Introduction: 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!

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

Ebay:

1x Vero board: http://rover.ebay.com/rover/1/711-53200-19255-0/1?...

1x 1Ω / 5W resistor: http://rover.ebay.com/rover/1/711-53200-19255-0/1?...

1x LM358: http://rover.ebay.com/rover/1/711-53200-19255-0/1?...

2x PCB terminals: http://rover.ebay.com/rover/1/711-53200-19255-0/1?...

1x IRLZ44N N-channel MOSFET: http://rover.ebay.com/rover/1/711-53200-19255-0/1?...

1x 500k potentiometer: http://rover.ebay.com/rover/1/711-53200-19255-0/1?...

Amazon.com:

1x Vero board: http://amzn.to/1OeN41p

1x 1Ω / 5W resistor:http://amzn.to/1FFJNEk

1x LM358: http://amzn.to/1OeMz7o

2x PCB terminals:http://amzn.to/1CuBQgM

1x IRLZ44N N-channel MOSFET: http://amzn.to/1H8jr1l

1x 500k potentiometer: http://amzn.to/1FFJxoT

Amazon.de:

1x Vero board: http://amzn.to/1yZ4k1J

1x 1Ω / 5W resistor:http://amzn.to/1yZ4u9x

1x LM358: http://amzn.to/1GtRhNH

2x PCB terminals:http://amzn.to/1GtRU9R

1x IRLZ44N N-channel MOSFET: -

1x 500k potentiometer: http://amzn.to/1yZ4Vkb

Step 3: 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!

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

http://www.youtube.com/user/greatscottlab

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

https://twitter.com/GreatScottLab

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Comments

author
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.

author
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...

author

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

author
Schnaugnfaisl (author)2017-07-21

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

author
Scuba_2 (author)Schnaugnfaisl2017-07-27

see my post

author
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!

author
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)

image.jpegimage.jpegimage.jpegimage.jpegimage.jpegimage.jpeg
author
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.

http://www.ko4bb.com/Test_Equipment/DynamicLoad/

Info on constant current load

http://mustcalculate.com/electronics/heatsink.php

Heat sink calculator

author
Chloe_Lemaire (author)2017-05-27

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

author
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:

author
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?

author
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!

author
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

author
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!

author
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 ^^

author
kamhagh (author)Chloe_Lemaire2017-06-01

Woops sorry! Wrong link xD try this one: https://www.instructables.com/id/Power-LED-s---simplest-light-with-constant-current/

LEDs are a pin to drive ;/

author
kamhagh (author)Chloe_Lemaire2017-06-01

https://www.google.com/amp/www.instructables.com/id/Super-simple-high-power-LED-driver/%3Famp_page%3Dtrue

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

author
vanmarleguus (author)2017-05-17

Can I use a irf44n instead of the irlz44n?

author
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

author
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.

author
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

author
Harie Amjari (author)2017-03-25

Where does the ground go in the schematic?

author

To GND?

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

Awesome!

image.jpgimage.jpg
author
Harie Amjari (author)AntekU2017-03-26

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

author
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.

temp_-1598772405.jpgtemp_-1888335233.jpg
author
ElenaV13 (author)Manoj BR2017-01-09

How can I increase the discharging current?

author
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!!

author
Harie Amjari (author)Iqbal Samin2017-03-25

A,B,C Stand for -
A = ALWAYS
B = BE
C = CAREFUL

author
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?

Thanks!

circuito_scarica_70A.PNGschema_circuito.PNG
author
Arduineer (author)ElenaV132017-01-11

No way don't touch it.

author
Iqbal Samin (author)ElenaV132017-01-09

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

author
ElenaV13 (author)Iqbal Samin2017-01-09

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

author
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?

author
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

author
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?

author
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.

author
khavoc (author)khavoc2017-03-21

pull-up*

author
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).

author
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. http://bit.ly/1G0N8fW

author
t3ch1t.io 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.

ConstantCurrentLoad.jpeg
author
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).

author
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?

author

It will work, but it will get very hot since it has a high drain-to-source resistance. It will also cause some voltage drop, so the amount of current calculated by the op amp might not be equal to actual current flow, so use an ammeter to measure the current flow (I might be wrong about it)

author
bob4analog (author)2017-01-24

Nice build! However, I don't think the circuit is supposed to oscillate. Any of the ready-made constant current circuits of the same design dont do that. Have you used any decoupling capacitors in your Circuit? This might help your circuit. Not trying to be critical, I just noticed this tho. ;-) Thanks, nice vid!

author
GreatScottLab (author)bob4analog2017-01-31

It will always oscillate in such a configuration. If you can even call this a proper oscillation. A capacitor does not change the properties of the OpAmp.

author
CF-Yannic made it! (author)2016-07-21

Based on your instructable I've created a 6 channel load: http://carforge.net/?p=83

Thanks for your input! :)

carforge_batt_test.jpg
author
CF-Yannic (author)CF-Yannic2017-01-17

New Link: https://www.carforge.net/49-2/

author
ElenaV13 (author)CF-Yannic2017-01-09

Hi! What can you do with 6 channels? Do you use it to discharge 6 cells at the same time or do you connect the channels in parallel in order to have an higher discharging current?

Please let me know! :)

author
CF-Yannic (author)ElenaV132017-01-17

I use the 6 channels to discharge 6 cells at the same time

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