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Internal Grounding in the Astron Power Supplies Compiled and HTML'd by Mike Morris WA6ILQ Formerly Maintained by Robert Meister WA1MIK (SK) Currently Maintained by Mike Morris WA6ILQ

Note: This used to be part of a Grounding and Metering page.

The term "Grounding" in an Astron power supply can refer to two different parts of the circuitry:

• On the AC input: Is the third prong of the power plug – the safety ground, the bonding ground, the green wire (pick your term) – connected to the cabinet?
• On the DC output: Is the negative side connected (grounded) to the cabinet or isolated from the cabinet?

The author has worked on a number of Astrons over the last 30+ years for himself and for friends, and occasionally has found that the paint (or power coat in the newer supplies) was not scraped away and it prevented a good electrical connection from the green wire of the power cord to the cabinet.
This is a safety item and should be checked and repaired immediately and is easily checked with a multimeter. Just put it on the ohms scale and probe from the round prong of the power plug to the cabinet of the power supply.
If you do not see zero ohms then FIX IT. Your survivors will appreciate it.

Enough said.

Is the negative side of the power supply output connected (grounded) to the cabinet or isolated from the cabinet?

Neither the positive side nor the negative side of any power supply should not be connected to the cabinet.

Over the years of production of the Astron supplies some models had the negative side of the DC output connected to the cabinet, some models had it floating from the cabinet and on a few models the negative output had a resistor between the negative output and the cabinet.

A good example of this is to look at the RS-35 schematics from 1987 and compare them to those from 1991. Personally, any supply that is not owned by me and goes across my bench gets a P-Touch label stating either "Negative side is floating from the cabinet" or "Negative side is grounded to the cabinet". Some Astrons have the AC fuse holder labeled with the fuse rating (amps), others do not. Another P-Touch label goes over the fuse holder in the second cabinet, for example, "Fuse: 3AB 10 amps fast blow".

Astron hasn't been consistent on wiring the negative side - some models were grounded, some were floating, and some were in between - the Astron RS-50 / RM-50 schematic that is dated March 1996 shows a 3K resistor from negative to chassis. The RM-50 schematic that is dated January 2000 shows a hard ground. And just to be different, I've had an RM-50 on my bench that had the negative side floating, and I know the history of that supply; I can definitely state that is how it came from the factory.

Arguments can be made for both situations. For example, some situations where one is working on a variety of both negative and positive ground equipment would require a bench power supply with both positive and negative to be floating. There have been times where I have used two 12 volt Astron supplies in series to temporarily power a 24 volt piece of equipment on the repair banch. This configuration works, but is not advised. Note that if one is powered up and the other is not you can have reverse voltage across the unpowered unit. For safety, you should put a high current reverse biased diode across the output of each of the two supplies.

Another argument for a floating negative terminal is that it allows you to power equipment designed for positive ground... I've run across that in telephone company ("telco") and microwave equipment. My personally owned power supplies all have the negative side floating from the cabinet - even if they didn't come that way. All of the Astron supplies in current manufacture have floating negatives.

Eric Lemmon WB6FLY (SK) ran into a situation of a grounded negative many years ago and wrote about it on the repeater-builder mailing list:

It has come to my attention that Astron has a built-in design flaw that may cause problems for some repeater operators.

I discovered this when I temporarily replaced a suspect Pyramid power supply at my mountaintop 220 MHz repeater with an Astron SS-12 switching power supply. When I got back home, and in a very quiet environment, I was shocked to hear a very prominent 60 Hz hum on the 220 carrier. Since this switching power supply uses a switching frequency up in the 40 kHz range, I could not understand how there could be 60 Hz hum!

The very next day, I took a known-good DuraComm switching power supply with me and returned to the repeater site and exchanged the two units. This time, I moved some distance away from the repeater building and tested with a handheld to ensure that the carrier was hum-free, and it was. I could not detect the hum on the first trip because the electrical equipment next to the repeater is quite noisy.

Once I got the Astron power supply on the bench, the cause of the hum was obvious: The negative output terminal was grounded internally! Although most large Astron power supplies such as the RS-20, RS-35, SS-25, and SS-30 have a black jumper wire between the negative terminal and the cabinet, the SS-12 uses a trace on the PC board to make the connection. I then e-mailed Astron Tech Support and received a schematic of the unit, along with advice as to where the offending trace was located. A quick bit of work with an Xacto-style hobby knife cut the trace, and floated the negative output lead. Problem solved!

Astron seems to be the only power supply brand that routinely grounds the negative lead; none of my units made by DuraComm, Samlex, Astec, or Pyramid have this connection. The hum was caused by a ground loop injecting 60 Hz into the DC source feeding the repeater. Since the radio, duplexer, and antenna feedline is always solidly grounded for surge protection, that means that the DC power source is grounded in more than one place- a very bad idea. I have modified all my Astron power supplies- both linear and switching - to remove any internal connections to ground at the negative DC output terminal.

I had a similar problem several months ago on my 6 meter repeater, which had a recurring problem with controller lockup. After I swapped the Astron RS-35M power supply out and put in a DuraComm supply, the problems went away. As you might expect, the Astron RS-35M was causing a ground loop, but this time it didn't cause audible hum. It did, however, corrupt some of the data signals going to the controller.

I strongly suggest that owners of an Astron power supply make a simple test with a V-O-M. With the output connections open and the power supply unplugged, measure the resistance between the grounding prong of the AC plug and the negative DC output terminal. If the reading is in the megohms, fine. If it is a short you know what to do...

73, Eric Lemmon WB6FLY

This 700kB PDF article by Jim Brown K9YC supports the ground bonding issue.

The author and current page maintainer Mike Morris WA6ILQ can be contacted here.

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Copyright © 2004 and and date of last update by by Mike Morris WA6ILQ

This web page, the hand-coded HTML on it, 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.