[Amps] MFJ products

[Randall, Randy]

Some linear Astrons came with over voltage protection built in.  I can't remember what models that did though.  Get a schematic for your particular model and pay attention to the production dates as there were various mods over the years.  If it has a SCR in the circuit, it likely has an over voltage crowbar circuit already installed.  You likely do not need a MFJ-4403 if running a rig on a properly sized and operating Astron supply especially if it has the over voltage protection.

Randy AB9GO

-----Original Message-----
From: Amps [mailto:amps-bounces at contesting.com] On Behalf Of Paul Kraemer
Sent: Wednesday, December 06, 2017 5:58 PM
To: Manfred Mornhinweg <manfred at ludens.cl>; amps at contesting.com
Subject: Re: [Amps] MFJ products

Does anyone have comments pertinent to using the MFJ-4403 power conditioner following either linear or switching supplies?
These claim to protect against overvoltage?
A good idea or just more stuff to fail?
I use only Astron linear supplies and never had a problem but who knows?
Paul K0UYA

-----Original Message-----
From: Manfred Mornhinweg
Sent: Wednesday, December 06, 2017 2:55 PM
To: amps at contesting.com
Subject: Re: [Amps] MFJ products

Jim, Don,

> It seems to me that this is a function of the design and construction
> of the regulator, including choice of components, not the method of
> AC/DC conversion.

The quality of design and construction of course is the most important factor in determining the likelihood of failure of a power supply, much more important than whether it's switching or linear. And I can't really tell which type fails more often, because in 37 years designing and building both types of power supplies, and repairing lots of them, I can only say that both types fail a lot. Even nicely built professional linear power supplies (such as US-made Kepcos) have landed on my repair bench.

But I would like to insist on my point, because it's factual, supported both by practical experience and by logic: In the event of a failure, switching power supplies almost always go DOWN in voltage (often to zero, sometimes to a lower-than-normal voltage, and often they start pulsing at low voltage), while linearly regulated power supplies very often fail with the voltage going UP. Of all failures I have seen in linear supplies, a good 30% has been with the voltage going up.

The technical fundamentation is simple: A linear power supply has a transformer, rectifier and filter, producing a voltage that's much higher than the nominal output voltage, followed by a regulator circuit with a series pass element. If this pass element fails shorted, the output voltage goes up to the full unregulated voltage. And the pass element works under high stress, because it has to dissipate a lot of power. So it fails easily.

Instead a switching power supply has a small, high frequency transformer, fed by a high frequency (25kHz to 1MHz) generated by power switching elements. The secondary voltage on that transformer is rectified and filtered, then delivered to the output without further regulation. There is no pass element, eliminating the main failure mode of linear power supplies that results in high output voltage. Regulation is done by adjusting the duty cycle of the switching elements. If the power devices of a switching power supply fail, the output will go to zero, no matter in what way they fail (shorted or open). That's an obvious safety advantage over linear supplies.

In both types a failure of the control circuitry can cause a rise in output voltage. But since the control circuitry works at very low power, it's not nearly as failure-prone as the power devices.

The bottom line is that switching power supplies are far less likely to expose a radio to overvoltage, although it's likely that their overall failure rate is higher than that of linear power supplies, owing to their higher complexity. Where there are more parts, there is more that can go wrong.

A high power linear power supply should always be fitted with a crowbar-type overvoltage protection. A switching supply can also be fitted with one, but it's less necessary.

Manfred

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