Working PSU for Philips Hue "Lightstrip" gives open-circuit (even at 200 M ohm range)

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NY

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Dec 6, 2021, 7:58:19 AM12/6/21
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I'm curious.

We found a Philips Hue "Lightstrip" that was not working, and it turned out
to be a duff 240 V AC to 24 V DC PSU. When I measured the resistance across
the mains input pins, it gave "open circuit" even at the 200 M ohm range of
my multimeter (trying both polarities). So maybe duff transformer or other
mains-voltage circuitry, I deduced.

But when I measured the same pins on a *working* PSU which drives the
lightstrip OK, it also gave open circuit.

Has anyone come across a SMPSU which has "infinite resistance" (ie open
circuit even on multi megohm range) and yet which works fine? Do they use
capacitors as "dropping resistors" (dropping impedances) on the mains pins?

Brian

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Dec 6, 2021, 8:18:55 AM12/6/21
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‘Seeing’ a very high resistance at the input of a SMPSU isn’t unusual.

From past experience, a common fault is ‘dry joints’ - often you can see
then, especially on ones which have mosfets / transistors which are through
hole - those are the pads which tend to fail. Clean away the old solder,
resolver and it may work. I usually do all the switching devices ( the ones
on the heat sink)

Obviously do power off etc.

NY

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Dec 6, 2021, 8:34:56 AM12/6/21
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"Brian" <no...@lid.org> wrote in message news:sol2js$ogq$1...@dont-email.me...
> ‘Seeing’ a very high resistance at the input of a SMPSU isn’t unusual.
>
> From past experience, a common fault is ‘dry joints’ - often you can see
> then, especially on ones which have mosfets / transistors which are
> through
> hole - those are the pads which tend to fail. Clean away the old solder,
> resolver and it may work. I usually do all the switching devices ( the
> ones
> on the heat sink)
>
> Obviously do power off etc.

I'd do that, but this PSU seems to have no screws (not even concealed under
labels) so the two parts of the case are evidently glued together or else
held by springy tabs that need ten hands to release them all simultaneously.

I also discovered that Philips made two different versions: some have a
barrel connector between the PSU and the control logic (and the LED strip
hard-wired to the controller) whereas others with a subtly different part
number have the PSU and controller hard-wired and a multi-pin connector
between controller and LED strip. By chance we have some of each type ;-)

John Rumm

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Dec 6, 2021, 9:17:57 AM12/6/21
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On 06/12/2021 12:46, NY wrote:
> I'm curious.
>
> We found a Philips Hue "Lightstrip" that was not working, and it turned
> out to be a duff 240 V AC to 24 V DC PSU. When I measured the resistance
> across the mains input pins, it gave "open circuit" even at the 200 M
> ohm range of my multimeter (trying both polarities). So maybe duff
> transformer or other mains-voltage circuitry, I deduced.

If its a modern "light weight" SMPSU, then the transformer is not
connected across the input pins like it was on old linear regulated
supplies. Quite often the first thing connected (excluding some EMI
filtering) is the rectifier.

Some meters may show a very high resistance in one direction at least (>
7Mohms on the one I just tried) - but that "resistance" is equally
likely to be the meter just charging the reservoir cap in the PSU.

(You might also see a different behaviour on a diode test mode if the
meter has one)

> But when I measured the same pins on a *working* PSU which drives the
> lightstrip OK, it also gave open circuit.
>
> Has anyone come across a SMPSU which has "infinite resistance" (ie open
> circuit even on multi megohm range) and yet which works fine?

Yup - especially if you are not applying enough voltage to forward bias
the rectifier diodes.

> Do they
> use capacitors as "dropping resistors" (dropping impedances) on the
> mains pins?

Not usually unless they are supplies designed for very low loads. Not
typical for your typical external wall wart.


--
Cheers,

John.

/=================================================================\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\=================================================================/

Rod Speed

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Dec 6, 2021, 9:25:21 AM12/6/21
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NY <m...@privacy.invalid> wrote

> I'm curious.

Don’t forget what that did to the cat.
Very likely.


NY

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Dec 6, 2021, 9:44:53 AM12/6/21
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"John Rumm" <see.my.s...@nowhere.null> wrote in message
news:sol62h$i4q$1...@dont-email.me...
>> Has anyone come across a SMPSU which has "infinite resistance" (ie open
>> circuit even on multi megohm range) and yet which works fine?
>
> Yup - especially if you are not applying enough voltage to forward bias
> the rectifier diodes.

Ah! I bet that's it. I wonder what voltage a multimeter typically applies to
a load when it is measuring resistance.

>> Do they use capacitors as "dropping resistors" (dropping impedances) on
>> the mains pins?
>
> Not usually unless they are supplies designed for very low loads. Not
> typical for your typical external wall wart.

I remember in Elec Eng at university we were given a project in teams to
design a SMPSU with certain criteria as regards permitted variation of load
voltage as more current was drawn, and permitted range of input voltages
(provided with a Variac for the test).

For some reason that escapes me, a dropping resistor was needed on the input
to the transformer - or maybe it was between bridge rectifier and switching
transistor. (This was an "old fashioned SMPSU" where the transformer was fed
at mains frequency rather than switching frequency as happens nowadays to
keep transformer as small as possible).

One of my team-mates (and I wish it had been me!) had the brilliant idea of
using a series capacitor instead of a series resistor to do this, with a
size based on impedance at the relevant frequency. This was a lossless
solution unlike a resistor, and meant we achieved significantly better
efficiency than any other teams. We won the prize!

(I remember that the switching transistor that we were required to use did
*not* like to be turned on permanently when full load was being drawn. The
feedback circuit limited the M:S ratio to a little less than 100% even with
heavy load. But it only took a loose wire during development, before
everything had been permanently soldered, and the transistor lost its
switching signal and was on permanently. We all did it at one time or
another. There was a noise like rustling autumn leaves for a couple of
seconds, and then the transistor blew up. Ours left a dent in the ceiling!
After that, we were all made to wear safety goggles just in case...)

Peeler

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Dec 6, 2021, 11:41:21 AM12/6/21
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On Tue, 7 Dec 2021 01:25:14 +1100, cantankerous trolling geezer Rodent
Speed, the auto-contradicting senile sociopath, blabbered, again:

<FLUSH the abnormal trolling senile cretin's latest trollshit unread>

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The Natural Philosopher

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Dec 6, 2021, 12:35:03 PM12/6/21
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On 06/12/2021 13:18, Brian wrote:
> NY <m...@privacy.invalid> wrote:
>> I'm curious.
>>
>> We found a Philips Hue "Lightstrip" that was not working, and it turned out
>> to be a duff 240 V AC to 24 V DC PSU. When I measured the resistance across
>> the mains input pins, it gave "open circuit" even at the 200 M ohm range of
>> my multimeter (trying both polarities). So maybe duff transformer or other
>> mains-voltage circuitry, I deduced.
>>
>> But when I measured the same pins on a *working* PSU which drives the
>> lightstrip OK, it also gave open circuit.
>>
>> Has anyone come across a SMPSU which has "infinite resistance" (ie open
>> circuit even on multi megohm range) and yet which works fine? Do they use
>> capacitors as "dropping resistors" (dropping impedances) on the mains pins?
>>
>>
>
> ‘Seeing’ a very high resistance at the input of a SMPSU isn’t unusual.
>
Not on a LED one, certainly but of course you didn't mean SMPSU, because
most LED power supplies are not that smart. see yer Big CXlive videos. a
series capacitor with a surge resistor into a bridge rectifier and a
smoothing cap - or maybe not - and then into a chain of series LEDS is
just cheap and works fine.






--
Outside of a dog, a book is a man's best friend. Inside of a dog it's
too dark to read.

Groucho Marx


John Rumm

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Dec 6, 2021, 5:46:24 PM12/6/21
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On 06/12/2021 14:43, NY wrote:
> "John Rumm" <see.my.s...@nowhere.null> wrote in message
> news:sol62h$i4q$1...@dont-email.me...
>>> Has anyone come across a SMPSU which has "infinite resistance" (ie
>>> open circuit even on multi megohm range) and yet which works fine?
>>
>> Yup - especially if you are not applying enough voltage to forward
>> bias the rectifier diodes.
>
> Ah! I bet that's it. I wonder what voltage a multimeter typically
> applies to a load when it is measuring resistance.

OOI I just connected my Fluke 117 back to back with my old Maplin
Precision Gold M125. If you put the fluke in auto range and measure
resistance, then the other meter measures around 0.5V and the Fluke sees
10 MOhms. If you put the fluke in manual range, then you can get a
maximum of about 2.5 volt out of it on some ranges (with the resistance
being out of range on all but the highest). Diode test mode uses 2.5V
all the time it seems.

The other way around gets similar results although it sees the input
impedance of the fluke at around 11 MOhms, and outputs about 0.4V

Brian

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Dec 6, 2021, 6:00:04 PM12/6/21
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Did you read the original post? He has a SMPSU. It isn’t unusual to power
LEDs by a proper SMPSU.

The dropper arrangement you describe works for some applications but isn’t
a SMPSU.



Brian Gaff (Sofa)

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Dec 7, 2021, 3:58:32 AM12/7/21
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This dry joint problem used to afflict old fashioned dimmers too. It must
have something to do with the two materials and the solder. You could at
least when I could see, see the kind of well around the pins after a few
years. Must be chemical or something I don't know, but some high power audio
amps had similar issues. Pioneer used to use wire wrap on the pins.
Brian

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Brian Gaff (Sofa)

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Dec 7, 2021, 4:05:06 AM12/7/21
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It also leaves the load at mains voltage, as there is no transformer to
isolate it.
I used to have a handy Pifco torch which directly plugged in to charge it
up. It was old school Nicads and traditional bulb. One day I came home to
find the living room full of smoke of the magic kind that smelt of melted
plastic and the like. The capacitor dropper had self destructed. It took
days to rid the place of the pong.
I don't know what they make them of, but back in those days it was
something less than pleasant!

Brian

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Dave Plowman (News)

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Dec 7, 2021, 9:08:44 AM12/7/21
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In article <son7lo$otd$1...@dont-email.me>,
Brian Gaff \(Sofa\) <bri...@blueyonder.co.uk> wrote:
> This dry joint problem used to afflict old fashioned dimmers too. It must
> have something to do with the two materials and the solder. You could at
> least when I could see, see the kind of well around the pins after a few
> years. Must be chemical or something I don't know, but some high power audio
> amps had similar issues.

It used to be a problem with things like line output transformers on early
TVs. A combination of heat and vibration caused the solder to fail.

--
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Dave Plowman da...@davenoise.co.uk London SW
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Dave W

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Dec 18, 2021, 5:15:46 PM12/18/21
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On Tue, 07 Dec 2021 14:08:22 +0000 (GMT), "Dave Plowman (News)"
<da...@davenoise.co.uk> wrote:

>In article <son7lo$otd$1...@dont-email.me>,
> Brian Gaff \(Sofa\) <bri...@blueyonder.co.uk> wrote:
>> This dry joint problem used to afflict old fashioned dimmers too. It must
>> have something to do with the two materials and the solder. You could at
>> least when I could see, see the kind of well around the pins after a few
>> years. Must be chemical or something I don't know, but some high power audio
>> amps had similar issues.
>
>It used to be a problem with things like line output transformers on early
>TVs. A combination of heat and vibration caused the solder to fail.

It's because there was too much vibration on the conveyor belt out of
the solder bath before the solder had set. This was a problem with
large tags that retain heat for two long. A crack forms all round the
spill, and goes open circuit some time in the future.
--
Dave W

Dave Plowman (News)

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Dec 19, 2021, 9:43:26 AM12/19/21
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In article <v2nsrg5oc2fjffp8r...@4ax.com>,
Dave W <dave...@yahoo.co.uk> wrote:
> >It used to be a problem with things like line output transformers on early
> >TVs. A combination of heat and vibration caused the solder to fail.

> It's because there was too much vibration on the conveyor belt out of
> the solder bath before the solder had set. This was a problem with
> large tags that retain heat for two long. A crack forms all round the
> spill, and goes open circuit some time in the future.

Ages ago at work we had loads of monitors all the same. Re-soldering the
LOPT got them going again. Until the next time.

--
*I will always cherish the initial misconceptions I had about you

Rod Speed

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Dec 19, 2021, 12:10:46 PM12/19/21
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Dave Plowman (News) <da...@davenoise.co.uk> wrote
> Dave W <dave...@yahoo.co.uk> wrote

>>> It used to be a problem with things like line output transformers on
>>> early
>>> TVs. A combination of heat and vibration caused the solder to fail.

>> It's because there was too much vibration on the conveyor belt out
>> of the solder bath before the solder had set. This was a problem
>> with large tags that retain heat for two long. A crack forms all
>> round the spill, and goes open circuit some time in the future.

> Ages ago at work we had loads of monitors all the same.
> Re-soldering the LOPT got them going again. Until the next time.

Yeah, the DEC VT1OO was notorious for that.

Peeler

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Dec 19, 2021, 1:15:34 PM12/19/21
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On Mon, 20 Dec 2021 04:10:38 +1100, cantankerous trolling geezer Rodent
Speed, the auto-contradicting senile sociopath, blabbered, again:

<FLUSH the abnormal trolling senile cretin's latest trollshit unread>

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