L-C Meter Adapter



General


Another project started with three others not finished. What would my Dad say? Well one thing he would say is, "Warn people that this project is under construction and has not been proved to work yet". You have been warned.

I had to use a signal generator, resistor, and oscilloscope to measure the value of an inductor for my battery-charger project. That was just irritating. So, after looking for a reasonably priced L-C meter and finding none, I started looking at articles for building one. There are many on the net, but I picked a circuit by Iulian Rosu. It looked the best for checking all sizes of inductors with minimum errors due to resistance and core saturation. The capacitance meter was thrown in for convienience and does not pretend to be a circuit in the league with a Gary Novak's femto farad circuit. It is just a simple capacitance checker.

The Circuit


If you look at the schematic below you will see that it is just a re-drawn version of the schematic for Iulian Rosu's adapter shown in the qsl.net webpage. I added one more position to the range switch to go to 20mH. There is probably a good reason why the increase in range won't work. I am hoping it is just accuracy. At 20 mH I am just looking for a range confirmation, not an actual measurement.

The other change I made to the inductance adapter circuit was to add a 10X gain stage after the detector. I wanted the inductance and capacitance adapters to use the same voltmeter scale and the capacitance adapter did not work so well on the 200mV scale.

Oh yes! I also had to change the clock to a 3.2MHz oscillator with division down to 100kHZ. It was more cost effective (cheaper) to use the 3.2MHzcrystal and a 4060 IC than to buy a 100kHz crystal.

The capacitance circuit is a triggered one-shot that is driven by the crystal clock. The clock is there, so why not use it? Simply pick the resistors to give the correct averaged output voltage for the available clock periods.

Because I wanted full-scale ranges from 2nF to 200uF, the resistor value range would have to be from 10 to 1 megohm, a couple of the timing resistors would have to be 1-Watt rated, and a separate dicharge power-transistor and driver would be needed and a 1-amp power supply would be needed. This would not work well.

I had to use two clock periods and two voltages in order to keep the resistor values in a reasonable range of values. The clock periods are a multiple of 100; that allowed me keep the resistor value for the 200uF range down to 510 ohms and still retain a reasonable 510k ohms for the 2nF range. By using 9V for the timer and timing resistors, I could use the low-time of the timing pulse for the measurement time and the high-time of the pulse for reset. A reset time is needed to get the big capacitors dicharged after measurement, but it is also needed to discharge a over-range capacitor that only gets partially charged during the measurement time. Without the discharge, a capacitor with value greater than the selected measurement range, would not be starting from 0V during the measurement period.





Schematic diagram of Adapter


The Board


I should have used double-sided board or planned layout to better fit an available box size. What I got was a 4.25" x 4.75" board. When you add a 12V transformer ( I don't like wall warts), two rotary switches, the test terminals,and the voltmeter terminals the adapter box is pretty big and there is a lot of wasted space in standard boxes. What to do?

Build a box? Too much work in metal. How about wood? Yuk! That looks like cr--. Okay, another project. make a wood box that doesn't look like a wood box.



Adapter Board Layout


The Box


This was not easier. It required too many steps. Five steps to glue it together then cut it in two, two steps to put in the alignment posts and drill holes for the screws, one more step to cut holes for the components, and finally, four steps to mask off areas and paint the box. That is 12 steps, so in a rush it would have taken two weeks. I think it took me two months at my pace.



One piece box assembly



Box assembly cut in half



Alignment pins added



Holes and probe connector



Box components wired



Switches wired



Partially assembled



Final box


Wiring


Keep the wires short is always a good rule. I chose to make assembly of the box top to the board more difficult in order to keep the wires as short as possible. The short wires do not allow the pc board to be removed from the box top without removing the nuts on the switches and potentiometer. The wire lengths in the wiring diagram below are exaggerated.



PCB Hole ID



Initial External Wiring



Final External Wiring



The Results




Copyright Dale Thompson,
November 29, 2008 through
last revision on November 14, 2010