Digest for sci.electronics.repair@googlegroups.com - 14 updates in 2 topics

Jezebels_couz <jbc3@three.net>: Dec 01 09:41PM -0500

In the process of finishing a 0-12 VDC @ 0-10 A switching power supply.
Fairly new at this and wanted to ask about the shunt resistor. It will
be in the ground line. Plans mentioned two options: either a custom
shunt made from constantan wire or similar, or simply two 5W 0.1 ohm
power resistors in parallel. Since I wanted to keep costs down, I am
going with the power resistors, but I have a concern: I will be running
12 gauge wire for the output lines, but when the ground wire reaches the
shunt, even the two resistors in parallel combined are far less than 12
gauge. Should I be concerned, or is the tiny resistor/ 12 gauge wire
distance (basically just for the 12 ga wire to resistor wire connection)
irrelevant to heating and losses?
Johann Klammer <klammerj@NOSPAM.a1.net>: Dec 02 08:47AM +0100

On 12/02/2020 03:41 AM, Jezebels_couz wrote:
> In the process of finishing a 0-12 VDC @ 0-10 A switching power supply. Fairly new at this and wanted to ask about the shunt resistor. It will be in the ground line. Plans mentioned two options: either a custom shunt made from constantan wire or similar, or simply two 5W 0.1 ohm power resistors in parallel. Since I wanted to keep costs down, I am going with the power resistors, but I have a concern: I will be running 12 gauge wire for the output lines, but when the ground wire reaches the shunt, even the two resistors in parallel combined are far less than 12 gauge. Should I be concerned, or is the tiny resistor/ 12 gauge wire distance (basically just for the 12 ga wire to resistor wire connection) irrelevant to heating and losses?
If it's not too long, the heat might dissipate through the adjoining traces.
(if they didn't lie about the resistor wattage, that is)
Bob Engelhardt <BobEngelhardt@comcast.net>: Dec 02 08:10AM -0500

On 12/1/2020 9:41 PM, Jezebels_couz wrote:
> gauge.  Should I be concerned, or is the tiny resistor/ 12 gauge wire
> distance (basically just for the 12 ga wire to resistor wire connection)
> irrelevant to heating and losses?
 
12ga wire is 1.6R per 1000 feet*. If your connection is 6", its
resistance is .0008R - <2% of your shunt & negligible.
 
* - https://www.engineeringtoolbox.com/copper-wire-d_1429.html
Ralph Mowery <rmowery28146@earthlink.net>: Dec 02 09:50AM -0500

In article <rq6ut4$jh7$2@dont-email.me>, jbc3@three.net says...
> gauge. Should I be concerned, or is the tiny resistor/ 12 gauge wire
> distance (basically just for the 12 ga wire to resistor wire connection)
> irrelevant to heating and losses?
 
The small wire of the resistors should handle it just fine. The main
reason for # 12 wire on the circuit is for the voltage drop.
"ohg...@gmail.com" <ohger1s@gmail.com>: Dec 01 08:52AM -0800

On Tuesday, December 1, 2020 at 11:20:44 AM UTC-5, Arie de Muynck wrote:
> The copper rotted away within a year. It may depend on the wax type,
> bees wax is supposed to be less agressive.
 
> Arie
 
I would think that candle wax would have dyes and perfumes added. Lots of coils, capacitors, transformers, etc. were potted in wax for decades without issues. I guess pure beeswax would be the safe bet though.
John Robertson <spam@flippers.com>: Dec 01 09:29AM -0800

>> bees wax is supposed to be less agressive.
 
>> Arie
 
> I would think that candle wax would have dyes and perfumes added. Lots of coils, capacitors, transformers, etc. were potted in wax for decades without issues. I guess pure beeswax would be the safe bet though.
 
I use beeswax for repotting some 1970s flybacks on old GE TVs that we
use in some classic video games made by Nutting Assoc. Works great and
hasn't led to any further trouble after upwards of ten years...I made a
simple silicon mold to hold the old core and just warm up and pour the
wax in.
 
John :-#)#
 
--
(Please post followups or tech inquiries to the USENET newsgroup)
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MOVED to #7 - 3979 Marine Way, Burnaby, BC, Canada V5J 5E3
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John Robertson <spam@flippers.com>: Dec 01 09:31AM -0800

On 2020/12/01 9:29 a.m., John Robertson wrote:
> simple silicon mold to hold the old core and just warm up and pour the
> wax in.
 
> John :-#)#
 
I should add that beeswax is likely too soft to hold the wire suspended
for any period of time and it melts at lowish temps, it would probably
be of no use to the OP for his application.
 
John :-#)#
 
--
(Please post followups or tech inquiries to the USENET newsgroup)
John's Jukes Ltd.
MOVED to #7 - 3979 Marine Way, Burnaby, BC, Canada V5J 5E3
(604)872-5757 (Pinballs, Jukes, Video Games)
www.flippers.com
"Old pinballers never die, they just flip out."
"Peter W." <peterwieck33@gmail.com>: Dec 01 09:40AM -0800

> > Arie
> I would think that candle wax would have dyes and perfumes added. Lots of coils, capacitors, transformers, etc. were potted in wax for decades without issues. I guess pure beeswax would be the safe bet though.
 
Carnauba wax (plant-based) Beeswax and Paraffin wax all have their negatives.
Carnauba wax and Beeswax are both highly acidic, albeit with 'weak' acids - but will attack bare copper and untinned copper wire over time. You will note that old waxed-paper caps used tinned copper or iron leads, not bare copper. You will also notice than when wax 'leaked' onto the chassis, there would be a permanent stain on the chassis - from the acids in the wax. The mix on the old paper caps and coils was, typically, 80/20 paraffin to beeswax - the admixture was more stabile than either alone, and easier to work than either alone.
Paraffin wax oxidizes, is highly flammable - more so even than beeswax- and while hydrophobic is sufficiently lipophilic as to attract fat-based contaminants out of the air - such as cooking odors, nicotine and so forth.
 
Transformers back in the day were 'potted' in either an asphalt-based tar (equivalent to modern non-plasticized hot roofing tar - that which is melted in tar-wagons), or in coal-tar pitch (awful stuff!). Both of which are stabile over a wide temperature range. Some few things were, in fact, potted in wax, but today that wax would be called "Jeweler's Wax) which, when cold is machinable and quite hard.
 
There are 'archival' waxes that are both acid free and self-extinguishing, but at $8 - $15 per ounce, not cheap.
 
I keep archival glue-sticks, electronics-safe glue sticks (compatible) and conventional glue-sticks (not compatible). I need two glue-guns as the change-over is quite wasteful otherwise.
 
Peter Wieck
Melrose Park, PA
Allodoxaphobia <trepidation@example.net>: Dec 01 06:09PM


> Not great advice, as most silicones cure with acetic acid, which
> corrodes circuit boards. If you must use silicone, use one that is
> specifically safe for electronics. There are only a few.
 
... specifically safe for electronics _and/or aquariums._
 
[Sorry - the first attempt went email...]
Jonesy
-- Marvin L Jones | Marvin | W3DHJ.net | linux
38.238N 104.547W | @ jonz.net | Jonesy | FreeBSD
* Killfiling google & XXXXbanter.com: jonz.net/ng.htm
Chuck <chuck445@yahoonospam.com>: Dec 01 01:32PM -0500

On 12/1/20 12:31 PM, John Robertson wrote:
> for any period of time and it melts at lowish temps, it would probably
> be of no use to the OP for his application.
 
> John :-#)#
 
Interesting. Not to change my own topic here, but I have a 125 KV high
voltage generator that I made myself fifteen years ago for
demonstrational purposes. The design is not mine, but uses two auto HEI
coils in opposite phase. To get the voltages needed and not destroy the
coils, they had to be placed under oil. However.....
 
Three years ago, after the device had been stored away for almost a
decade, I removed it and its storage container (I always keep any oiled
components in an extra container to contain possible leakage later).
Well, good thing I had the extra container as there was some leakage.
Not much, but enough that I decided to drain and switch out containers.
Done and restored.
 
Another option at the time of the original build was, instead of using
oil, to place the coils in wax. I never did this originally, but
decided to make a spare HV section (which only contains the two coils).
This time, I used white canning wax with no additives. I did this last
year. The year before, I was worried that the canning wax might crack
upon exposure to heat and cool, so I poured a block and placed it into
an environment with a lot of temperature extremes for a year. No
cracking after a year, so that's when I decided to pour the wax spare
generator section.
 
Now, just as a precaution, I'd never take the unit in a hot car, warmed
up from summer heat let's say, for any length of time as I'd be
concerned that the wax might start melting, so that's where the oil
filled one would be used. Where the wax one is stored might vary from
15-100 F, so confident at those temps.
 
Oil can sure creep right through and out of things! I have another
homemade device, a high voltage probe, consisting of many resistors in
series. It can measure up to 100 KV and uses a standard multimeter with
its very high resistance. I made it 20 years ago. However, being
stored on the shelf above me right near where I sit here, I noticed some
suspicious spots of what I thought was water at first on the rug this
past summer. I was perplexed at first and thought my roof might be
leaking until I decided to finger the wet drop one day and rub fingers
together to see if it was in fact water. I then knew for sure that it
was oil. Turns out that when I made my HV probe, I exited the resistor
PVC enclosure with a piece of HV multi-stranded TV wire of the kind
often seen coming out of TV flybacks. Well, although I sealed around
the wire to PVC exit, I didn't properly seal the strands themselves. It
took 19 years, but oil made its way through that wire and was dripping
from the end plugged into the meter. It's not much, maybe five drops in
several months, but definitely a leak. Not having time to get to this
yet to remedy, I covered over the wire end with a plastic bag and had
this hang out over the shelf side to contain any further leaking. As of
today, the bag probably contains an ounce of oil.
 
As for the circuit board, I probably won't use wax to hold the wires. I
like the hot glue idea, which I have in abundance here, and I can always
reheat later to the wires come right back off the board. I just have to
be sure not to get any of the glue on the soldered traces themselves.
Chuck <chuck445@yahoonospam.com>: Dec 01 01:39PM -0500

On 12/1/20 12:40 PM, Peter W. wrote:
 
 
> I keep archival glue-sticks, electronics-safe glue sticks (compatible) and conventional glue-sticks (not compatible). I need two glue-guns as the change-over is quite wasteful otherwise.
 
> Peter Wieck
> Melrose Park, PA
 
What would be wrong when using conventional glue sticks?
"Peter W." <peterwieck33@gmail.com>: Dec 01 11:11AM -0800

> What would be wrong when using conventional glue sticks?
 
Conventional glue-sticks have a much lower plastic temperature than the others - Anything lower than about 120 F should be fine. But understand that the melting point is about 175 F. Archival glue-sticks melt from 250 F to 380 F depending on the application. Electronics glue-sticks are designed for hot-temperature application or 350 F to 380 F. In a solid-state device with low voltages and (always) low ambient and operating temperatures, conventional glue-sticks should be fine unless labeled otherwise. Low-voltage unless the sticks carry a UL-94 listing.
 
Peter Wieck
Melrose Park, PA
Chuck <chuck445@yahoonospam.com>: Dec 01 02:35PM -0500

On 12/1/20 2:11 PM, Peter W. wrote:
 
> Conventional glue-sticks have a much lower plastic temperature than the others - Anything lower than about 120 F should be fine. But understand that the melting point is about 175 F. Archival glue-sticks melt from 250 F to 380 F depending on the application. Electronics glue-sticks are designed for hot-temperature application or 350 F to 380 F. In a solid-state device with low voltages and (always) low ambient and operating temperatures, conventional glue-sticks should be fine unless labeled otherwise. Low-voltage unless the sticks carry a UL-94 listing.
 
> Peter Wieck
> Melrose Park, PA
 
Ok, thanks because I wasn't sure. That part of the circuit won't be
under heating so I'm going to go ahead and use the sticks I have.
Chuck <chuck445@yahoonospam.com>: Dec 01 03:34PM -0500

On 12/1/20 1:32 PM, Chuck wrote:
> like the hot glue idea, which I have in abundance here, and I can always
> reheat later to the wires come right back off the board.  I just have to
> be sure not to get any of the glue on the soldered traces themselves.
 
Incidentally, to follow up a bit, I concluded that the reason why the
original oil container leaked was due to thermal expansion of the oil
through the container. The coil containment container was sealed with
gaskets (at the top only) and I think the oil pushed through the gaskets
when temperatures in the environment were higher. Not sure, but the
real test this time will prove/ disprove this as I refilled with oil but
leaving space between the oil and top/ gasket seal.
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