Didier
Your comments regarding SMPS are very valid, but there are SMPS designs and
then some !
I bought a Wavetek 395 Function Generator with a totally blown up SMPS for
approx $100. I attempted to repair this unit but the PCB was badly
damaged/burned and one 8-pin Dil device was blown to bits - plus burnt out
resistors. I tried to obtain a replacement from the manufacturers without
success. After hooking up external supplies and proving that it was
otherwise OK, I contemplated building a linear supply within the box - it
would have been heavy and difficult to fit. This latter point reinforces
your comment about size and power dissipation. The original unit was
identical to the average small PC PSU, but with very different outputs.
After about 10 months of looking for a solution - I thought I recognized a
Wavetek 395 Function Generator in Bob Mokia's Lab. photo, and ask him if he
could find me a spare PSU, after a short while he came back with a
replacement for around $50 - presto I now have a fully working 395 for $100,
which I notice sells for $1500 in the US.
Footnote:
No axe to grind, but like others in the Group,I have found Bob Mokia to be a
"straight" and helpful dealer.
Roy
----- Original Message -----
From: "Didier Juges" <[email protected]>
To: "'Discussion of precise time and frequency measurement'"
<[email protected]>
Sent: Tuesday, June 02, 2009 3:24 AM
Subject: Re: [time-nuts] Thunderbolt - any negatives ?
Most likely failures on power supplies are with the power components.
Failure of the pass transistor in a linear supply is likely to result in
overvoltage at the output, while failure of the switch on a switchmode
supply will blow the fuse instantly.
It is been my experience (after 30 years in the field) that a properly
designed switchmode supply is at least as reliable as a linear supply of the
same output power, if for no other reason than the lower dissipation and
resulting reduced failure rate.
By using integrated controllers with lots of protection features built-in,
switchmode supplies tend to be smarter than linear ones, and their failures
tend to cause fewer damage to other circuits.
Of course, your mileage may vary...
Didier KO4BB
-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of Robert Atkinson
Sent: Monday, June 01, 2009 2:46 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Thunderbolt - any negatives ?
Most switch mode power supplies actually run a voltage
doubler on the input when running on 110V. This puts over
300V across the transformer and switch. Also the regulation
loop crosses the isolation barrier introducing more failure
points that can result in overvoltage.
Robert G8RPI.
--- On Mon, 1/6/09, [email protected] <[email protected]> wrote:
> From: [email protected] <[email protected]>
> Subject: Re: [time-nuts] Thunderbolt - any negatives ?
> To: [email protected]
> Date: Monday, 1 June, 2009, 6:17 PM
> Hi there,
>
> A switcher has much more stresses on the components, since
it usually
> switches the primary side rectified 110/220V high-voltage across a
> transformer.
> Thus the switching FET has to be very high voltage capable (about
> ~170V DC in the US), and the second component under stress is the
> primary high voltage capacitor, because it sees a very fast AC
> switching current on it (current draw is on when the FET is on, and
> off when the Fet is off). Also there has to be a fast
snubber network
> to prevent the back-emf from destroying the primary Fet with
> over-voltage.
>
> A linear supply has none of these fast current/voltage
transients on
> it, only a couple of diodes switching the 60Hz secondary onto a
> capacitor at low voltage.
>
> A secondary concern is thermally induced stress, switchers will
> usually be packed into a very small enclosure with very high power
> capability/density.
> This is not possible for linear supplies, since the
transformer size
> will usually determine overall sizing. Compare a Laptop
power supply
> size (usually these have between 40W and 90W rating!) to a similar
> rated linear supply.
>
> bye,
> Said
>
>
> In a message dated 6/1/2009 09:48:29 Pacific Daylight Time,
> [email protected]
> writes:
>
> Is there something I don't understand in this area? What makes a
> linear supply more reliable than a switcher?
>
> My first guess would be a switcher would be more reliable
because it
> would run cooler.
>
> That's probably assuming the same amount of design effort which is
> probably not a valid assumption if I'm comparing a brand-X linear
> with a brand-Z switcher. A quick glance at the general
construction
> might give a better answer.
>
>
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