Was this the first LED test light?

This is the circuit board out of my first multi-voltage tester when I was an apprentice in the 1980's. It was quite an expensive unit at the time, but is designed for professional use.

The circuit seems simple enough with just a single sided circuit board and traditional through hole components, but it was an arduous thing to reverse engineer due to the very weird way the components are used to achieve a lot with the components of the era.

There are two distinct sections. The simple two-LED polarity indicator with PTC resistor to regulate the current thermally, and the main section that does devious things with standard transistors.

I think the PNP darlington is used to provide more current to the 12V LED, which has its own circuitry.

The 39V zener seems to be capping the LED-transistor supply to protect against overvoltage. The LED transistors themselves are being used in a non-standard way that possibly allows them to cascade progressive LEDs in series for maximum intensity at low current.

The LED supply is created by a fixed 120K resistor in parallel with a PTC thermistor that allows higher current to flow at lower voltages, but thermally regulates the current down at higher voltages.

In the future I'll reverse engineer a generic Chinese multi-voltage test light to see if they have copied the circuitry from what I believe to be an original Steinel design.


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