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  Adding an Inrush Current Reducer
to an Astron Linear Power Supply

By Robert W. Meister WA1MIK
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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 surge 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:

  1. 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.
  2. 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.
  3. 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 relay. It can be installed inside or outside the power supply and doesn't require any other components. All switching power supplies have a thermistor in series with either the incoming AC line or the output of the bridge rectifier before the filter capacitors. This site has a lot of good information about thermistors, when to use them, how to select them, etc.

I opted for choice #3 above. I purchased some 2.5 ohm 11 amp devices from Mouser, part number 871-B57364S259M, for about $2US each. Digikey has them as well, part number 495-3063-ND. I had also purchased some five-lug terminal strips for another project, Mouser part number 158-1005, for about $1US each, and used one of these in my Astron RS-35M power supply. A few inches of #16 stranded hookup wire completed the job. I modified an older supply, one that has the line cord permanently attached. You may have to alter the procedure a bit if yours is newer and has the IEC input connector. You may also have to choose a slightly different thermistor depending on the current rating of your power supply.

The original Astron linear power supply primary (AC input) wiring is shown in the diagram below. The thermistor should be installed between the fuse and the power switch.


Here's a step-by-step procedure for adding the thermistor inside the supply.

  1. Disconnect and unplug the supply.
  2. Remove the cover screws and slide the cover off the supply.
  3. Compare your AC wiring to the diagram above and make notes or a drawing if it differs significantly. The terminals on the switch are actually numbered along the side, from 1 to 3, bottom to top.
  4. Remove the fuse from the fuse holder. You don't want to apply heat to the fuse holder's terminals with a fuse inside.
  5. Unsolder the black wires going to the fuse holder.
  6. Remove the fuse holder from the rear panel.
  7. Unsolder the green and white AC power cord wires going to the small two-lug terminal strip near the AC input. If your supply has the IEC input connector, you may have to unsolder the wires coming from it.
  8. Remove the remaining wires from the terminal strip. On my supply, one went to the top terminal (3) of the front panel power switch (to light the internal lamp) while the other went to the transformer.
  9. Remove the AC power cord strain relief from the rear panel and remove the cord.
  10. Remove the (now empty) terminal strip but save the mounting hardware; it will be used to install the new terminal strip.

I'll identify the lugs and holes of the terminal strip by numbers: 1 through 5, going from left to right, with the mounting hole facing away from you. The lugs stick up above the phenolic strip; the holes are in the middle of the phenolic strip. The terminal strip will be mounted in the existing hole in the chassis with lug 1 toward the rear of the power supply. Now proceed with the reassembly.

  1. Attach the thermistor to lugs 1 and 2 of the terminal strip. Leave the leads about 1 inch long and position the thermistor upright and away from anything else.
  2. Attach the wire from the middle terminal of the front panel switch that used to go to the fuse holder, to lug 1 of the terminal strip and solder this lug.
  3. Attach a two-inch piece of insulated stranded wire to lug 2 of the terminal strip and solder this lug.
  4. Thread the AC power cord through its hole in the rear panel and attach its three wires to the holes in the terminal strip: green to hole 3, black to hole 4, and white to hole 5. Solder all three of these holes. Note that the power cord wires are soldered into the holes in the terminal strip, NOT the lugs that stand above it.
  5. Attach the two black wires that used to go to the original terminal strip (the top terminal (3) of the front panel switch, and one wire from the power transformer), to lug 5 of the new terminal strip and solder this lug.
  6. Attach a two-inch piece of insulated stranded wire to lug 4 of the terminal strip and solder this lug. At this point, lugs 1, 2, 4, and 5 should be soldered, and holes 3, 4, and 5 should be soldered. Lug 3 and holes 1 and 2 should be empty and unused.
  7. Mount the terminal strip with the hardware that held the original strip to the chassis.
  8. Reinstall the AC power cable strain relief if present.
  9. Reinstall the fuse holder.
  10. Solder the wire coming from lug 2 of the terminal strip to the rear-most lug on the fuse holder.
  11. Solder the wire coming from lug 4 of the terminal strip to the end lug on the fuse holder.
  12. Reinstall the fuse.
  13. Reinstall the cover.

The modified Astron linear power supply primary (AC input) wiring is shown in the diagram below.


Here's a photo from the left side looking at all the AC wiring. The terminal strip lugs are numbered.


Here's a photo looking down into the AC wiring. The thermistor has been identified. You can't see too much of the terminal strip in this shot.


My brother read this article and suggested that I could have just lifted one wire off the fuse holder and installed the thermistor in series with it, but that would have left the thermistor supported by just one lead, and I didn't think that was acceptable. Since I had the terminal strip, I unwired everything and built it back up fresh. Alter the procedure if circumstances warrant it.

Inspect your work. Make sure the wires all run nicely and won't touch the cabinet when you reassemble the power supply. Position the thermistor away from everything, as it will get hot. Use an ohmmeter and verify the following conditions:

  1. One side of the line plug to ground: infinity with the power switch on and off.
  2. The other side of the line plug to ground: infinity with the power switch on or off.
  3. One side of the line to the other side: infinity with the power switch off.
  4. One side of the line to the other side: 2.5 ohms with the power switch on.

I also modified an Astron LSRM-25A (28V, 25A) supply, which also had a permanently attached power cord. The AC input was wired a bit differently: the varistor was installed at the fuse holder rather than at the transformer, but there was still the same two-lug terminal strip mounted near the line cord to deal with the ground and white power wires. I didn't have to modify the AC input wiring since there was plenty of room inside the unit. I installed an additional terminal strip (although I only used two lugs on it), cut the black wire between the fuse and the power switch, and installed the thermistor in series. This photo shows the original AC input area. The AC line cord connects to the rear of the fuse holder and the terminal strip below it.


Here's the schematic of this supply. The wire going from the fuse holder to the power switch's middle terminal (2) got cut where indicated (red color on the wire) and the thermistor was inserted where the wire had been cut. Note that this schematic is for the LSRM-25A (28V supply), which differs from the 14V supplies.


Here's a photo of the completed modification. The new terminal strip utilized the same hardware and hole as the existing strip. I soldered the wires to two lugs of the new strip and used the respective holes for the thermistor. I positioned it over the ventilation slots in the bottom of the cabinet since this device can get hot.


I modified a newer RS35M that had an IEC AC input power connector on the rear. There was no terminal strip present but the wiring was much easier to deal with. The LINE, NEUTRAL, and EARTH terminals are well marked and you only need to deal with the LINE terminal, which goes directly to the rear contact of the fuse holder. The side contact of the fuse holder goes to the power switch then to the transformer primary. Here's what the IEC connector looks like. Note the "L", "N", and "E" markings.


Here's the schematic diagram of the stock power supply with the IEC connector:


The thermistor gets installed in series with the wire from fuse holder to the power switch, AFTER the fuse holder. Similar to the above procedure, I unsoldered the wire at the fuse holder, installed a terminal strip using the ground screw as a mounting point, added a new wire from the fuse holder to one lug of the terminal strip, connected the wire previously unsoldered to a second lug of the terminal strip, and installed the thermistor between the same two lugs. Modifying one of these IEC-equipped supplies is much easier than one with a hard-wired AC cord. Here's what the supply looks like when completed:


Here's the schematic diagram of the modified power supply with the IEC connector:


While I've got the supply open, I add a bleeder resistor across the main filter capacitor, a 3900 ohm, 5 watt metal film component, Mouser part number 286-3.9K-RC. I just solder it directly to the pads or holes at the outer edges of the regulator circuit board and let it sit about 1/16 inch above the board. It's pointed out in the photo below.


I recently acquired a relatively new VS20ML supply. This has an IEC-style power input receptacle with a built-in 5x20mm fuse. The rear of this receptacle has the usual 1/4 inch quick-disconnect tabs, and the wires run directly to the power switch. You may need to cut the wire going from the Line terminal to the switch terminal #2, or cut the push-on terminal off the wire and add one to a short piece of wire to run to the terminal strip that will hold the thermistor. In other words, get creative. You probably won't need to add the thermistor to a small supply like this one, but Astron could certainly use this type of input receptacle on larger supplies.

The author can be contacted at: his-callsign [ at ] comcast [ dot ] net.

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This page originally posted on 11-Jul-2011

Diagrams, photos, layout, and hand-coded HTML © Copyright 2011 and date of last update by Robert W. Meister.

This web page, this web site, the information presented in and on its pages and in these modifications and conversions is © Copyrighted 1995 and (date of last update) by Kevin Custer W3KKC and multiple originating authors. All Rights Reserved, including that of paper and web publication elsewhere.