DIY: Relatively simple digital LC meter circuit and construction details

[nzo]

Thanks to Phil Rice VK3BHR

Details here

Includes source code, board art, faq and panel art.

I have not built this as I already have an Altronics LC meter.

[Mechatrommer]

i've done the simplified version of it. the heart is in the Ccal and C and L next to it. the problem is drifting with temp will change the frequency, and there's no hard math on it, i doubt it will follow the formula 1 / sqr(LC)? cant remember what, have to dig my archive. i will get back to this next time.

[amspire]

I prefer the version with the Nokia display that costs a few dollars on ebay.

http://projectproto.blogspot.com/2009/09/lc-meter-pic16f876a-n3310lcd.html

From what I have read from people who have properly tested these meters, when calibrated, they can often do better then 1%, but sometimes they can be as much as 4% out, particulary if you have a lossy capacitor or inductor. Unlike a true LCR bridge, these meter tend to depend on the part behaving perfectly.

Definitely a useful thing to have, but if you want readings you can trust, you need a bridge or one of the 2 value LCR meters.

Also you are up against off the shelf meters like these:

http://www.aliexpress.com/product-gs/498780603-New-LCR-RCL-Inductance-Capacitance-Resistance-Meter-Free-Shipping-TS37-wholesalers.html
http://www.aliexpress.com/product-fm/452868186-wholesale-price-DM4070-3-1-2-Digital-LCR-meter-w-self-discharge-2000uF-Free-Shipping-wholesalers.html
http://www.aliexpress.com/product-fm/510614018-New-MS8222H-LCR-Meter-DMM-compared-w-FLUKE-CE-CATII-wholesalers.html

and I think this one is based on the same circuit that you were going to make:
http://www.aliexpress.com/product-fm/495648379-LC200A-handheld-inductance-capacitance-table-inductance-table-capacitance-table-digital-electric-bridge-on-site-LCR-meter-wholesalers.html

[amspire]

And here is a USB design that works with a PC:

http://projectproto.blogspot.com/2010/10/pic18f-usb-lc-meter.html

[Mechatrommer]

and I think this one is based on the same circuit that you were going to make:
http://www.aliexpress.com/product-fm/495648379-LC200A-handheld-inductance-capacitance-table-inductance-table-capacitance-table-digital-electric-bridge-on-site-LCR-meter-wholesalers.html

http://www.ebay.com.my/itm/New-LC200A-Inductance-Capacitance-L-C-Multimeter-Meter-/300582408167?pt=LH_DefaultDomain_0&hash=item45fc1b8fe7

* "Modle":LC200A
* 500KHz test freq.
* 0.01 pF - 100mF
* 0.001 uH - 100H
* Measurement accuracy:1%
* with kelvin clip
* US $48.88 (free shipping)

sounds tempting?

[amspire]

and I think this one is based on the same circuit that you were going to make:
http://www.aliexpress.com/product-fm/495648379-LC200A-handheld-inductance-capacitance-table-inductance-table-capacitance-table-digital-electric-bridge-on-site-LCR-meter-wholesalers.html

http://www.ebay.com.my/itm/New-LC200A-Inductance-Capacitance-L-C-Multimeter-Meter-/300582408167?pt=LH_DefaultDomain_0&hash=item45fc1b8fe7

* "Modle":LC200A
* 500KHz test freq.
* 0.01 pF - 100mF
* 0.001 uH - 100H
* Measurement accuracy:1%
* with kelvin clip
* US $48.88 (free shipping)

sounds tempting?

The kelvin clip is not used as a kelvin clip - it is just used as bad SMD clip. It would be better to get SMD tweezers without the strong spring and big teeth. That clip is designed for wire, not SMD parts.  It looks like the same as my Chinese one, but your price is a bit lower.

The trouble with these meters in general is they genuinely can do 1% (as long as a very stable calibration capacitor has been used). But they are not accurate for all capacitors and inductors, so you can never be sure if it is 1% accurate or not.  Once you know it is accurate for a certain type of capacitor, then you can probably trust it, but it sounds like you need a better LCR instrument as well to confirm the results each time you come across a new type of capacitor, or a new range of inductors. Inductors are the least perfect of our passive components, and so it will be inductance readings that are the most doubtful.

Since the reading is wrong for lossy components, it will probably be bad for testing old electrolytic capacitors that may have bad ESR. You may get totally fictitious readings.

Richard.

[BravoV]

Has anyone every tried this yet -> Visual Analyzer ZRLC -> http://www.sillanumsoft.org/ZRLC.htm (freeware)

Its using the pc's sound in/out audio ports, and two op-amps and single resistor, got this link by one of the member here Jim, I'm planning to tinker with it later this month if I had a time, pretty simple though I wonder how it's accuracy. 

[amspire]

A very interesting idea. A lot of potential.

I think it will have limited accuracy until the circuit is improved a bit. in particular, a PC input is not designed for accuracy, so I think it needs an automated calibrate step to match the two channels. I would also like to see a differential amp across the component under test at least to give a 4 wire measurement.

I like the way they take fourier transforms to eliminate the effects of everything but the test frequency. That is nice. Since most new PCs can sample at 96KHz or higher, there is a chance it could work at 100/120Hz and 10KHz as well as 1KHz. With the right circuit, I am sure the range could be extended from the miserable 0.01uF to 100uF to a full 0.1pF to 10,000+ uF.

To test anything over 100uF, it will need an amplifier to boost the signal power from the computer.

Also to cope with notebooks that often only have a mono Mic in, it would be good if the input could switch between the two measured signals.

Richard.

[luky315]

I experimented with the "original" circuit and it works fine, but now I need improved accuracy for small caps (<=100pf) and inductors (<22µH).
I get the impression that this circuit is not optimal for that, but what else could I use?