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Blog Project14's DIY Test Equipment: DIY Constant Current Load
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  • Author Author: ipv1
  • Date Created: 3 Sep 2017 6:18 AM Date Created
  • Views 3721 views
  • Likes 14 likes
  • Comments 7 comments
  • diytestequipch
  • constant current
  • ip_iot
  • diy
  • constant-current
  • project14
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Project14's DIY Test Equipment: DIY Constant Current Load

ipv1
ipv1
3 Sep 2017

This is my submission to the Element14 Project14 DIY

 

The problem

I have recently been doing extensive experimentation with recycled batteries as well as voltage regulators and line control circuits. The customary approach requires a programmable DC load to the connected such that the output can be tested under various conditions. Unfortunately, these commercial solutions are quite expensive in addition to being big and bulky. Due to a constant fear of the spouse with regards to unexplained expenditures as well as a lack of bench space, I set out to find a DIY solution to the problem.

 

The result is a simple yet elegant project that can be assembled in a manner of hours and is useful for all electronic hobbyists and professionals alike. Due to a reduction in size and very low BOM cost, the device can be used for field testing of a number of equipment as well. In the writing below, I will try to explain the research process, accepted approach, assembly as well as a quick demo of the project.

 

The search

Doing a google search for ‘constant current source’ comes up with a number of interesting results including publications in instructables. Though these may not inspire much confidence, they are a source of test data that an approach was in fact used. My search lead me to a youtube video by Dave Jones of the EEVBlog where he uses a simple circuit to make a DIY Constant Current Load. Further investigation revealed that many more had tried the approach and indeed was a usable. Hence, I decided to recreate the circuit shown below due to it’s simplicity.

 

image

 

 

The tweaks

The selected low-side sensing and control approach was solid however, I had to improvise a bit in my approach with the first change being the MOSFET itself. The MTP3055 was not something I had to hand so I decided to use the IRFZ44N. This had a cascading effect on the design with the Vcc being migrated to 9V and eventually two 9V cells. I used the LM358 which is half the op-amp the LM324 is however retains all the characteristics. Lastly I only had 1/4watt resistors which I had to make do with but did not make a big difference in the long run. I also did not want to spend on a dedicated meter hence resorted to other means. The final improvisation in the assembly is the use of a multi turn preset in place of the suggested multi turn pot which is cheaper and more compact.

 

The Circuit Diagram

Below is a shot of my notebook with the circuit diagram, specs and a layout. I used separate connectors for the Op-Amp supply and the load. In order to monitor the set current, tapped into the non-inverting input of the second op-amp. This connected to a multimeter would give us the set current without the need for a dedicated meter thus reducing the BOM cost of the project. An extension of this technique could tap into the inverting input to get the value of actual current being drawn. In my case, I employ a dedicated meter for the task to ensure accuracy.

 

image

 

 

Adding a fuse or switch seemed trivial though can be a useful feature. I do not plan to make a PCB for this since it is a stepping stone to the next level which I will discuss at the end. I made a layout of the design before I switched on the soldering iron so that I knew exactly what I would be aiming for. I aimed to made the device as small as possible and in true DIY fashion.

 

The Assembly

Putting humpty-dumpty together was not a challenge thanks to a bit of planning and the end result is presented in the image below.

 

image

 

With my 3D printer lacking the necessary filament, I decided to add some DIY stand offs to the resulting circuit board. Pencil erasers cut up in the right size and attached using hot glue were a successful endeavour allowing for a bit of ground clearance. This would be helpful in case of solder blobs or unsheathed wire trims were loose on the workbench. The heatsink was a non branded one though it did seemed big enough and did it’s job well. The end result is shown below without it’s connectors as well as in complete setup.

 

image

 

 

Testing

My tests were conducted with a Tenma Bench Power supply at 12 Volts though I did do the math before hand. Below is a capture of my workbook reflecting the same.

image

 

Sincethe IRFZ44N is capable of working up to 175’C and 50Watts, my tests would be a piece of cake. Below is a video of the entire project along with a demo of the final product.

 

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One point to be noted is that at 1 Amp of current, the drop on Rsense would be 1 Volt and with the Vgs required to be more than 10V for the IRFZ44N and 4.5V for the MTP3055, the gate voltage should be as high as possible. This would also go on to effect the switching times with the Vgs rise time not being able to meet it’s intended mark.

 

 

More Notes and future work

I designed this module to work as a standalone project however there are a number of things that can be done in the future. The first being the addition of an Totem-Pole Driver stage to the MOSFET to increase switching speed. This is easier said than done however it addresses the issue raised in the previous section.

 

The second modification is using a DAC to digitally control the set current. The buffer could be converted into a gain stage and it could be digitally controlled. Next would be the addition of an ADC to monitor the actual current flowing. An INA219 could be used directly if the circuit were to be made anew and on a dedicated PCB.

 

Lastly, a self resetting fuse and temperature sensor could be a handy feature for long term use. A DS1307 could be used for the RTC and store current and voltage data until it is retrieved. This could be furthered by the addition of a memory card hence the possibilities are endless.

 

 

Robert Peter Oakes Jan Cumps and jw0752 already have an extensive project in the making for the more serious user. This is a quick and dirty solution for the procrastinating maker and enthusiasts and I hope it finds use in your hobby shop.

 

 

Happy Hacking!

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Top Comments

  • jw0752
    jw0752 over 7 years ago +4
    Hi Inderpreet, This is an excellent DIY build and very practical. I liked how you included your note book entries. I also use notes on each of my projects that in the end turn into a crude owners manual…
  • ipv1
    ipv1 over 7 years ago in reply to jw0752 +2
    Thank you sir, I would like to add that the soldering station you sponsored for the Secret Santa is being put to good use and I appreciate the support. I have been using notebooks since 2006 for making…
  • michaelkellett
    michaelkellett over 5 years ago +2
    One point to be noted is that at 1 Amp of current, the drop on Rsense would be 1 Volt and with the Vgs required to be more than 10V for the IRFZ44N and 4.5V for the MTP3055, the gate voltage should be…
  • michaelkellett
    michaelkellett over 5 years ago

    One point to be noted is that at 1 Amp of current, the drop on Rsense would be 1 Volt and with the Vgs required to be more than 10V for the IRFZ44N and 4.5V for the MTP3055, the gate voltage should be as high as possible. This would also go on to effect the switching times with the Vgs rise time not being able to meet it’s intended mark.

    I looked at the data sheet for the IRFZ44N from IR, especially this graph on page 3:

    image

     

    You can see that typically you will need a lot less than 5V Vgs to get 10A of drain current - so you should be able to run you op amp from a 9V supply.

     

    Another point that may be helpful, whenever I put a buffer in the feedback loop of an op amp I add some defensive components:

     

    image

    R1 helps to protect the op amp from the capacitive load presented by the MOSFET gate.

    It also slows things down, make it too big and the MOSFET will take ages to turn on.

    C1/R3 reduces the gain in the loop at high frequencies, many op amp/MOSFET combinations are unstable.

    The best way to tune (in Spice or hardware) -

    start with the values shown, drive with pulses or square wave.

    Adjust R1 to get fast enough response (check op amp specs for recommended minimum series R driving a capacitive load).

    Adjust C1.R3 to get best (smallest) overshoot (increase R1 or C3), and best response time (decrease C1 or R3)

    As long as your op amp is unity gain stable you can tune this design to be stable with pretty much any MOSFET and op amp - high Q inductive loads  can be a problem.

     

    All the above applies if you are using the MOSFET as a current sink or a voltage follower where R2 is the actual load.

     

    MK

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  • balearicdynamics
    balearicdynamics over 7 years ago

    Well done Inderpreet. Thank you for the exhaustive documentation making it fully replicable and easy to understand for all. Bookmarked for future applications.

     

    Enrico

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  • ipv1
    ipv1 over 7 years ago in reply to mcb1

    Thank you DAB sir,

     

    mcb1 sir,

    That solution works if you have a good power supply at hand. My in-field Variable Power supply regulation testing had a sort of spot-welding machine as a load. The idea was to see if a fluctuation load could cause a reset in other sub-systems thereby necessitating the need for more filter caps. I ended up giving each local voltage regulator an array of large polar capacitors at the output side. Fortunately, I still have my paper journal from the time with the nitty-gritty details.

     

    Thanks for sharing,

     

    Sincerely,

    IP

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  • mcb1
    mcb1 over 7 years ago

    Well done.

    I do like the Inderpreet CAD package you use ... works well.

     

    Way back in Telecom days the boys needed to test power supplies and couldn't find anything like this commercially, so they built one that was essentially one of our variable power supplies operating into a very low resistance.

    It doubled as a heater on cold days, but certainly worked well.

     

    Mark

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  • DAB
    DAB over 7 years ago

    Very good project.

     

    Well documented and written.

     

    DAB

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