Part 4 - Owens Bridge Inductance Measurement Device with Variable AC Frequency and Level


In this part, we add a variable AC source. This is used instead of the 24 volt transformer shown as a "placeholder" in part 2. Thus, the connections to the part 2 instrument are the output of AC source (shown below) replaces the 24 volt winding of T1 (removing T1 of part 2) and "mains" to power the unit. The AC source consists of a variable frequency oscillator and a power amplifier.

Caution: The amplitudes and power available from this generator can be lethal. Use extreme caution. Make sure the AC is OFF before connecting of disconnecting inductors or transformers when using this option. Treat the output of this generator with at least the same respect as the mains power!



To develop the AC signal, both an oscillator and a power amplifier are needed. The oscillator section starts out with a classic op-amp triangle wave generator (U1B). This is buffered and ampified slightly and fed into 4 diodes. These diodes simply smooth off the top/bottom of a triangle wave to produce a reasonable sine wave. The quality of the sine wave is approximately independent of the frequency. With the values shown, the sine wave distortion is approximately 2%, which is perfectly adequate for this application. The diodes are isolated by another buffer. The output of this buffer is about 0.6V RMS. There are sine wave oscillators available. I chose to derive the sine wave as a teaching example as well as to allow you to use very commonly available parts. There is nothing particularly wonderful about the op-amp picked. Almost any common one will operate in this circuit.

The oscillator switches ranges by changing a capacitor value, and within a range. the frequency is adjusted by a pot. The range is approximately 10:1 adjustment per range, and the oscillator is set to provide frequencies from 20-200Hz, 200-2kHz, and 2kHz to 20kHz.

There is also provision to use an external oscillator, if one is available. The external oscillator needs to provide about 0.6V RMS in order to fully drive the power amplifier. The load on this external oscillator is over 15k ohms.

Incidentally, this is also a poor mans audio oscillator. Simply disconnect T1 (and the power amplifier) and get up to 4 volts of sine wave from pin 1 of U1. Remember that U1 pin 1 has 12 volts of DC on it, so capacitively couple the output.

The 4th section of the quad op-amp is a driver. This drives the primary of T1. The frequency response is purposely restricted so as to avoid saturation effects of low frequencies and high amplitudes in the transformers of the power amplifier. About 4V RMS is needed at the driver output to provide 240 volts out of the power amp.

Caution: The amplitudes and power available from this generator can be lethal. Use extreme caution. Make sure the AC is OFF before connecting of disconnecting inductors or transformers when using this option. Treat the output of this generator with at least the same respect as the mains power!

The power amp is a class A push-pull amp using IRF3315 devices. Amveco/Talema toroidal input and output transformers are used. These can provide 240 volts at frequencies above 50Hz. C1 purposely introduces a pole at about 50Hz so that maximum amplitude available decreases below this point. This keeps the output transformer (T2) from saturating. If you are primarily interested in testing transformers and inductors at low voltages, simply reverse the primary and secondary connections to T2 (red/vio/yel/blk windings towards the transistors, red/grn/blu/brn windings towards the bridge. This will then allow you to place about 10 volts across the inductor.

The IRF3315 devices are biased at approximately 200mA each (400mA for the amp). The resistors in the source are intended to keep the circuit reasonably balanced. If you have grossly mismatched transistors, the transformers might become saturated and not work well. Make sure the current unbalance is not over about 20mA (by monitoring the voltage at the source resistor: it should be the same for both FETs).

The Talema toroids will operate at 10-20kHz, although the maximum voltage available will be slightly less than midband frequencies.

The power supply for this oscillator/amplifier is very simple: it is a bridge rectifier with CRCRCRC filter, and two zeners to produce 12 and 24 volts for the op-amps. The 12 volt "tap" provides the reference for all op-amp stages.

Operation / Calibration / Specs

In operation, WITH THE LEVEL DOWN, connect up the inductor or transformer you want to test. Then set the frequency and level you want to test at. Make sure to TURN DOWN THE LEVEL before connecting or disconnecting the inductor or transformer you want to test.

Use a frequency meter to calibrate the frequency of the instrument. With accurate parts, the range should simply change the frequency by a factor of 10 (fringe effects on this circuit and values is essentially non-existent). So all you need is to place frequency calibration for pot settings. Something like 20 40 50 60 100 150 200 ought to be sufficient. You can then interpolate or keep a frequency counter handy when you monitor the level on the Owens bridge.

Similarly, you can add rough voltage calibration on the level pot, but it is usually best to monitor voltage right at the inductor itself. The circuit of part 2 already has this provision.


Three ranges: 20Hz to 200Hz, 200Hz to 2000Hz, 2000Hz to 20000Hz. Sine wave distortion 2%.

External source can be used: External input requires 0.6V into about 15k ohms.

Maximum power available: about 30 watts.

Maximum voltage available: 240 volts above 50Hz, decreasing to 100 volts at 20Hz, and decreasing above about 10kHz to about 180volts at 20kHz.

Parts List

In the parts list shown below, part numbers are from the Digi-Key catalog (Oct-Dec 2003). Parts are usually in stock at Digi-Key ( I have been told that the Amveco/Talema toroids are also readily available in Europe.

AC Addition Parts List
ref designator qty value manufacturer Part Number estimated cost (USD)
C1,C2 2 2.2u 25v Panasonic E1225-ND 2.94
C3-C7,C10 10 1000u 50V elec Panasonic P5186-ND 9.00
C8 1 1.2u 25v Panasonic E1125-ND 1.00
C9 10 0.12u 25v Panasonic P4526-ND 1.98
C11 10 12n 25v Panasonic P4583-ND 0.84
D1,D2,D5,D6 4 1N4148 Fairchild 1N4148FS-ND 0.24
D3,D7 2 12v .5w zener Microsemi 1N5242-BMSCT-ND 2.40
D4 4 1N5404 GS 1N5404GICT-ND 1.28
J1 1 RCA jack CUI CP-1412-ND 0.71
Q1, Q2 2



heat sink

Intl Rect






R1,R10 5 39.2k 1% YAGEO 39.2KXBK-ND 0.54
R2 5 1.3k 1% YAGEO 1.30KXBK-ND 0.54
R3,R9 5 2k 1% YAGEO 2.00KXBK-ND 0.54
R4,R18,R22-23 5 10.2k 1% YAGEO 10.2KXBK-ND 0.54
R5,R8 2 5 ohm 1w 1% Huntington ALSR1F-5.0-ND 3.90
R6 5 43k 2w BC BC43KW-2CT-ND 1.60
R7 5 2.21k 1% YAGEO 2.21KXBK-ND 0.54
R11 5 15.4k 1% YAGEO 15.4KXBK-ND 0.54
R12,R20 2 100k pot CTS CT2241-ND 5.54
R13 1 680 1w Huntington ALSR1J-680-ND 1.59
R14,R16,R19 3 0.15 ohm 2w Huntington ALSR3J-.15-ND 4.20
R15,R21 5 100k 1% YAGEO 100KXBK-ND 0.54
R17 5 33.2k 1% YAGEO 33.2KXBK-ND 0.54
R24 5 30.1k 1% YAGEO 30.1KXBK-ND 0.54
S1 1 SPDT switch NKK 360-1057-ND 4.20
S2 1 SP 3 posn sw. CANNON CKC7002-ND 5.58
T1 1 115+115:22+22 3.2 w Talema TE62014-ND 13.68
T2 1 115+115:22+22 50w Talema TE62085-ND 18.83
T3 1 115+115:15+15 50w Talema TE62083-ND 18.83
U1 1 TL064 T.I. 296-1773-5-ND 0.68

Thus, the total cost of this addition is about $116.52. This can be reduced considerably by shopping around or by having a good "junk box" of parts on hand.