2011-12-21 LC Meter Model LC100-A

From my watsonseblog dated 2011 Sep 07

2011 Sep 07 LC Meter Model LC100-A

In the middle of August I got on eBay and bought a LC meter model LC100-A, which is actually just a PC board, an LCD display, 2 very short clip leads with alligator clips, and a short USB to mini USB cable to supply power to it. There is no case, just the PC board. In my earlier blog, I purchased a “4070L” LCR meter which was much more professional with a case and real clip leads. But I was disappointed that it gave poor results with inductors below 1 millihenry. The LC100-A easily reads inductors below 1 millihenry, and below 1 microhenry, just like my AADE LC Meter IIb, but at 1/4 the price – $33.00.

A few weeks after I ordered the LC100-a I received a package in the mail from China (fortunately I didn’t have to go to the post office to retrieve it like I’ve had to in the past).  I opened it up and cut the antistatic bag open, and found a PC board and display somewhat similar but not the same as the one in the bid listing,  I got on eBay and sent a message to the seller saying that the item I received didn’t look like the one in the listing.  The seller sent back an email asking that I take a picture of it and send it.  I took a picture of it and attached it to an email, and the seller replied that they had made a mistake and would send me the correct item, if I would send my mailing address.  Apparently the seller wasn’t looking much at the picture, because I took it with the wrong item setting on the package with my address on it.  That delayed the package another day due to the email exchanges.  A few weeks later – it’s already the second week of September – I finally received the correct item in the mail.

I was disappointed that the seller sent the wrong PC board the first time.  But I didn’t have to return it, so I have a spare display and PC board laying around for another day.  The seller was apologetic and everything worked out okay, it just took longer than I expected.  I do have to say that this is getting to be so typical of items I buy on eBay; it seems that more often than not there is a major problem with purchases I make on eBay.  I am talking about purchases (“Buy It Now”) where the item is sold as new, not used or ‘as is’.  I have had major problems with sellers who are putting used items up for bid, and often the sellers are inexperienced with few transactions.  But in this case, the seller is very experienced with thousands of transactions, and this kind of mistake should not have happened.  This mistake cost me a few weeks delay, but it cost the seller the price of the wrong item, because it did not get returned to the seller.  You can’t do business if your inventory dwindles away because of stupid mistakes your staff make when shipping stuff.

Checking out the meter

I’ve had just a short time to play, er, experiment with the LC100-A and compare its values with the values of coils I measured with the LC Meter IIb. The reason I’m doing this is because both meters are supposed to measure with an accuracy of 1 percent. They are very close, just a few percent, but they are not exactly the same. I don’t expect them to measure the same, because an inductor may have slightly different inductance depending on the frequency at which it is measured. But I’m pleased with this little PC board; it does a much better job of measuring coils that I use in Joule Thiefs and other small projects such as RF oscillators. The earlier LCR meter I bought was clearly not designed to measure coils in this range, and the higher inductance ranges are nearly useless because those larger inductances are seldom found in electronics.

They didn’t send any instructions or any paperwork with the PC board (in my brief search online I couldn’t find any, but I did find a few specifications). They don’t tell you that when you power it on, you should short the alligator clips together and press the Zero button in and hold it in until it says OK. I had to find this out by trial and error, and without my previous experience with the LC Meter IIb I may have never known that this was necessary. I don’t think the new purchaser with no experience would have any clue that this was necessary. I need to download an instruction manual for this LC meter to see what the Func button is supposed to do. I press it while there is a coil being measured and it gives a frequency that differs depending on the coil.

When I first got it I plugged the USB cable into the PC board and into my desktop PC. There is a small power switch that has to be turned on to apply the power. After I zeroed it and measured a few coils, I found that I wanted to use it in other locations not close to my desktop, so I had to make up a 5 volt power cable for it. The PC board comes with two power connectors, the mini USB connector and a standard 5.5 mm power socket. I have several 5 volt, 0.7 amp regulated AC adapters (it must be regulated 5VDC) but they have the wrong connector on the end. So I had to cut that off and solder on the correct connector to the end and put heat shrink tubing over the splice to make it look halfway decent. Now I can plug the PC board meter into the power strip and be independent of the desktop or any other PC.

Update Sep 10 – I measured a few dozen coils with both meters. The LC100-A is consistently lower than the LC IIb by a fraction of a percent to 1 or so percent. I don’t remember measuring one that was higher on the LC100-A. I measured some coils that were factory made, and these usually have values that are standard. For instance I measured one toroid on the LC IIb and it measured 220 uHy, which is most likely its true value. When I measured it with the LC100-A, it measured 210.6 uH, which is most likely low. I calculated that as 4.3% low, but the actual percent differs depending on the value of the coil. Most coils seem to measure 1 to 2 percent lower on the LC100-A.

Update Sep 11 – I measured several 470 pF, 1% silver mica capacitors with both meters. Yes, I zeroed both meters before I started making the measurements. The LC Meter IIb measured them just about right on. The LC100-A measured them consistently about 4 pFs higher.

I then measured several 1 uF, 5% ‘J’ plastic film capacitors. The LC Meter IIb measured close to 1 uF. The LC100-A could not measure them on the regular range, I had to push the Hi C button. I thought this was odd because the online specifications say that it is supposed to be able to measure up to 10 uF on the low range. It measured the 1 uF caps a few percent low, which is the opposite of the 470 pF measurements which were higher. This “higher on the low end, lower on the high end” error seems to indicate that the LC100-A should measure capacitors with no error somewhere in between. I also thought it might be possible to reduce the error by performing a mathematical correction on the value depending on the value. Of course this could also be done on the L measurements to reduce the error.

Conclusion
One conclusion I can make is that this LC100-A meter does not meet its advertised specifications of an accuracy of 1 percent. Since I purchased only a single unit, I cannot make any judgment on whether this particular meter only has the error, or whether all of the meters have this same error. I can’t find any manual that might tell me how it works, or if there is way to adjust for the error. As it stands, the meter is consistently inaccurate, so I can measure inductors and adjust slightly for the error, and I will get a fairly decent accuracy.

Back to experimenting…