# Question about Theory then details will follow

#### arishy

Joined Apr 26, 2014
123
The reason for the question is based on "commercial" device sold today which convert 240 AC to 120 AC with 2K Watt capacity

This is basically a dimmer circuit using BT136.

Here is my question (you really no need for details here). If I measure voltage AFTER the dimmer it is STILL 240v irrespective of any adjustment of the duty cycle which is suppose to deliver REDUCED something for the load. I am aware of the theory of reduced power using PWM. The circuit does not change the peak voltage it is still 240v. Magically if you use high value high wattage resistance across output you can READ a reduced VOLTAGE.
I know what you will say; it is a simple voltage divider circuit .

Did I answer my question or there is more to it than that.

#### Jon Chandler

Joined Jun 12, 2008
1,098
(you really no need for details here)
And nobody has any real need to answer here. Excellent.

#### Ian0

Joined Aug 7, 2020
10,276
If you measure it with a 10MΩ voltmeter, you will read the same as the input voltage because of the leakage current. Don't forget you only need 24uA for a10MΩ voltmeter to read 240V.

#### LowQCab

Joined Nov 6, 2012
4,295
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#### crutschow

Joined Mar 14, 2008
34,842
I know what you will say; it is a simple voltage divider circuit .
No, that's not what I would say.
Did I answer my question or there is more to it than that.
You did not and there is more.

That apparently is a phase-control TRIAC which only transmits part of each half of the AC cycle to reduce the average voltage, but not necessarily the peak voltage.
It is not PWM, and it's not a voltage divider.

Such a voltage reducer may cause a problem with some types of appliances or electronic devices, since the output has a high peak voltage and is not a sine-wave.

#### arishy

Joined Apr 26, 2014
123
No, that's not what I would say.
You did not and there is more.

That apparently is a phase-control TRIAC which only transmits part of each half of the AC cycle to reduce the average voltage, but not necessarily the peak voltage.
It is not PWM, and it's not a voltage divider.

Such a voltage reducer may cause a problem with some types of appliances or electronic devices, since the output has a high peak voltage and is not a sine-wave.
I knew I got it all wrong and here is the place that will put it right.
So, the peak voltage did not "budge" WoW ( 240v and 330v peak)
My meter reading is NOT right. If I get professional one; it will read the 240/330v and not the 110v.
People are plugging in 110v devices and they work!!!!! and that I need an explanation for...please
Some devices that are sensitive to distorted signal ( definitely not a sine wave) will possibly suffer or may be damaged.
If I plug a lamp rated 110v, it will work because it is a resistive load. Is that right? Motors is another story.
If I put a bridge rectifier, it 's output is 330v ( 240*1.4) the smoothing capacitor must be rated 400v Is that right?

In my view this type of "universal" power adapters ( 240v to 110v ) should be banned.

#### WBahn

Joined Mar 31, 2012
30,296
SOME 110 VAC devices will work (or at least appear to work acceptably well for awhile) IF they can tolerate the very high peak voltages and the highly distorted waveforms that result. Some devices can tolerate both of those just fine. Others will be destroyed almost instantly. Then there are lots that will kinda, sorta work while being damaged and eventually failing, perhaps by just not working any more, or perhaps by going up in flames, or perhaps something in between.

#### crutschow

Joined Mar 14, 2008
34,842
My meter reading is NOT right. If I get professional one; it will read the 240/330v and not the 110v.
No, professional meters read the RMS (averaged over the waveform period) not the peak value.

Also 240/330V is the RMS value of the total input sinewave, not the peak.

#### arishy

Joined Apr 26, 2014
123
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Of course I need a reply; especially if all my "answers" were WRONG. YOU; can put me right. Can you imagine the amount of knowledge I gained from this post. I am sure you can teach me to be better in what I do.

#### arishy

Joined Apr 26, 2014
123
And nobody has any real need to answer here. Excellent.
I love this type of answers. But may be I wonder how such a "recommendation" benefit the members.

#### WBahn

Joined Mar 31, 2012
30,296
No, professional meters read the RMS (averaged over the waveform period) not the peak value.

Also 240/330V is the RMS value of the total input sinewave, not the peak.
If 240 VAC is the input, that is (almost certainly) RMS. But the 330 V is (almost certainly) referring to the peak voltage of such a waveform (the actual peak is nominally 339 V. A sine wave with a 330 V peak would have an RMS value of 233 V.

#### LowQCab

Joined Nov 6, 2012
4,295
"" In my view this type of "universal" power adapters ( 240v to 110v ) should be banned. ""
I totally agree.
The number of truly practical applications for Triacs is very limited.
But they are so cheap, and so simple, that they may never completely go away.
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#### Ian0

Joined Aug 7, 2020
10,276
Legally, it's a difficult thing to regulate. It would require a 230V country to have electrical standards which apply to 110V supplies and connectors. I see quite a few Americans who have brought adaptors with them which would be illegal in Britain, but have been bought legally in USA, because USA has no standards for 230V BS1363 connectors.
Fortunately, there is more often than not a large distance between 230V and 110V countries, and high current appliances are heavy and bulky so few people would transport one, and require the device described by the TS.
No-one would bring an American kettle to Britain - the British just couldn't wait that long for a cup of tea.

#### WBahn

Joined Mar 31, 2012
30,296