Nuts & Volts - June 2007

Probing Cores

version assumes that the op-amp’s output and supply currents change in step and that the driver stage has a well-defined gain. It uses modified PNP and NPN current mirrors.

Each power transistor (Q2 and Q4) has a large emitter resistor that drops over a volt at the current peaks. These transistors are driven by opposite polarity emitter followers so the base-emitter voltages of each pair roughly cancel. The drivers’ bases are connected to the amplifier’s power pins and to the power rails via 51 ohm resistors (R12 and R13).

The current gain of this combination is 23. Apart from about 3 mA of static current, the amplifier’s power currents equal the current flowing through its combined 100 ohm load. That is, a 100 mV amplifier input generates 23 mA at the driver output. SW1 selects a suitable drive voltage for each current range. C6 reduces noise and stray feedback and makes the current ramp much cleaner.

The op-amp’s static current causes the output transistors to pass nearly 100 mA when the output current is zero. I tried reducing this, but only introduced cross-over distortion. We must just live with about 1.6 wasted watts. The regulators’ and transistors’ peak dissipation is around 5W, but no heroic heatsinking is required.

The output transistors’ base currents are adjusted to balance out offsets. Short the inductor terminals and set the switch to its grounded input position. Measure the voltage across R18 with a multimeter on its 200 mV scale and adjust VR1 until the meter reads zero. The specified output transistors have current gains exceeding 150 at 500 mA. Many popular power transistors have lower gains and won’t work unless R14, R15, and VR1 are reduced to supply more base current.

■ FIGURE 5. The unboxed B-H tester board.

pliance, so bipolar 5V power rails might have sufficed. However, at a current peak the input to the current mirror is 1.3V less than the positive or negative power rail. If the main power were ±5V, the net supply to the op-amp would be about 8.5V. Since the TL082 is rated for 10V minimum, I boosted the power rails to ±8V to avoid risking odd non-linearities. This means hunting down 7808 and 7908 three-terminal regulators. By the way, I used 2,200 µF reservoir capacitors for space reasons, 4,700 µF ones would be better.

I/V Sensing

One end of the inductor under test is grounded. This makes measuring the voltage across it easy, but you need a floating current-sensing resistor (R18). A differential amplifier (U5a) drives the scope’s X input with a scale factor of 5V per amp. The inductor voltage goes to the integrating amplifier (U5b) whose output drives the scope’s Y input. As mentioned above, an adjustable amount of the X voltage is fed to the integrator input to compensate for the inductor’s winding resistance.

The control switch has an off position that shuts off the ramp generator. Ideally, it should be put in this position before removing the inductor under test. The output voltage swings from one power rail to the other if you don’t.

plastic or metal box at a later date. I’d already cut a 2. 5” by 4. 6” board, so I juggled things to fit. To simplify making changes, I used wire-wrap sockets for all the small components. This adds considerably to the board height and you might be more comfortable using PC sockets and sleeved bus wire.

The back edge of the board is fitted with a length of 0.5” by 0.75” aluminum angle. This supports the power input socket and the two BNC sockets that connect the unit to the oscilloscope. It also acts as a heatsink for the power regulators and the two output transistors. If you put the board in a case, this angle fits flush against one wall so use a metal box.

Power comes from a 9 VAC wall transformer. Mine is an old RadioShack one rated at 780 mA. These ratings are about the minimum acceptable for this application. If you use a 12 VAC or higher voltage transformer, you’ll need a bigger heatsink.