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Testing a Selenium Rectifier

Posted by Peter Hiscocks on 10/25/2016

Before the invention of the silicon diode, designers had the choice of germanium point contact diodes for low currents, copper-oxide rectifiers in instruments, and selenium rectifiers for power applications. The selenium rectifier replaced the vacuum tube rectifier in the period 1950 onward and was in use until the 70's. Selenium rectifiers are recognizable by the cooling fins, which are often painted green or orange.

We were given a three-phase selenium rectifier, taken from an ancient Honda 150 motorcycle. The alternator is wired to produce 3 sine wave outputs, each 120 degrees apart. The rectifier consists of three diodes with one common terminal, so that the output is a reasonable approximation of direct current.

The owner of the bike had replaced the selenium rectifier with a modern three-phase silicon diode array, a much smaller and probably more efficient device.  Was the selenium rectifier still good?  We measured the forward and reverse resistance with a multimeter, and the results were inconclusive.

The Wikipedia page on Selenium Rectifier mentions that a 'forming current' may be required after a long period of disuse, and long period of disuse applies in this case: it had been out of service 40 years or more.  The forming current is a minimum current to regenerate the proper operation. So we connected it to a CTR-101 curve tracer to run the forward characteristic curve. 

Unlike a multimeter test, the curve tracer exercises the device under test over a range of voltage and currents, so it's a much more informative measurement.

As you can see from the graphs, the forward characteristic is a classic diode curve with a threshold about 0.2 volts, increasing to 0.8 volts at 1 amp forward current.  Switching on the measurement cursors, the forward resistance over the linear region is 0.37 ohms.

The reverse characteristic shows a reverse current of 800uA or so up to a reverse voltage of 30 volts, also very respectable.

This is the measurement of one of the three diodes: the other two have a similar characteristic.

So yes, the selenium rectifier is still functional. And the Honda 150 engine also runs just fine, and is being used to drive a home-constructed sawmill.

Yes, we test IGBTs

Posted by Peter Hiscocks on 10/6/2016 to Software
We were asked the other day whether our curve tracer CTR-101 can test IGBT devices.

What's an Insulated Gate Bipolar Tranistor (IGBT)? If you live in a world of small analog signals (that's us), then you probably haven't used an IGBT. But if you work in high power applications such as variable speed drives for induction motors or welding controllers, you've probably used them.

The IGBT is one of those hybrid devices that is 'the best of both worlds'. It combines the high input impedance of the MOSFET with the low saturation voltage of the bipolar transistor.  The IGBT is used almost exclusively as a switching device, so in the ON state, the power dissipation is proportional to the saturation voltage.  Smaller saturation voltage results in less power dissipation and simplified cooling requirements.

We ordered some of the IGBT model IRG4PF50 to test on the curve tracer. The specifications are impressive: 900 volt breakdown voltage, 51 amps current, 200 watts, all for $6 from Digikey.  Even allowing for the limitations of heatsinks and thermal resistance (which always make the actual power less than shown on the spec sheet), this is an impressive device.

The CTR-101 can measure up to 30 volts or so at a test current of 1 ampere: it's intended for small signal devices.  But we can test the behaviour in the small signal region.  This would be useful, for example, if you needed to match one or more units.

The figure shows the results: on the N-MOSFET setting of the curve tracer, we get a family of curves similar to an enhancement mode MOSFET.  There is certainly enough information here to determine the gate threshold voltage and match device characteristics.  So yes, the CTR-101 can measure the characteristics of an IGBT.

A transfer characteristic (drain current vs gate-source voltage) might also  e useful: if you have some interest in that, give us a shout and we'll add that feature to the software.

Peter Hiscocks
October 2016



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