# [EVDL] 240vac to 120vac converter: It's a diode!



## EVDL List (Jul 27, 2007)

OK. If somebody has a good idea on how to provide 200w of 120VAC from the
240VAC available from the J1772 system please let us know. I'm not too keen
on 10-20lbs of transformers or smoking wall warts. The parallel/series
switching of resistive loads (the battery heaters) I'm thinking would
require exactly matched heaters to achieve even heating. This would have
to be tested out. Please don't make me plug in the J1772 then have to hunt
down a 120VAC outlet to plug in a separate cord!

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## EVDL List (Jul 27, 2007)

The main concern of the designer is cost reduction. Consider a 240 rms sine
wave pass it thru a half wave rectifier and then thru a series capacitor,
the output is a 120v rms sine wave with a lot of harmonic distortion in the
alternation the rectifier clipped off. Because peak to peak is rms times
2.828 then a half wave rectifier blocks one alternation reducing the peak
to peak to half divided by 2.828 for rms gives half the input voltave and a
series capacitor restores some symetry but one alternation is flat topped
so the harmonic distortion is very high although not as high as a square
wave's is compared to a sine wave.
Regards,
*Dennis Lee Miles*
+++++++++++++++



> Lee Hart <[email protected]> wrote:
> 
> > On 4/15/2012 8:32 AM, Willis "Skip" Korhonen wrote:
> > > I think a pretty basic MOSFET switch circuit acting as a rectifier would
> ...


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## EVDL List (Jul 27, 2007)

Dennis,

The factor of 2.828 for peak-to-peak to RMS is only true for a sine wave;
just removing half the sine wave with a diode without using a capacitor
will have the same peak-to-peak value but a very different RMS value.

For an arbitrary waveform, you can use calculus: RMS of f(t) =
sqrt(mean(f(t)^2)), calculating the mean by integrating over one cycle of
the waveform.

As Lee said, 240V through a diode gives you a half cycle then 0 repeating,
which is 170V RMS. If you block the DC component with a capacitor, this is
106V RMS (assuming the capacitor acts as a short at 60Hz).

However, a capacitor that has low enough impedance at 60Hz to let power
through will probably be much more expensive than a TRIAC dimmer circuit
that Lee described.

-Morgan LaMoore



> Dennis Miles <[email protected]> wrote:
> 
> > The main concern of the designer is cost reduction. Consider a 240 rms sine
> > wave pass it thru a half wave rectifier and then thru a series capacitor,
> ...


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## EVDL List (Jul 27, 2007)

Am I missing something, or aren't we talking about a transformer? That's
what I would probably use if I wanted to get 120vac sinewave from a 240vac
sinewave....

Z



> Lee Hart <[email protected]> wrote:
> 
> > On 4/17/2012 9:34 PM, Morgan LaMoore wrote:
> > > As Lee said, 240V through a diode gives you a half cycle then 0
> ...


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## EVDL List (Jul 27, 2007)

> Lee Hart <[email protected]> wrote:
> > On 4/17/2012 9:34 PM, Morgan LaMoore wrote:
> > > As Lee said, 240V through a diode gives you a half cycle then 0 repeating,
> > > which is 170V RMS. If you block the DC component with a capacitor, this is
> ...


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