PDP-8 h724 Power Supply Information

The PDP-8/E computer used the h724 linear power supply. This machine was the last that used a linear power supply. The later 8/M and 8/F machines used the h740 switching power supply. Linear supplies were used in older machines since the electronics for them was simple. When the cost of semiconductors dropped and performance improved then the switching supplies became more cost effective. The advantage of switching supplies are that they are more efficient, smaller and lighter than linear supplies when large amounts of power is required.

In linear supplies the transformer runs off the input AC line to step down the voltage to the levels somewhat higher than needed by the computer. The lower voltage AC is then converted to DC and run through a linear regulator to generate the controlled output voltage. The linear regulator dissipates the difference between the transformer output voltage and the desired voltage as heat. The input transformer is a large part of the 45 pound weight of the h724 power supply and the power transistors take a large amount of space.

In the h740 switching power supplies the internal voltage is stepped down to the final output voltage by switching the current on and off into an output inductor at about 10 kHz. This process is more efficient so less power (heat) is dissipated in the power supply. Also less internal voltages are needed since the switching output regulator can run off a wide range of voltages. Both of these reasons allows the input transformer to be smaller than in the linear supply.

In modern switching supplies the AC is frequently rectified to a high voltage DC and then stepped down to the output voltage using a small inductor or transformer running at a high frequency (10 kHz to 200 kHz). This gets rid of the large input transformer entirely.

The first h740 power supplies had problems but later ones were better. All modern computers use switching supplies for the power supply.

The h724 power supply does not provide enough +5VDC power to have a fully loaded system with M series cards. The original design was assuming most of the slots would be filled with memory.
h724 Specifications
Voltage Current Provided  Fuse Rating  Usage
+5VDC 20A 25A Logic power supply
+8VDC (unregulated) 2A 2.5A Front panel lights
+15VDC 1A 1.5A Core memory
-15VDC 8A 10A Core memory
14VAC (unregulated)  Unknown .5A Line frequency real time clock 
Unknown Unknown .125A Internal power relay drive

Links to h274 pictures

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