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Author Topic: curious! switcher vs linear power supply? what you use and why?  (Read 630 times)

G8FXC

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Re: curious! switcher vs linear power supply? what you use and why?
« Reply #15 on: November 14, 2022, 07:52:22 AM »

I also learned the value of a switcher with good crowbar protection the hard way. The linear supply had a pass transistor short delivering almost 36 volts to my Icom. Needless to say it was fried.

I now use battery in the shack and a switcher when portable. Similar set up as NA4IT.

You have my sympathy! I would not say that it is a common problem but, when linear supplies go bad, they usually do push a lot of volts into the load. Switchers are more likely to "fail safe" anyway, and the better ones come with crowbar fitted...

Martin (G8FXC)
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K9AO

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Re: curious! switcher vs linear power supply? what you use and why?
« Reply #16 on: November 14, 2022, 08:34:12 AM »

A crowbar circuit is just going to fire on overvoltage and blow the fuse. FOr that to work it has to be after the protection fuse, so any fuse in the power cord to the radio externally from the power supply is not going to fulfil the requirement. There needs to be a fuse internal to the PS. Generally this is the case.

However this won't protect against the case where the radio fails. What you want is full foldback current limiting. Ordinary current limiting will simply limit the current to a set max value by decreasing the PS output voltage to not exceed that value. Meanwhile the radio fries away and whatever failed keeps drawing current. With full foldback current limiting the overcurrent event causes the PS to go into extreme current limiting, practically zero in fact. This shuts down the power to the radio after the event and does not let the failed device keep drawing excess current.

Of course if this is a 50 amp PS and your radio only needs 20 amps then the PS protection circuit isn't going to fault at values reasonable over the 20 amps draw of the radio. So in that case a PS better matched to the needs of the radio is safer.
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G8FXC

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Re: curious! switcher vs linear power supply? what you use and why?
« Reply #17 on: November 14, 2022, 10:43:23 AM »

There are many possible failure modes - we have to protect against the most probable and most expensive! The Astron SMPSUs are amongst the best specified - according to the sales brochure, they have

  • Fold Back Current Limiting
  • Crowbar Over Voltage Protection
  • Internal Fuse Protection
  • Over Temperature Shutdown

which should cover most bases...

What I was trying to do as best as I could was to insure against expensive failures that will not be covered by warranty... In the case I described, a £3,000 radio that was just a few weeks old was destroyed by a PSU failure and Yaesu washed their hands of it. If an internal fault in the radio makes it draw 50A and melt itself, the warranty will cover that!

Martin (G8FXC)
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AK4YA

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Re: curious! switcher vs linear power supply? what you use and why?
« Reply #18 on: November 17, 2022, 10:14:59 PM »

can most any switching power supply noise not be solved with enough toroids and ferrites?
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KD6VXI

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Re: curious! switcher vs linear power supply? what you use and why?
« Reply #19 on: November 18, 2022, 03:28:46 AM »

can most any switching power supply noise not be solved with enough toroids and ferrites?

I use both.

Currently I'm using a 30A Jetstream switcher.  I haven't really noticed any noise on it.  That is connected to the shack radios.

On my test bench I have a pair of 60A Tripp-Lite power supplies married together (1 control board controlling all pass transistors) for 120A.  I also have a homebrew 600A benchtop supply (iron based, weighs in the hundreds of pounds).

Whatever I have on hand.  I don't buy junk, so it doesn't matter which kind of power supply I use.

I do notice in 30 years of using switchers in PCs, ham shacks, helping CBers with their stations, etc. that switchers have a lot (I mean a LOT) more breakdowns.  And when they break, it's better to usually trash them.

I also use an external crowbar via back to back zeners so when they fail, if they fail with 120 on the output, they blow a fuse instead of blowing up my unobtanium 2879 finals.


--Shane
WP2ASS / ex KD6VXI
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