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240->120V AC adapter horror

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.

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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.

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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

--
"it's the network..."                          "The Journey is the reward"
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Reply to
Spehro Pefhany
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That should be 500K, of course.

Best regards, Spehro Pefhany

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Reply to
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..

Reply to
Jamie

"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.

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

Reply to
Phil Allison

Maybe the silicone has sticky stuff on both sides?

John

Reply to
John Larkin

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.)

Yup. sqrt(2) * 240 * sin(114) = 310V vs. 340V peak.

Yup.

Hosers, clearly.

Best regards, Spehro Pefhany

--
"it's the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
Reply to
Spehro Pefhany

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

--
"it's the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
Reply to
Spehro Pefhany

"Spehro Pefhany"

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

The equivalent Aussie expression is " Bogan" :

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A fairly recent term, as colloquialisms go.

BTW

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

Double whammy.......

..... Phil

Reply to
Phil Allison

Then why bother with the heat sink?

John

Reply to
John Larkin

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

Best regards, Spehro Pefhany

--
"it's the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
Reply to
Spehro Pefhany

"Spehro Pefhany"

** 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

Reply to
Phil Allison

On a sunny day (Sat, 08 Jan 2011 20:36:30 -0500) it happened Spehro Pefhany wrote in :

If it is just for *resistive* loads, then maybe a simple diode would work better? In case of light bulbs that may create visible 25 Hz though.

Reply to
Jan Panteltje

Yes. The voltage drop across the triac is more-or-less fixed,so average current (mostly) determines power dissipation.

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)

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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.

Best regards, Spehro Pefhany

--
"it's the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
Reply to
Spehro Pefhany

A rectifier would will put 41% more RMS voltage on the load than it is designed for. Hence the phase control.

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

Best regards, Spehro Pefhany

--
"it's the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
Reply to
Spehro Pefhany

On a sunny day (Sun, 09 Jan 2011 08:01:28 -0500) it happened Spehro Pefhany wrote in :

Oops:-) Was thinking half the power, sorry about your bulbs :-)

Reply to
Jan Panteltje

"Spehro Pefhany"

** Anyone see a problem here ??

.... Phil

Reply to
Phil Allison

Marketing.

Cheers! Rich

Reply to
Rich Grise

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

Ed

Reply to
ehsjr

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