s, supposedly 2150mAh each, has been on it for just over 8 hours now. The voltage is at 2.85, I'm gonna cut it off at 2.75. They're older batteries from a laptop, so I don't expect the full capacity.

Victor2900 (author)2017-05-05

Hi nice project!

Can you explain more the graf with constant current in the video. Why are you using 500mA in your calculation when the current in the load resistor is 1A?. Is the 500mA current the current that goes into the op-amp?

Thanks for your help

Harie Amjari (author)2017-03-25

Where does the ground go in the schematic?


AntekU made it! (author)2017-03-21


Harie Amjari (author)AntekU2017-03-26

Do you have an idea where does the ground go in schematics?

Carl Hauschke (author)2015-08-29

I think I wired up everything correctly but the circuit only draws a maximum of 20ma. What could be wrong with my circuit?

ThomasVDD (author)Carl Hauschke2015-09-02

I've got the same problem! It doesn't draw more than 178mA for me; everything wired up fine though. :s

Carl Hauschke (author)ThomasVDD2015-09-02

I noticed that my MOSFETs are conducting even if nothing is connected to the gate. Sometimes if I apply 5V to the Gate I can toggle the MOSFETs but that's pretty much random. Do your MOSFETs have the same problem?

khavoc (author)Carl Hauschke2017-03-14

you may need a pulldown resistor if you have an open collector on your op amp. otherwise you will get readings all over because the gate is floating.

khavoc (author)khavoc2017-03-21


ThomasVDD (author)Carl Hauschke2015-09-03

Mosfets often conduct when nothing is connected, but it shouldn't when the gate is connected to ground. Mine work fine: open when gate is connected to ground and closed when it's connected to VCC (5V in this case).

Carl Hauschke (author)ThomasVDD2015-09-22

I manaaged to fix the problem. The output of the LM358 was not high enough with 5V on the input. Just increase the voltage delivered to the LM358. According to the data sheet everything up to 32V is safe. made it! (author)2017-03-15

Great instructions made for an easy build.

Still working out how to set different currents for different voltages. Is there an equation for how to set if to get different currents. I know the video said 1V = 1 Amp. I guess that means 0.5V = 500ma. If I decrease the value of the pot can I get finer adjustments?

Seems easy to draw a lot of current by accident with the 500k pot. Set it too high once and the resistor started to glow orange. Cut the power quick and it seems everything survived my mistake.

agr00m (author)t3ch1t.io2017-03-17

The equation is in the video: I = Vin / R.

I = Current

Vin = Voltage from potentiometer

R = Resistance of load resistor (R1)

If you increase R to say 10 ohms, then at 1 volt you'd have a current of only 100mA (1/10 = 0.1). If the maximum voltage is 5v, then the most current for this resistance would be 500 mA.

If you know the maximum current you want to achieve, then use this formula to choose the value for R: R = Vin / I.

So if you want a 1A maximum load, then R would be 5 ohms (5v / 1A = 5ohms).

shivakumar1995 (author)2017-02-15

I tried going through datasheets of both IRLZ44n and IRF510. they seem similar. can i replace the IRLZ44n with IRF510?

About This Instructable




Bio: Awesome Electronics Tutorials, Projects and How To´s
More by GreatScottLab:Make Your Own Crude Cocktail MachineMake Your Own Solid State RelayMake Your Own Simple Theremin
Add instructable to: