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Internal Metering 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.

If you have a meterless Astron and have the front panel space for a meter or two I highly recommend adding at least a voltmeter if not a current meter as well. Acquiring matching voltmeters and ammeters is easy. A single meter can be used when wired with a switch to select current or voltage. For the wiring just refer to the metered version of your supply or of a similar model. Personally, I tend to use analog meters as they are much easier to install and connect, they are quite adequate for the job (usually high precision is not needed) and (depending on the meter) some digital meters can't measure their own power source.

Both the current and voltage meters in a stock metered Astron supply use a single turn open frame potentiometer that can get dirty or contaminated and that can cause inaccuracy over time. A replacement totally enclosed 10 turn precision pot costs pennies.

The factory Astron voltmeter is a low current meter that is calibrated in volts and uses a series variable resistor (potentiometer) to limit the current. Some of the earlier Astron metered supplies did not have an adjustment (they had a fixed resistor) which means that one needs to replace the fixed resistor with a pot in order to calibrate the meter.

Depending on age of the power supply the Astron ammeter uses either a 500 ohm or 1k ohm pot for calibration.

Tony King W4ZT bought a used astron that had both meters "stuck". He replaced both with digital meters and created a web page about installing digital meters. I was sent a PDF of it, which can be found here. Later I found the original web page mentioned above.
The digital ammeter he bought required an isolated power source, and the article includes a schematic and photo of the isolated 5vDC source that he built on perfboard.
There is another Tony King article on a power supply test load further down on this page.
By the way, digital ammeters that do not require the isolated supply can be found.

From another email to repeater-builder on power supply metering:

The typical Astron power supply ammeter is NOT a load current meter in the classic sense - it is calibrated in amps but is wired as a voltmeter, and actually measures the voltage drop across one of pass transistor emitter ballast resistors (also called an emitter swamping resistor, or a load balancing resistor). This technique only works properly if all of the pass transistors are absolutely identical in gain (not just the same part number) AND all of the emitter ballast resistors are exactly the same resistance (not just the same marked value). Then and only then is the voltage drop across the one resistor directly proportional to the entire load. Neither the pass transistors nor the emitter resistors are that closely matched, so the resulting displayed current value is only approximate.

Another problem is that the mechanical grip of the pins power transistor sockets (the base and emitter pins) will fatigue with age and the constant heating and cooling and that will result in eventually loosening of the pins. This WILL cause an intermittent connection or a higher resistance connection. Since the current meter is dependent on current flow through the emitter pin of one pass transistor any change in resistance (like with temperature) will result in misleading current readings. There have been cases where the supply has been delivering about 10 amps and the current meter is displaying about 2 amps just becasue the pass transistor that drives the current meter had a high reisitance on the socket pins.

That said, who really needs that accuracy in a low-priced test bench supply? In many cases all you are doing is looking for a reasonable value, for a peak or a dip and the internal Astron metering is fine for that. The Astron method of reading the voltage drop across the ballast resistor that carries 1/2, 1/4, or 1/8 of the total current is "good enough" for any rough measurement. If you need better, you are probably not using the internal metering in an Astron.

You have three options:

• Use an external current meter in series with the supply output.
• Buy (or make) a proper meter shunt and position it in series with the total power supply output, and rewire the Astron meter (it's a one milliamp full scale meter) across it, then adjust the calibration pot to set the meter to the correct value, or...
• Get a meterless Astron and cut holes, then mount digital meters in the front panel plus mount a properly rated current shunt and wire it in series with the output connection as mentioned above. You can get decent digital panel meters (DPMs) for under US$20 from several sources.

More on power supply metering, from an email to repeater-builder from Robert Meister WA1MIK (SK):

There are plenty of 3-1/2 digit LED and LCD meters that can be purchased from electronics and surplus businesses. For example, Marlin P. Jones & Associates has several in the US$9-$12 range. Make sure that the meter you buy will work with a common ground for its power input and meter input. Not all meters can do that. In particular, many of those that require a 9V supply often state they can't measure their own power supply. Get one that operates on 5V and add a LM78M05, LM7805 (or similar) dedicated regulator to run the meter(s) off the Astron's internal unregulated DC supply across the main filter capacitor. The 5 volt regulator can be mounted almost anywhere.

You may also be content with a small analog meter. There are plenty of 0-15vDC meters for under US$10 that will do quite nicely. You can also use just about any meter you have and add an appropriate resistor in series to give you the scale you want; Astron themselves uses 1mA DC meters in their supplies for both voltage and current. See any of the power supply "M" schematics for the details. Or use a 0-5v meter and "stack" it on top of a 10vDC reference, then calibrate it as 10v at the bottom end and 15v at the top.

For test purposes I disconnected one end of both of the two meters on an RS-35M and connected a Fluke digital multimeter, a 10k resistor, and a 10v power supply all in series. Both meters went to just about full scale, and the DMM read 995uA, so these are definitely 1mA DC full scale meters.

There's about 15k ohms in series with the voltmeter. This is accomplished with a small trimpot soldered in series with one terminal on the back of the meter. I suspect the full-scale resistance is in the 20-25k ohm range.

There's about 360 ohms in series with the ammeter on my supply. This is also done with a small pot soldered in series with one terminal on the back of the meter. I suspect the resistance is in the 500-1000 ohm range, but it will definitely vary depending on the current rating of the power supply.

I calibrated my supply by setting the voltage to 14.00 on an external digital meter and adjusting the Astron's voltmeter to 14 volts. I then hooked a pair of 1.0 ohm 250 Watt resistors in parallel across the output terminals, and adjusted the Astron's ammeter for 28 amps. I had to work fast, those resistors get real hot in a hurry (with almost 400 watts being dissipated).

Contact Information:

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

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This page originally created in August 2000 by Kevin Custer W3KKC.
Totally rewritten 14-Oct-2004 and a lot of material added by Mike Morris WA6ILQ.
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.