LCR Bridge

By Cor van Rij

Finished LCR bridge Building electronic equipment is fun! Especially if you have access to some instruments that will help you troubleshooting. The best instrument available to everyone are your own brain and your own senses. When you smell smoke you have a problem and with your eyes you can see where it is comning from. Even without smoke you have the "finger-test"; if it is too hot to touch you have a problem (DO NOT try this with equipment connected to mains, whether ~115V or ~230V or any voltage above 24V!). The standard external instrument that everyone should have is a volt-current-ohm meter (VOM). This will help you measure voltages and currents. In the resistance mode it will help troubleshooting short-circuits in your circuit, test diodes and resistors, check for open connections in inductors and cables. Nowadays they're available dead cheap. Check out for example FutureLec (not associated!). With a price of 5$ it is hardly worth buying a replacement battery. Some people even use them as a cheap alternative for ready DVM modules. Check out the web-site of Hans Summers. These digital VOM do have an internal resistance which is typical like 10MOhm/Volt and an accuracy of <5%.

The next thing you probably want to measure is value of a certain capacitor or if you planning on building radios the value of an inductor. Unfortunately most of the cheaper multi-meters, which are perfectly adequate for the job, don't have this option. The more expensive models do have an option for measuring capacitance. However, only the really expensive models do have the ability to measure the inductance of a coil. This is where the bridge circuit comes in. Put an unknown capacitor in series with a capacitor with a known value. Take two resistors (one adjustable) and vary the ratio until you do not measure a voltage difference between the capacitors and the resistors. If you want to have more information check it 1,2,3, in the literature below.

Circuit diagram

OK, enough talking. Time to get to the interesting part. Below you will see the block diagram of the device. An oscillator on the left, an audio transformer salvaged from an old transistor radio, the actual bridge circuit (the taper resistor and the two condensators, one as reference the other as unknown) and finally an amplifier with meter reading. It can easily been seen that if teh ratio RA/RB and RC/RD are equal, there will be no voltage across the bridge. Circuit diagram
If we now go to the actual circuit diagram, the block diagram is easily to recognize. The astable multivibrator (AMV) with the two transistors on the right is the oscillator. The 10K potmeter are resistor RA and RB, of the two condensators one is the reference, the other is the "unknown". The right half of the circuit is the detector with a simple old VU-meter as indicator.
Circuit diagram
The AMV oscillates a at a frequency of f=1/(2* 0.69*R*C), which is circa 609Hz. A transformer in the collector of one of the transistors is used to isolate the bridge from the oscillator. In this way it is possible to connect the node between two resistors to earth and use the other bridge connection to measure the voltage difference e.q. the bridge unbalance.
A J-Fet 2N3819 is used as a buffer with a high input impedance and a low output impedance. The input impdeance here is solely determined by the 1MOhm resistor. This resistor is required sothat the gate sees a path to earth.
Finally an opamp is used to multiply the signal 100x or 10x depending on the resistor value used in the feedback leg. In the final build a SPDT switch was used sothat the sensitivity could be changed between normal and '10x'. This feature is handy sothat in the 'normal' setting the dip can quickly be found and the final tunining is done with the '10x' setting.
The condensator in the feedback leg is used sothat the DC amplification is 1x. The diodes in rectifier are germanium diodes, with their lower Vd a very small dead range is created.
On the right you can see a simple RC lowpass filter in the powerline of the detector. This is used to isolate the detector from the switching noise from the AMV (see also improvements). Later on, I decided to improve this with a single transistor. While the original filter does have a cut-off frequency of fc=1/(2*pi*R*C)= 28Hz the modified circuit does have a cut-off frequency of fc=1/(beta*2*pi*R*C)= 0.013Hz an improvement of more then 2000x.

Circuit diagram

OK, now how do we set up the selection of reference values? After some sketching I came up with two different possibilities. See the diagram below. The first has the advantage that you need only one pair of socket to connect the unknown impedance to but it requires the use of switch with two main contacts. The other option uses three sockets but uses a selector switch with only one main contact. The choice was finally made for the latter since this does have the advantage of 'pairing' components.
Reference value selection

After some tinkering I decided to go for the scale of 0.1x - 1x - 10x. This means that the middle has a 1x ratio, when the potmeter is turned fully counter-clock-wise the multiplier ratio is 0.1x and when the potmeter is turned fully clock-wise the multiplier is 10x. So, with the refrence values as given on the left you get the following ranges:
Impedance ranges

Construction

Now that we have the circuit for all sub-parts and have the values for reference reistors, inductors and capacitors we can start building. For this time I decided first to sketch the PCB set-up before heating up my soldering iron. I've found that this doesn't really add anything in terms of speed or compactness, however. So in later projects I've abanded this step alltogether.
Component Layout 1/2 Component Layout 2/2
Click to enlarge
Click to enlarge
 
As the housing for the LCR-meter I obtained a Tea-bag stirage box from Xenos (a household goddy stores in Holland). I removed the plastic-glass and used some coating to change the look to a darker more old look. A couple of screw were glued to the bottom as stand offs for the print and for the battery. As protective shielding the inside is covered with household silverpaper.
Inside box, painted Inside box, mounted print and shielding
 
Next several sketchus were made for the front-panel. The one in the middle is my final sketh before I draw it up in CAD and exportedit to a laser-printer (right).
Front panel, several sketches Front panel, final layout Front Panel CAD drawing
Click to enlarge
Click to enlarge
Click to enlarge

And finally, the final product:

Finished LCR bridge

Calibration

Calibration is fairly simple. Take a couple of precision (1% tolerance) resistors. A good selection would be to use values from the E-12 range. For example 1K, 1K2, 1K5,1K8, 2K2, 2k7, 3K3, 3K9, 4K7, 5K6, 6K8, 8K2, 10K, 12K, 15K, 18K, 22K, 27K, 33K, 39K, 47K, 56K, 68K, 82K, 100K, which together covers one sweep of the dial with the 10K build-in reference resistor. Put the calibration resistor accross the terminal and turn the dial on the right until lowest signal is measured, then mark this on the right dial. Repeat this for each calibration resistor. The accuracy of your measurement now only depending on the accuracy of you reference values.

Improvements

So, would I do anything dofferent the next time I would build a bridge to measure resistors, capacitors or inductors. Well, as a matter of fact there are a couple of things I would change:
  1. To measure resistance I would power the bridge with DC. This would enable to really measure the pure ohmic resistance of a part.
  2. Actually, I am a bit disappointed in the ability to measure inductors. There are properly thjree reasons for this:
    1. Inductors are "dirty" components the parallel capacitance and the series resistances are often not neglectable
    2. The complex impedance is fairly low at a frequency of 1KHz. As you now the impedance of an inductor is ZL=2*pi*fc*L. For an inductor of 1uH and a frequency of 1KHz this means an impedance of 0.00628 Ohm. An inductor of 1mH an impedance of 6.28 Ohm.
    3. As seen in the previous points, the impedance of an inductor is fairly low and they are "dirty" components e.q. do have a "large" series resistance. A, so called, Hays bridge offers the possibility to compensate for this. For more information see the literature
  3. I would set up the bridge differently, so that only a couple of precision resistors and one single precision capacitor is required. A precision capacitor is much easier to obtain, cheaper. Also the money otherwise spend on inductors could now be spend on one quality reference capacitor. See 1,2,3, in literature for more information.
Does it mean this is a worthless piece of equipment? No, not all, usefull handy and very cheap. However, over time one learns this device shortcomings and finds ways to improve.



Have fun
Cor

Literature:

  1. "The Impoverished Radio Experimenter, Volume 3, Lindsay Publications
  2. "Universal LCR Bridge", H.Moorshead, Practical Wiresless, December 1972, link
  3. "LCR Bridge", M. Fisher, Practical Wiresless, December 1965, link
  4. "Meetapparaten, ontwerpen en gebruiken", A.J. Dirksen, De Muiderkring BV
  5. "Kapaciteitsmeetbrug", Elex, June 1985
  6. "RC-meetbrug", Elex, May 1990,
  7. "RC-meetbrug met melodie-chip", Elex, June 1986,