LCR bridge

After acquiring a Digital Multimeter, the next thing you probably want is to measure are capacitances and inductances. A must when building receivers or transmitters yourself. This nifty LCR bridge helps you do precisely that! It is based on the simple bridge circuit. It works by putting an unknown component in series with on of a known value. Take two resistors (one adjustable) and vary the ratio until you do no longer measure a voltage difference between mid-points of the capacitors and the resistors. Apply the ratio to the known capacitor/resistor/inductor to obtain the value of the unknown part.

As you can see above, the set-up is pretty straight forward. An oscillator on the left, an audio transformer (salvaged from an old transistor radio), the actual bridge circuit and finally an amplifier with VU meter as a bridge balance indicator. It can easily be seen that if the ratio RA/RB and RC/RD are equal, there will be no voltage across the bridge.

The various blocks are easily recognized in the actual circuit diagram. Transistors Q1 and Q2 form an oscillator that runs at around 610Hz. A transformer in the collector of Q1 is used to isolate the bridge from the oscillator. In this way it is possible to connect the node between the two resistors to ground and use the other bridge connection to measure the voltage difference eg. the bridge balance/unbalance. A JFET 2N3819 (Q3) is used as a buffer with a 1MΩ input impedance and a low output impedance. Resistor R5 is required so that the gate always sees a path to ground. Depending on the position of switch SW1, Op amp A1 is used to multiply the signal by 10x or 100x. This feature is handy so that in the ‘normal’ setting the dip can quickly be found and the final tuning is done with the ‘100x’ setting. C6 ensures that at DC, the amplification is 1x. The bridge rectifier diodes are germanium, though Schottky diodes will work just as well. Transistor Q4,R11,C7 form a low pass filter circuit on the amplifier supply, to isolate the detector from the switching noise from the oscillator. The filter has a cut-off frequency of fc=1/(β2πR11C7) ≈ 0.013Hz.

The chosen bridge setup requires a range selector switch with only a single mother con-
tact and also features the option of “pairing” components. The scale used is 0,1x – 1x – 10x. This means that the middle has a 1x ratio, when the pot-meter is turned fully counter-clock- wise the multiplier ratio is 0.1x and when the pot-meter is turned fully clock-wise the multiplier is 10x. So, with the reference values in the left column you get the following ranges:

Reference ValueLow-end scaleHigh-end-scale
L10 uH1 uH<->100 uH
1 mH100 uH<->10 mH
100 mH10 mH<->1 H
C 100 pF10 pF<->1 nF
10 nF1 nF<->100 nF
1 uF100 nF<->10 uF
R100 Ohm10 Ohm<->1 kOhm
10 kOhm1 kOhm<->100 kOhm
1 MOhm100 kOhm<->10 MOhm


A tea-bag storage box was used as a housing. The plastic-glass in the lid (with the reference to it’s original function) was removed. In the corners some small pieces of wood were glued for mounting the front panel. Two layers of walnut stain were used to darken the wood. Boiled linseed oil was applied for a durable finish. A couple of bolts were glued to the bottom as standoffs for the print and the battery. Household ‘silver’ paper, aluminium foil) was glued to the inside as shielding material. The front panel is made from 3mm birch plywood. More foil was glued on the back panel. A front panel was drawn on the computer, printed out and glued to the front side. Finally the controls were mounted.

To calibrate use a couple of precision (1% tolerance) resistors. A good selection would be to use values from the E-12 range spanning 2 decades eq. 1K, 1K2, …, 8K2, 10K, 12K, 15K, … 82K, 100K. Put the calibration resistor across 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.

For low inductance values, the performance can be disappointing. There a probably a couple of causes for this. Low inductance values give low impedance. This could be remedied by increasing the oscillator frequency a twenty old increase should easily be achieved. Another reason could be that
inductors tend to be “dirty”components with lots of series resistance. This could possibly be remedied by changing the bridge layout to a Hays or Maxwell bridge setup. Any way, enough room for experimenting
and learning!

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