Project14's DIY Test Equipment: DIY Constant Current Load

RE: Project14's DIY Test Equipment: DIY Constant Current Load

michaelkellett — Mon, 06 Jan 2020 10:20:21 GMT

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:

[View:/resized-image/__size/426x375/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-a375dca5-7cdd-4061-94d1-04f498753396/contentimage_5F00_205638.png:426:375]

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:

[View:/resized-image/__size/341x325/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-a375dca5-7cdd-4061-94d1-04f498753396/contentimage_5F00_205639.jpg:341:325]

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.

RE: Project14's DIY Test Equipment: DIY Constant Current Load

balearicdynamics — Mon, 04 Sep 2017 06:38:01 GMT

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

Enrico

RE: Project14's DIY Test Equipment: DIY Constant Current Load

mcb1 — Mon, 04 Sep 2017 05:44:59 GMT

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

RE: Project14's DIY Test Equipment: DIY Constant Current Load

DAB — Sun, 03 Sep 2017 18:24:46 GMT

Very good project.

Well documented and written.

DAB

RE: Project14's DIY Test Equipment: DIY Constant Current Load

jw0752 — Sun, 03 Sep 2017 06:34:16 GMT

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. Without the notes I would have trouble trouble shooting my own designs. The design that you used is basically the same as the load that is at the heart of the one I put together.

John