Wednesday, April 27, 2011

Kit review – High Accuracy LC Meter

Hello readers

Time for another kit review. Lately one of my goals has been to make life easier and in doing so having some decent test equipment. One challenge of meeting that goal is (naturally) keeping the cost of things down to a reasonable level. Unfortunately my eyesight is not the best so I cannot read small capacitor markings – which makes a capacitance meter necessary. Although I have that function within my multimeter, it is often required to read resistors in the same work session.

Thus the reason for this kit review. A day trip to Altronics saw me return (amongst other things) with their High Precision LC Meter kit. The details were originally published in the May 2008 issue of Australia’s Silicon Chip magazine. The meter specifications are:

  • Capacitance – 0.1pF to over 800 nF with four-digit resolution;
  • Inductance – 10 nH to over 70 mH with four-digit resolution;
  • Accuracy of better than +/- 1% of the reading;
  • Automatic range selection, however only non-polarised capacitors can be measured.

The power drain is quite low,  between 8 (measurement) and 17 milliamps (calibration). Using a fresh 9V alkaline battery you should realise around fifty to sixty hours of continuous use. At this point some of you may be wondering if it is cheaper to purchase an LC meter or make your own. A quick search found the BK Precision 875B LCR meter with the same C range and a worse L range for over twice the price of the kit. Although we don’t have resistance measurement in our kit, if you are building this you already have a multimeter. So not bad value at all. And you can say you built it :)

Speaking of building, assembly time was just under two hours, and the kit itself is very well produced. The packaging was the typical retail bag:

The first thing that grabs your attention is the housing. It is a genuine, made in the US Hammond enclosure – and has all the required holes and LCD area punched out, so you don’t need to do any drilling at all:

The enclosure has nice non-slip rubberised edging (the grey area) and also allows for a 9V battery to be housed securely. The team at Altronics have done a great job in redesigning the kit for this enclosure, much more attractive than the magazine version. The PCB is solder-masked and silk-screened to fine standard:

There are two small boards to cut and file off from the main PCB. We will examine them later in the article. All required parts for completion were included, and it is good to see 1% resistors and an IC socket for the microcontroller:

At first I was a little disappointed to not have a backlit LCD module, however considering the meter is to be battery operated (however there is a DC socket for a plugpack) and you wouldn’t really be using this in the dark, a backlight wouldn’t be necessary. Construction was easy enough, the layout on the PCB is well labelled, and plenty of space between pins. Lately I have started using a lead-former, and can highly recommend the use of one:

Assembly was quite simple, just start with the lower profile components:

… then mount the LCD and the larger components:

… the switches and others – and we’re done:

The only problem at this point was the PCB holes for the selector switch, one hole was around 1mm from where it needed to be. Instead of drilling out the hole, it was easier to just bend up the legs of the switch and keep going:

At this stage one has to cut out two supports from the enclosure, which can be done easily. Then insert the PCB and solder to the sockets and power (9V battery snap). Initial testing was successful (after adjusting the LCD contrast…):

If you look at the area of PCB between the battery and the left-hand screw there are eight pins – these are four pairs of inputs used to help calibrate and check operation of the meter. For example, by placing a jumper over a pair you can display the oscillator frequency at various stages:

Furthermore, those links can also be used to fine-tune the meter. For example one can increase or decrease the scaling factor and the settings are then stored in the EEPROM within the microcontroller. However my example seemed ok from the start, so it was time to seal up the enclosure and get testing. Starting with a ceramic capacitor, the lowest value in stock:

Spot-on. That was a good start, however trying to bend the leads to match the binding posts was somewhat inconvenient, so I cut up some leads and fitted crocodile clips on the end. The meter’s zero button allows you to reset the measurement back to zero after attaching the leads, so stray capacitance can be taken into account.

Next, time to check the measurement with something more accurate, a 1% tolerance silvered-mica 100 picofarad capacitor:

Again, the meter came through right on specification. My apologies to those looking for inductor tests – I don’t have any in stock to try out. If you are really curious I could be persuaded to order some in, however as the capacitance measurement has been successful I am confident the inductance measurement would also fall within the meter’s specifications.

As shown earlier, there were two smaller PCBs included:

The top PCB is a shorting bar used to help zero the inductance reading, and the lower PCB is used to help measure smaller capacitors and also SMD units. A nice finishing touch that adds value to the meter. The only optional extra to consider would be a set of short leads with clips or probes to make measurement physically easier.

When reading this kit review it may appear to be somewhat positive and not critical at all. However it really is a  good instrument, considering the accuracy, price, and enjoyment from doing it yourself. It was interesting, easy to build, and will be very useful now and in the future. So if you are in the market for an LC meter, and don’t mind some work – you should add this kit to your checklist for consideration. It is available from Altronics stores and currently a good deal at Au$76 including GST.

As always, thank you for reading and I look forward to your comments and so on. Furthermore, don’t be shy in pointing out errors or places that could use improvement. Please subscribe using one of the methods at the top-right of this web page to receive updates on new posts, follow me on twitter or facebook, or join our Google Group for further discussion.

High resolution images are available on flickr.

[Note - The kit was purchased by myself personally and reviewed without notifying the manufacturer or retailer]

Otherwise, have fun, be good to each other – and make something! :)

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