Adding an Inrush Current Reducer to an Astron Linear Power Supply

Those of you who own large 500 watt and higher Astron linear power supplies, and even large HF amplifiers, know all too well that `GNNNNnng` sound that they often make when turned on. This is due to the high inrush (surge) current flowing into the transformer when the filter capacitor is discharged. Depending on the size of the supply, this surg
Adding an Inrush Current Reducer to an Astron Linear Power Supply - schematic

e can exceed the rating of the circuit breaker on the outlet the supply is plugged into, but as it`s so brief, the fuse on the back of the supply rarely blows during these surges. There are several ways to prevent this: Leave the supply on all the time. Even if it`s left on all the time, there will still be momentary power outages and interruptions, so this really isn`t a true solution. Purchase a commercial inrush current limiting device for about $80US. Here`s one that plugs into the wall, has its own on/off switch, and provides an inrush current limited outlet for any load. Unfortunately you must use the power switch in the external box and leave the supply`s conveniently located power switch turned on all the time. You can make a similar device yourself with a relay and a resistor and you can install it between the power switch and the transformer inside your unit. Add a thermistor in series with the input of the supply. This is a small negative temperature coefficient device that gets wired in series with the transformer. It has a high initial resistance (high compared to the resistance the transformer presents when power is first applied) but as current is drawn and the device gets hot, the resistance decreases (hence the negative temperature coefficient). It regulates itself, providing a current limiting resistance when cold and almost no resistance when hot. It does this with no moving parts, unlike units that use a...

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