Paralleling Two Astron Linear Power Supplies

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Linear Power Supplies
By Robert W. Meister WA1MIK

I have noticed on the larger supplies, such as the 14 Volt RS70 and the 28 Volt LS35, there's a red binding post terminal on the rear that's labeled "FOR PARALLEL OPERATION ONLY". None of the schematic diagrams ever showed such a connection and it's been a mystery to me for quite some time. Well, the mystery is finally solved, thanks to a reader who needed help repairing an RS70M supply.

This terminal is only present on the highest-current, non-adjustable power supplies, those capable of 50 to 70 Amps at 14V, or 30 or more Amps at 28V. If you have a supply rated at a lower current and need more current capability, Astron would prefer you to buy a higher current supply and not parallel them. If you need more current than their single highest-current supply can put out, then you would need to parallel two of them. It's entirely possible you could parallel more than two but I wouldn't recommend that. Buy a supply meant to handle the desired current.

Here's a photo of the rear of an Astron RS-70A showing the binding post terminal. On some supplies this terminal may be located higher up, in the "expansion" area.

This terminal is wired directly to the LM723 regulator IC pin 10, which feeds regulated drive power to the TIP29 driver transistor on the regulator circuit board. There's an unused solder pad reserved just for that wire. See the photo below of a recent regulator board component side. Click on the image for a larger view.

I've marked up an RS70M schematic showing the connection for the parallel binding post. Click on the image for a larger view.

So, how do you physically connect two supplies in parallel? It's not difficult.

You must adjust the output voltage of each supply, using the internal pot, to be extremely close, as in within 10 millivolts DC; the closer the better. You probably don't want to rely on VS supplies with variable voltage adjustment pots on the front, as they're too easy to touch and change, and none of Astron's VS-series have the paralleling terminal. Whichever supply has the higher voltage will end up supplying most of the current, so getting them equal is key to balancing the load.
With both supplies shut off, use some rather thick stranded wire, such as #4, to connect the two positive output terminals together and connect the two negative output terminals together. Use good quality crimp lugs and I'd suggest soldering the lugs onto the wires after you crimp them. Make all wires as short as possible to minimize voltage drop. These are the green and red wires in the photo below.
Use a piece of #20 or #22 gauge stranded wire to connect the two red binding posts together. It is carrying just a few hundred milliamps so it doesn't need to be very large. This is the yellow wire in the photo below.
Connect your load to the positive output terminal of one supply and the negative output terminal of the other supply. This is to equalize the current draw from both supplies.
Turn on both power supplies.

Remember that each power supply is capable of drawing 12A from the AC line, so you probably want to run each supply from its own dedicated 15A or 20A circuit with nothing else plugged into either one.

Contact Information:

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

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This page originally composed on 10-Oct-2016

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