I wrote an article on this topic in 2000 that was published on Circuit Cellar Online. Despite what the alarmists may tell you that nothing is ever deleted on the Internet, I have evidence to the contrary. I believe that the basic information that I wrote about is still relevant and so I am re-writing the article as a blog. I must acknowledge the role Ernesto Gradin, my mentor, played in the realization on the magnetics of the project. Any errors are as a result of my misunderstanding.
My current employer, Emphatec Inc., has been through several guises, but we still design and manufacture several sensors for an AC current. The AC current is measured with either a current transformer, or in-line resistor and then converted to a signal friendlier to instrumentation like a 4-20mA current loop. If you look through my blogs both here on Planet Analog and on MCU Designlines, you will find several references to such devices. The latest was Figure 2 in Potentiometers: Mechanical & Electronic, Part 2. Integral to the manufacturing process is calibration and test necessitating the generation of an AC current to stimulate the input of the module under test.
The currents that need to be generated for our products can, in some cases, exceed 100Amps AC. Resistive techniques typically generate excessive heat. In addition, they are also potentially dangerous to the test technician since there could be 120V present at the module inputs. A further disadvantage is that the test is restricted to the line frequency and subject to the unpredictable changes of the line voltage.
One solution in the industry is to use a toroidal transformer with only a few turns on the secondary. For all transformers, the power presented at the primary is equal to the power delivered at the secondary, and the output voltage is proportional to the turns ratio of the secondary winding to the primary winding. It is possible by using these two properties to construct a toroidal transformer to generate a high output current at a low output voltage. I am simply going to refer to this as a “transformer” from now on.
We started out using an existing transformer with a 120VAC primary and drove it with a variac. A few turns were made through the toroid as the secondary and shorted together, and this is a very simple start, but we could never get the output current to be stable over any length of time. We attributed this to mains variations, but we never had the instrumentation to get to the bottom of it.
The next attempt was to use a sine wave oscillator driving a power amplifier that was connected to the primary of the transformer. In this configuration it was possible to change the AC frequency, but it suffered from another problem. The transformer secondary current is affected by the loop resistance, which includes the terminal resistance of the module and the wire length. In addition with higher current, the secondary loop warmed and affected the overall loop resistance.
We decided to control the output in a closed control loop built on dedicated PCB that included PC setup and monitoring, an 8051 micro, and a couple of chips that are now obsolete. Because of the obsolescence I will not include that in my description, and if I were to repeat this project, I would certainly consider a PID loop made up of commercially available parts.
Because we were going to use a custom driver, we had to wind our own transformer. We will cover that in the upcoming parts of this blog.