In parts 1 through 3 I described the construction of the transformer we needed to generate the 10Amps AC. We still had to drive the transformer. Although I said I wasn’t going to discuss the complete circuitry, I feel a little guidance would be helpful.
I tried several power amplifiers and managed to destroy them all until Ernesto suggested the LM12 which proved indestructible. It appears to be recently obsolete, so you may want to consider the LM3886 or the OPA549 although I have not tried either. Please add your suggestions to the comments below. Figure 4-1 shows how we connected the driver to the transformer.
So there you have the knowledge necessary to generate an AC current. The only heating is in the windings of the transformer and there is a very low output voltage so that it will be safe for an operator. All you have to add is a control loop- not a trivial task, but has much more published literature than the actual AC current generation.
Before I go I thought there is a little more in the way of the practical application of this. In our products, often the input to the DUT is a toroidal current transformer as you saw with the Bicron B5303 discussed in earlier blogs. To close the loop of the secondary of the AC current generator requires you to thread the wire through the DUT’s toroid and then close the connection in some form of terminal. It is really nice if you could make this a high speed connection. This topic may appear vaguely familiar to you as it was the subject of a blog by Martin Rowe in The Connecting Edge. Unfortunately the forum is long gone without any public residual content and so I will have to repeat the basis of the blog.
One of the dirty secrets of two part connectors (plug and socket) is the low number of make/break cycles; often it is only in the tens of cycles! The contact resistance can also be a factor in generating AC currents. We came across a range of Y-series range of products from Hypertronics that offers high current rating, low insertion force and a large number of make/break cycles. Let me hasten to add that it is extremely difficult to get information on this product range. I will be happy to send the pdf of the catalog to whomever asks, but the safest is to deal with their distributors. Often you have to get one item from one distributor and the mating part from another. Nevertheless there aren’t a lot of alternatives. Again, if you know of any please contribute a comment below.
Figures 4-2 and 4-3 show one of our arrangements to allow a quick and reliable connection in order to calibrate the DUT.
Now I’m done. There are a few commercially available devices which do generate AC currents like the CA Instruments 3000CS, but I don’t know if they use the same technique that I have described. Aside from my original article (which is now lost in the ether) I know of no other text which deals with this subject. I hope you have enjoyed the show.