Javascript® Electronic Notebook by Martin E. Meserve Inductance Meter
I was doing some work with toroids and needed a inductance meter, so that I could check my work and to help identify some unknown cores. To do that job, I had purchased a Inductance Adapter at a ham fest. I was going to build one, but this one was all put together and ready to go. While it sat on my desk next to a small DVM (Cen-Tech 7-Function Digital Multi-Meter from Harbor Freight), I had an idea. I thought, wouldn't it be nice if the Inductance Adapter could be integrated into the meter. Then I wouldn't have a extra box laying around my work bench.
I thought I had a unique ground breaking thought, but I was wrong. After I worked everything out, I came across a similar idea on the internet. There were slight differences between their idea and mine, but essentially, it was the same thing. The on-line idea sacrificed a switch position and was built directly in the meter. But I didn't want to sacrifice a switch position and I wanted to mount the adapter external to the meter.
So, I outlined my thoughts.
I am not going to give wire by wire construction details. So please reference the Physical Construction drawing below. It should give you a good idea of the process. On the left, the drawing shows a scaled sketch of the meter and added adapter. On the right, the drawing shows how the power switch on the Inductance Adapter is wired into the meter. The second drawing is a schematic of the Inductance Adapter, for reference.
The Inductance Adapter fit very nicely on the back of the meter. The circuit board is 1-5/8" high and 2-1/2" wide. Two 1/8" holes were drilled and the board was mounted on 1/4", 4-40, aluminum spacers. Smaller spacers could have been used, but that size was what I had on hand. I kept the mount a little loose, because I would need to remove it to attach my wires. I had thought of cutting away the back of the meter so that the adapter's PCB could be flat with the case, but that would have eliminated the top clips that are molded into the case.
The adapter's test points are mounted on the top of the meter. These are the test points where the "Unknown Inductor" is connected. I first removed the two wires and cleaned up the solder lugs under each test point. Two 1/8" holes were drilled for the screws that hold it in place, and two larger holes for the test point lugs. I had to be careful to avoid the plastic protrusions that are intended for the rear of the meter case. The rear of the meter case clips into these protrusions first, and then closes at the bottom. After drilling the four holes I used a Exacto knife to elongate the holes slightly. This gave me some wiggle room and helped avoid the protrusions.
I looked into burrowing power from the meter's internal battery. However, the meter's internal battery negative terminal is not referenced to the COM input terminal. So I simply used some velcro to attach the 9 Volt battery to the rear of the meter, just below the Inductance Adapter.
Except for the wires to the 9 Volt battery, all the other wires on the Inductance Adapter need to be removed and replaced. The Inductance Adapter needs to be unmounted, because the new wires need to be inserted from the top of the PCB and then soldered on the back. Overall there will be 5 wires needed. Two for the inductor test points, and three for the adapter's output and On/Off switch. I used a insulated #22 stranded wire with a Teflon jacket. But you can use almost any kind of wire you have on hand.
In the first drawing, the connection to the meter's input jacks is detailed. In the meter that I have, the PCB for the jacks is separate from the main PCB and connected together with solder bridges. So to separate the V/Ω/ma input jack from the main PCB, all you need to do is remove the solder bridge. Then it is a simple matter of running the wires from the adapter's On/Off switch to the available pads.
