Constant Current Load - Adjustable

Other Hardware General Electronics
1

Hello everyone,

I am trying to build a capacity tester for 18650 cells. I'm looking to use an Arduino to control the discharge rate through PWM. I started with the "standard" constant capacity circuit but instead of a potentiometer to adjust the current I want to use PWM. Schematic is attached.

My issue is that I am unable to reach the desired discharge rate of 1A. I am limited to a range of approx. 0.5 - .9A. I am unsure why this is happening. I even tried replacing the PWM input circuit with a potentiometer and voltage divider and saw the same behaviour. Any suggestions?

Thanks!

Constant Current Load.png
Constant Current Load.png841×477 4.66 KB

2

An LM358 does not have rail2rail outputs. There might only be ~3.5volt at it's output with max PWM (measure it).
So did you use that IRLIZ44 mosfet, or a different (non-logic level) one.

Is the battery circuit able to carry 4Amps (no thin wires or crappy breadboard)
Leo..

3

Hi,
Welcome to the forum.

Please read the first post in any forum entitled how to use this forum.
http://forum.arduino.cc/index.php/topic,148850.0.html .

OPs diagram.

Constant Current Load.png
Constant Current Load.png841×477 4.66 KB

What model Arduino are you using?

Thanks.. Tom.. :slight_smile:

4

TomGeorge:
Hi,
Welcome to the forum.

Please read the first post in any forum entitled how to use this forum.
http://forum.arduino.cc/index.php/topic,148850.0.html .

OPs diagram.

Constant Current Load.png
Constant Current Load.png841×477 4.66 KB

What model Arduino are you using?

Thanks.. Tom.. :slight_smile:

I am using an Arduino Uno. PWM circuit is working as expected.

PWM Value of 51 outputs 1V
PWM Value of 128 outputs 2.52V
PWM Value of 255 outputs 5.02V

I checked the LM358 Pin 1 Output and I am seeing 3.76V at max PWM (5.02V). The MOSFET (IRLZ44) is also generating a significant amount of heat compared to the resistor.

Any suggestions on a different op-amp to use to correct this issue? LMV358?

Thanks!

5

One way to fix it is to use a higher supply voltage for the opamp.
9volt (battery) would do.
Must also use decoupling cap from opamp VCC to opamp ground.

The fet is used in linear mode, so will get hot.
It could dissipate 1watt (max) on a 4volt battery, and that's a temp rise of about 65C above ambient (TO-220).
Use a small clip-on heatsink.
Leo..

6

Thank you for the quick response. I will test out the 9V battery and report back. If I was set on only supplying 5V to the circuit, what would your recommendation be? Just considering my options for when I scale up the design for 32 cells.

7

If the fet is getting hot it means it is not switching fully on , probably because the gate voltage is not high enough

8

Maybe you can use a charge pump powered by PWM pins in your final circuit.
Or use opamps with rail2rail input and outputs.
The search engines of Mouser, RS components etc. (depending where you live) might help you there.

@hammy
The fet is supposed to work in linear mode (getting hot).

The problem is that you run out of headroom because of the increasing voltage on R3 and the voltage loss of the opamp.
Leo..

9

Update on progress.

Supplied op-amp with 9V and circuit is now functioning as expected. I now have full control of discharge rate.

Also, MOSFET is generating significantly less heat.

Thanks again for the help!

10

dills_b:
If I was set on only supplying 5V to the circuit, what would your recommendation be?

Could use a 5>9volt (micro-power) boost converter for those opamps.
Leo..