This is a second build of of a Simple DC Electronic Load project using information that I learned about curing Oscillations in a separate discussion:
The knowledge that I received from other element 14 members as well as things that I learned from the bad decisions that I made on the first build made me want to give it another try. Here is the blog from the first build:
This time I had a much more stable control circuit design:
Check out the first link above about curing oscillations for credit where it is due with my mentors.
There are three differences that have eliminated the oscillation problem over the entire voltage / current range for this Load. The first change was to put resistance R2 (4K7) between the sense resistor and the inverting input of the OPA192 instead of the previous direct connection. This allows the second improvement, a 200 pF capacitor at C2, to work properly and dampen any oscillations with negative feedback to the Op Amp. Resistor R3 was also lowered from 100R to 47R following another recommendation from a friend.
I wanted to improve the reference voltage stability so the zener diode used in the first design was changed to the use of a TL 431 which is a precision programmable reference. I hoped to improve temperature stability with this change so that the load would be stable without a warm up stabilization period.
The final addition to the build was the addition of simple polarity protection on the input. While the IRFZ 34 does have an internal protection diode I wanted to eliminate the chance that it would be damaged an accidental polarity reversal. My fix for this was to put a fuse in the circuit and a reverse biased 10 amp diode so that a fuse would blow if the polarity were to be reversed. I also have a red LED to indicate that the fuse has blown and that the input is reversed. I had experimented with the idea of using a forward biased diode in series with the input but this had an effect on how the unit handled lower input voltages. There are always side effects and compromises become choices that have to be made.
This time I wanted to put the power supply and the control circuitry on the same board. I am using a small AdaFruit protoboard. With good fortune I found that I had duplicates of the heat sink, enclosure, transformer, and ammeter. This build will use an OPA 192 rail to rail op amp as recommended. Here is the finished control and power supply module:
One of the things that I learned from the previous build and problems with oscillations was that I did not want to hard wire the control module into the enclosure. It makes it too difficult to service or make modifications. Therefore Micro JST plugs and jacks were used as well as a molex connector for the mains connection to the transformer. I don't anticipate any problems needing modification but that was how I felt last time too and it was a real challenge to work on the hard wired circuit board in the first unit.
I was happy with the front panel layout of the first unit so it was duplicated on this unit. The only change was the lowering of the power LED to accommodate a reverse polarity warning LED. Here is a picture of the front panel:
Since it is fun to look at pictures here are a few of the process of the build:
The last two pictures illustrate that I do not bundle the wires until after all preliminary tests are completed. This saves time not to mention nylon straps which must be cut if a problem is noted. In the picture on the right the wires have been bundled. I try to plan for bundling and carefully measure wires and plan their final route even though I do not actually route them until the testing is done. This enclosure is particularly nice as the air vent holes provide excellent tie down ability by looping a small nylon strap through the holes and around wires.
This unit turned out very stable and much more to my liking than the first. Subsequently I have gone back to the first unit and made the modifications necessary to have it work as well as the second unit.