Motorola TPN1154 and TPN1136 Desktop Power Supplies

Posted on Feb 6, 2014

The regulated output voltage varied as the unregulated DC voltage changed, and that changed as the load current or the incoming AC line voltage was varied. There was no easy fix to the existing circuit that would solve this problem. Eric WB6FLY had already run some load tests on these supplies and mine acted just like his did. The problem is that the unit

Motorola TPN1154 and TPN1136 Desktop Power Supplies
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is designed poorly. The unregulated DC voltage is used to provide power for the regulator circuit and that voltage decreases as the load current increases, so the regulation suffers. Varying the incoming AC line voltage also changes the unregulated DC voltage, so it too has just as much effect on the regulated output voltage. The graph below shows how bad things are. I did some investigation and noticed that the line regulation spec for the common LM7812 three-terminal regulator is in the order of tens of millivolts, far better than what the Motorola power supply was ever capable of producing. The downside is that these three-terminal regulators are only capable of about 1 amp of current, far less than the Motorola power supply was designed to do. Studying the data sheet, I think I figured out a way to use an LM7812 regulator in place of most of the original regulator circuitry. I tried to implement this as a drop-in replacement, where the original circuit board is removed and a new one with a few simple parts is installed. The original supply was non-adjustable; I decided to make mine adjustable, at least within about +/- 15% of the nominal 14VDC output. I did all of this for a parts cost of under $7US. Here`s the original schematic of the 14-amp power supply, taken from the Maxar-80 manual. The 8-amp supply has only one series regulator transistor (Q4) that feeds the output directly; it does not have R12 or R13, or Q5...

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