Part 3- Owens Bridge Inductance Measurement Device with DC current Capability


Last time we talked about a real instrument that you can build. It is self contained, except that it does require an external AC voltmeter as the null indication device. This version adds capability to measure the inductor with settable DC in the inductor. For the Lx10 range, 0 to 100 mA is supported; for the Lx1 range, 0 to 200mA is supported; and for the Lx0.1 range, 0 to 400mA is supported. No modifications to the basic bridge are needed, as indicated last time. This addition uses the "mains" to derive a relatively clean DC source, and a constant current device to place a constant current across the inductor test. Thus, mains and J2/J3 are the only connection points to the part 2 instrument.

The Circuit:


The circuit is a very simple RCRCRC filtered supply and a current source. The filtered supply provides 174 bolts at no load, and drops to 106 volts at 400mA. Ripple must be relatively low. In this supply, ripple is about 2 mV at 400mA load, so it is sufficiently filtered. Available voltage ranges from 154 volts at 100mA to 136 volts at 200mA to 106 volts at 400mA.

The current source consists of a single IRF730 with a degeneration resistor in series with the source. About 14 volts maximum is placed on the gate, and with 4 volts gate-to-source voltage, a 100 ohm resistor in the source will develop 100mA. That resistor is range switched . 100 ohms is used for the Lx10 range, allowing 100mA maximum on that range, but an additional 100 ohm resistor is paralleled for the Lx1 range to allow 200mA, and an additional 50 ohms is paralleled on the Lx0.1 range for higher current.

There are a number of limit conditions that have to be accounted for though. The current source needs about 15 volts across it to operate OK. The basic bridge has 1k in the Lx10 range (so at 100mA, 100 volts is dropped across that 1k), 100 ohms in the Lx1 range (so that at 200mA, 20 volts is dropped across that resistor), and the Lx0.1 range has a 10 ohm resistor (so that at 400mA, 4 volts is dropped across that resistor. Thus, it is evident that on the Lx10 range, 39 volts is available for the inductor, so the system will be useful at 100mA as long as the inductor resistance is less than 390 ohms. Similarly for the Lx1 range, 101 volts is available, so the system is useful at 200 mA as long as the inductor resistance is less than 500 ohms. Likewise, for the Lx0.1 range, 87 volts is available so 217 ohms is supported at 400mA.

I have placed a push button switch to add the DC current. This is a safety issue, so that you don't get zapped connecting or disconnecting the inductor with current flowing.

I've also added some transzorbs across the FET and a MOV across J2/J3. These parts are intended to absorb the transient associated with removing current across the inductor.


Operation is similar to the previous version. The only difference is to add the current, push the pushbutton, set the current to the desired test current, and then (while still holding the button), adjust the bridge for null as previously described. Also see the calibration section.

Parts List

As before, I have not included chassis, line cord and hardware items. I'll let these up to your imagination. Source part numbers were from recent catalogs (July 2001) and may have changed by the time you read this. In the table below, all part numbers refer to the Mouser catalog (

RefDesig Qty Value Mfgr Part# Est.Cost(USD) Notes
C1,2,3 1 1000uF 200V CDE 5985-250V1000 20.73
C4 1 100uF 25V Xicon 140-XAL25V100 00.30
D1,2,3,4 4 1N4007 Rectron 583-1N4007 00.16
D5 1 30V 0.5W zener Gen Semi 625-1N5256B 00.07
D6 1 14V 0.5W zener Gen Semi 625-1N5244B 00.07
D7,8 2 100V 1.5kW Gen Semi 625-1.5KE100A 01.08
Q1 1



Heat Sink







R1 1 10 ohm 10W Xicon 280-CR10-10 00.55
R2 1 39 ohm 10W Xicon 280-CR10-39 00.55
R3 1 47 ohm 10W Xicon 280-CR10-47 00.55
R4 1 10k 5W Vishay 71-CW5-10K 00.52
R5 1 3.3k 0.5W Xicon 30BJ500-3.3K 00.22
R6 1 25k POT Xicon 31VC403 01.20
R7,8 2 5.1k 0.5W Xicon 30BJ500-5.1K 00.44
R9,10,11,12 4 100 ohm 3W Vishay 71-CW2B-100 01.60
RV1 1 250V MOV Xicon 1492-250V20 00.54
S1 1 PUSHBUTTON NKK 633MB201101 05.10
T1 1 mains-isolation XFMR Magnetek 553-N68X 19.40

Totals: 55.79USD

Setup / Calibration

There are two ways of calibrating the current through the inductor.

1. You could place A DC meter in the path indicated in the schematic. This is the most accurate, but requires a second meter to be available. If you don't do this, be sure to short the connection between J1, J2 of this schematic as shown.

2. Place a current meter in place of the inductor in the Owen's bridge. Now, mark calibration points on the potentiometer's knob at appropriate points (5 mA, 10mA etc). Then, to use the instrument, just set the pot to the predetermined spot.


Inductance measured:
0 to 3.21Hy in 10 mH steps

200 ohms at 400mA DC

0 to 32.1Hy in 100mH steps

500 ohms at 200mA DC

0 to 321Hy in 1Hy steps

390 ohms at 100mA DC

An Example

Here's the results you can get. I measured a Hammond 193C (20Hy, 100mA filter choke) on the bridge. Since inductance is a function of the DC current and AC voltage across the inductor, I sued both the Lx1 and Lx10 ranges. The Lx1 places about 1VRMS AC across the inductor, and the Lx10 places about 10VAC across the inductor. Here's the Results

Inductance of Hammond 193C
DC Current (mA) Inductance at 1VRMS (Lx1 range) Inductance at 10VRMS (Lx10 range)
0 16.7 24
20 14.7 20
40 14.0 19
60 13.4 18
80 12.8 17
100 11.8 16
120 10.6 14

The same thing can, of course, be applied to transformers. It's revealing to see how much inductors and transformers change with both DC and signal level.

What's Next

In part 4, we'll add variable AC level and frequency. This allows you the ability of determining how the inductance changes with signal level or with signal frequency.