Bowmar 901D / TEN Power Supply

The Bowmar 901D uses a very sophisticated power supply circuit because the main calculator chip TMS 0107 requires negative voltage power rails. Pin 15 of the chip requires a negative 9 V whilst pin 28 positive 9 V. A dc-dc converter circuit consisting of a transformer and an oscillator generates the voltage rails. Toroidal transformers are usually more efficient and their size significantly smaller than a conventional transformer, hence making it ideal for this application. The colour-coded transistors operate in matched pairs for the oscillator circuit.

DC-DC Converter

Back in the 1970s, computers were the size of fridges, and their power supplies occupied an even bigger space, hence it was normal in those days to use negative voltage rails. However, this was a pocket-sized calculator; hence, they had to find a novel way of producing the necessary voltages.

An oscillator circuit consisting of a matched pair of transistors drives the secondary windings of the transformer.

A pair of diodes D connected to the secondary windings in reverse bias mode allows the negative voltage through, from the negative half of the power cycle.

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