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'[EE]:Transformerless PSU (+ question for repairmen'
2001\07\30@163245 by Jinx

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> > for energising a relay at its nominal voltage and keeping
> > it closed with a holding voltage, which can be 50% of the

>Thanks Jinx, but reading your page, came to me the idea I
> said about the 24V relay connected directly to the 110V mains,
> using a diode and a resistor to drop the current. I think I got a
> winner here, I'll do some tests today ;o)

With the opto-triac for on-off control not much to go wrong there.
My other thought would have been to have a cap supply for just
the relay so there are no glitches on the 5V supply. I can usually
find large (1-2uF) ones that I need in old SMPS or TVs

I'd like to know this though. Which has the lower MTBF - X2 cap
dropper or high watt resistor dropper ? I suspect the hi-W R for
heat reasons but that's just intuitive and probably has more to do
with PCB layout

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2001\07\31@061217 by Roman Black

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Jinx wrote:

> >Thanks Jinx, but reading your page, came to me the idea I
> > said about the 24V relay connected directly to the 110V mains,

> With the opto-triac for on-off control not much to go wrong there.
> My other thought would have been to have a cap supply for just
> the relay so there are no glitches on the 5V supply. I can usually
> find large (1-2uF) ones that I need in old SMPS or TVs
>
> I'd like to know this though. Which has the lower MTBF - X2 cap
> dropper or high watt resistor dropper ? I suspect the hi-W R for
> heat reasons but that's just intuitive and probably has more to do
> with PCB layout


Really, you could design either type for NO
failure. Depends on the size and cost requirements.
:o)

I'd go for a good capacitor, with 10% of the
drop done by a good qual series resistor running
no more than 15'C temperature rise. Reasonably
efficient, but gentle on the cap. Also use a snubber
across the mains with a good mains cap and a smaller
resistor. I've got units like this running for
15 years and never failed.
-Roman

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2001\07\31@064631 by Vasile Surducan

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On Tue, 31 Jul 2001, Roman Black wrote:

> resistor. I've got units like this running for
> 15 years and never failed.
> -Roman
>
 How old are you, Roman ? You have good advices...

Vasile

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2001\07\31@070629 by Jinx

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> > I'd like to know this though. Which has the lower MTBF

> Really, you could design either type for NO
> failure. Depends on the size and cost requirements.
> :o)

I think I may be biased towards Rs as they seem to look
cruddier after a few years than caps. But as you say (and
what I should have mentioned) is that you can over-spec
a part to minimise stress

> resistor. I've got units like this running for
> 15 years and never failed

Possibly an appliance as a whole was expected to live
a shorter time than the PSU so some components were
chosen for cost and simplicity. Say for example you have
a TV that craps out after 10 years. The manufacturer may
consider that a reasonable lifetime. Owner gets the fault
repaired and off goes the TV for few years more, and all
the while the power-handling components are going well
past their expected service life. Although perhaps that
was the case in the past - in these cheap import throwaway
days that might not be so

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2001\07\31@112428 by Roman Black

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Jinx wrote:

> > resistor. I've got units like this running for
> > 15 years and never failed
>
> Possibly an appliance as a whole was expected to live
> a shorter time than the PSU so some components were
> chosen for cost and simplicity. Say for example you have
> a TV that craps out after 10 years. The manufacturer may
> consider that a reasonable lifetime. Owner gets the fault
> repaired and off goes the TV for few years more, and all
> the while the power-handling components are going well
> past their expected service life. Although perhaps that
> was the case in the past - in these cheap import throwaway
> days that might not be so

So is Mr. Jinx planning a cheapy product? ;o)
I would pull apart a few cheapy mains micro
products, like electronic timers etc, look at how
they do the Xc supply and make yours just that
bit better. Should be good training for
certification needs of the product too. Might
even lead you to the right brands for the caps
and semis too for cheap pricing.
-Roman

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2001\07\31@135425 by Mike Kendall

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My experience as a repair technician with over 19 years, is that the two
most commonly failed items are power supplies and moving parts.  If you
live in an area with extremely high incidences of lightning storms such as
New Orleans or Florida, your power supply may very well fail long before
it's expected service life is up.
Regards,
Mike
{Original Message removed}

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