Wednesday, January 18, 2017

Testing the precision rectifier circuit...are those parts marked right?

My boards came back from OSHPark late yesterday afternoon, so I went ahead and built one up this morning. Most of the parts came from new stock or the shop's parts inventory, but the diodes did not - germanium diodes are easy to get if you buy packs of surplus Soviet parts on eBay, but buyer beware!

As with the other boards I've purchased from OSHPark, all the text was nice and clear, holes were clear, and the solderability of the board was excellent. It went together quickly and easily.

(In retrospect, I should have noticed the diodes weren't right, but I haven't worked with Ge diodes for so long...)

Some quick checks on the board revealed that I had made a stupid wiring error. The input voltage was reversed. I have no idea why, other than I just didn't run the traces correctly. That was a simple fix, cut a track and run a couple of jumpers. After that, it came right up, all LEDs lit and no smoke!

Second issue was the LEDs are drawing way too much current. While 20mA isn't much, I want to keep the total draw down as much as possible, so I'm going to increase the current limiting resistors to 4.7k. I need them lit, not lighting up the inside of the box.

So, those problems are taken care of, and I've got it hooked into the electrical system here at the shop for testing, but it's not acting like I think it should. Everything is floating around way too much, and the outputs are going negative...

That shouldn't happen. I immediately suspect the diodes.

Off to the bench, a quick meter check reveals that the bands on the diodes are actually on the anode! Looking closer, I can see the point contact to the Ge is under the bands. Go figure! (Someone mentioned that it was marked like a selenium device...)

Here's a closeup, and you can see the cup with the wire coming out, and just make out the semiconductor material under the bands. What I find odd about this whole thing is I've seen other examples of this diode, and they were marked correctly. Oh well, now I know.

(The Russian D9E, or so the package says!)

With the diodes in correctly, it goes back to the test rig. Now, everything is working just as it should!

Perfect. The output jumps when my device activates!

The current transformers are just clipped on to the input to the box. I'm not worried about how much, just the absence or presence of current in this case. Calibration will come when the devices are secured and can't move around.

This test is successful. Time to button up the panel and get ready to build one for myself, and think about where things are going to be installed.