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240->120V AC adapter horror

S

Spehro Pefhany

Jan 1, 1970
0
An associate bought this for me (from a kind of flea market/surplus
shop), just because he thought it looked amusingly gawd-awful. I have
to agree.

It's an allegedly 1600W 240->120VAC adapter for resistive loads.

http://www.speff.com/pcb_view.jpg
http://www.speff.com/resistor.jpg
http://www.speff.com/side_and_cover.jpg
http://www.speff.com/top_view.jpg

The circuit is straightforward.. a simple preset phase control. The
firing angle is determined by a 15K fixed resistor in series with a
50K trimpot and a 0.1uF film cap (it should be around 115 angular
degrees). There's a diac and a BT137 triac.

http://www.nxp.com/documents/data_sheet/BT137_SERIES.pdf


First thing, the cover just lightly snaps on/off; a child or careless
adult could easily remove it. Looks like cheap non-fire-retardant
styrene.

The gold anodized heat sink housing is nicely made, perhaps thicker
than it needs to be. There is no reason for the 15K resistor to be as
large as it is.. it's just acting as a jumper. The triac is
more-or-less held in contact with a ~1/3mm thick silicone heat pad and
thus the heat sink, but only by the springiness of the leads. Given
that the triac is a barely heat-sinked 8A triac.. I suppose it's a
good thing that the fuse in series with the output is 5A. At least
it's a 250VAC fuse. Either the insulation around the resistor or that
on the triac could easily slide out, leaving the metal case connected
to the mains. The bottom of the PCB has a thin (~0.004") insulator,
and creepage distance at the pins is well under 2mm. Yet they bothered
to put thread lock on both the nuts holding the pins to the PCB.

Total horrorshow. The really sad thing is that they could have used
similar materials and components and made a pretty good (and fairly
safe) 500W converter for not much different cost.


Best regards,
Spehro Pefhany
 
S

Spehro Pefhany

Jan 1, 1970
0
On Sat, 08 Jan 2011 20:36:30 -0500, the Spehro Pefhany

50K trimpot and a 0.1uF film cap (it should be around 115 angular
<snip>

That should be 500K, of course.


Best regards,
Spehro Pefhany
 
J

Jamie

Jan 1, 1970
0
Spehro said:
On Sat, 08 Jan 2011 20:36:30 -0500, the Spehro Pefhany



<snip>

That should be 500K, of course.


Best regards,
Spehro Pefhany
Nice converter, I wouldn't be plugging anything valuable into it.

I am not even sure if switching supplies would operate correctly
from a phase angle chopped source like that!..

How ever, on a second note, I was given a Marine 1500 C/3000P watt
converter today for parts, because it didn't output anything.. Nice
unit, has amp and source LED graph bars. Is modified sine wave, what
would you expect otherwise..

After a close inspection of the GFCI duplex mounted in it, I found
it not to reset.. Was able to use a household duplex receptacle as a
replacement, which I had lying around :)

Jamie..
 
P

Phil Allison

Jan 1, 1970
0
"Spehro Pefhany"
The circuit is straightforward.. a simple preset phase control. The
firing angle is determined by a 15K fixed resistor in series with a
50K trimpot and a 0.1uF film cap (it should be around 115 angular
degrees).


** That is just a tad ambiguous.

The CONDUCTION angle needed is 66 degrees for conversion of a sine wave at
240 volts to 120 volts rms.

The resulting train of pulses have peak values of +/- 310 volts - ie only
a little under the usual peak values of a 240 volts AC supply.

Also - if there is any electronics associated with the load presented to
such a converter (ie timers or power controllers) - they will probably very
soon be fried.

BTW:

I have seen examples of ( Canadian ) folk using one of these with their
domestic audio gear brought out to Australia with them.


..... Phil
 
S

Spehro Pefhany

Jan 1, 1970
0
"Spehro Pefhany"



** That is just a tad ambiguous.

The CONDUCTION angle needed is 66 degrees for conversion of a sine wave at
240 volts to 120 volts rms.

In the power biz, firing angle is measured from the zero crossing, so
yes you're right, the _conduction_ angle is 180 minus 66 or 114 (more
like 113.83 if I did the math right) to get RMS output of 0.500 the
input (ignoring triac drop etc.)
The resulting train of pulses have peak values of +/- 310 volts - ie only
a little under the usual peak values of a 240 volts AC supply.

Yup. sqrt(2) * 240 * sin(114) = 310V vs. 340V peak.
Also - if there is any electronics associated with the load presented to
such a converter (ie timers or power controllers) - they will probably very
soon be fried.
Yup.

BTW:

I have seen examples of ( Canadian ) folk using one of these with their
domestic audio gear brought out to Australia with them.

Hosers, clearly.
.... Phil


Best regards,
Spehro Pefhany
 
S

Spehro Pefhany

Jan 1, 1970
0
Maybe the silicone has sticky stuff on both sides?

John

Nope. Clean and slippery. There's a slight embossed "tread"-like
pattern on the triac side, while the other side is smooth.


Best regards,
Spehro Pefhany
 
P

Phil Allison

Jan 1, 1970
0
"Spehro Pefhany"
Hosers, clearly.


** Had to look that unfamiliar word up - ain't Wiki wonderful or such info.

The equivalent Aussie expression is " Bogan" :

http://en.wikipedia.org/wiki/Bogan

A fairly recent term, as colloquialisms go.


BTW

As I recall, the particular Canadians were of Irish extraction.

Double whammy.......



...... Phil
 
S

Spehro Pefhany

Jan 1, 1970
0
Then why bother with the heat sink?

John

Maybe a copy of a copy of something that once worked okay?



Best regards,
Spehro Pefhany
 
P

Phil Allison

Jan 1, 1970
0
"Spehro Pefhany"
It's an allegedly 1600W 240->120VAC adapter for resistive loads.


** Makes the * rms * output current 13.3 amps at 120 VAC.

But the *average rectified value* is just over half that number

- meaning that an 8 amp rated triac is OK.

Should dissipate about 10 to 12 watts - at most.



..... Phil
 
S

Spehro Pefhany

Jan 1, 1970
0
"Spehro Pefhany"



** Makes the * rms * output current 13.3 amps at 120 VAC.

But the *average rectified value* is just over half that number

Yes. The voltage drop across the triac is more-or-less fixed,so
average current (mostly) determines power dissipation.
- meaning that an 8 amp rated triac is OK.
Should dissipate about 10 to 12 watts - at most.

1) There's a 5A glass fuse in series with the output, so we
don't need to worry about much more than 6~7A RMS
for any length of time (AGC5's take ~10 seconds to go at 7A)

http://www.cooperbussmann.com/pdf/b040436b-cbc7-495d-95a6-99fe85202786.pdf

2) With that idiotic "heat sink" setup, it
looks like a toss-up to me whether the triac fails
before the fuse or the other way around. When the
triac goes, it puts 240VAC across the output.. and
hopefully the fuse then goes, but that depends on the load,
obviously). The heat sink might be barely adequate for 5-10W
if the triac was actually firmly attached to it.

.... Phil


Best regards,
Spehro Pefhany
 
S

Spehro Pefhany

Jan 1, 1970
0
If it is just for *resistive* loads,
then maybe a simple diode would work better?

A rectifier would will put 41% more RMS voltage on the load than it is
designed for. Hence the phase control.
In case of light bulbs that may create visible 25 Hz though.

At 170VAC on a 120V lamp, not for long.



Best regards,
Spehro Pefhany
 
P

Phil Allison

Jan 1, 1970
0
"Spehro Pefhany"
1) There's a 5A glass fuse in series with the output,


** Anyone see a problem here ??



..... Phil
 
E

ehsjr

Jan 1, 1970
0
Spehro said:
An associate bought this for me (from a kind of flea market/surplus
shop), just because he thought it looked amusingly gawd-awful. I have
to agree.

It's an allegedly 1600W 240->120VAC adapter for resistive loads.

http://www.speff.com/pcb_view.jpg
http://www.speff.com/resistor.jpg
http://www.speff.com/side_and_cover.jpg
http://www.speff.com/top_view.jpg

The circuit is straightforward.. a simple preset phase control. The
firing angle is determined by a 15K fixed resistor in series with a
50K trimpot and a 0.1uF film cap (it should be around 115 angular
degrees). There's a diac and a BT137 triac.

http://www.nxp.com/documents/data_sheet/BT137_SERIES.pdf


First thing, the cover just lightly snaps on/off; a child or careless
adult could easily remove it. Looks like cheap non-fire-retardant
styrene.

The gold anodized heat sink housing is nicely made, perhaps thicker
than it needs to be. There is no reason for the 15K resistor to be as
large as it is.. it's just acting as a jumper. The triac is
more-or-less held in contact with a ~1/3mm thick silicone heat pad and
thus the heat sink, but only by the springiness of the leads. Given
that the triac is a barely heat-sinked 8A triac.. I suppose it's a
good thing that the fuse in series with the output is 5A. At least
it's a 250VAC fuse. Either the insulation around the resistor or that
on the triac could easily slide out, leaving the metal case connected
to the mains. The bottom of the PCB has a thin (~0.004") insulator,
and creepage distance at the pins is well under 2mm. Yet they bothered
to put thread lock on both the nuts holding the pins to the PCB.

Total horrorshow. The really sad thing is that they could have used
similar materials and components and made a pretty good (and fairly
safe) 500W converter for not much different cost.


Best regards,
Spehro Pefhany

Well, not quite a _total_ horrowshow. There's one potentially
good thing about it: it could give Joerg an opportunity for
sound effects. :)

Ed
 
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