Electronics Forum / Basics / February 2007

You can't make the water hotter except by reducing the water flow. To make
it cooler, you need a 20 amp dimmer control which is safe to use on 220 V.
You will have to adjust it before using the shower - it would be very
dangerous to use one while in the shower.

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Phase control involves turning on a latching device, like an
SCR or TRIAC at the same fraction of each half cycle, and
the device remains on till the next current zero crossing.
Power is varied by changing the timing of where in each half
cycle that power is switched on.

Integral cycle control involves turning on the switching
device just after a voltage zero crossing, so that an entire
half cycle of line voltage is passed, so that there are no
fast voltage rises in the waveform.  Power is controlled by
changing the number of complete cycles, in some time
interval, that are passed to the load and the remainder that
are blocked.  Usually, the cycles that are passed are all
together in a group, and the ones blocked are also together
in a group, but the power control is smoother if the
groupings are eliminated and the ratio of passed and blocked
cycles are mixed as well as possible.  With a microprocessor
that has a line cycle input, to synchronize the timing to
the line zero crossings, this process can be done almost
entirely in software, as can the feedback PIC control
function, whose output decides the fraction of power cycles
passed.

While it is not so important that each positive half cycle
delivered is immediately followed by the negative half
cycle, it is important that it is impossible for the
controller to supply only positive or only negative half
cycles, because this would put a DC component back into the
line power source, and that isn't good for the transformers
in the distribution system.

See:
http://www.athenacontrols.com/pages/fund.html
http://www.ccipower.com/support/control_modes.php
http://www.patentstorm.us/patents/6849834-description.html

There are also special purpose solid state relays that
accept a control voltage and generate the variable duty
cycle burst of line cycles in proportion to the control voltage.
http://www.crydom.com//userResources/productFamilies/50/crydom_pcv.pdf

There is a similar switch here also but the switch isn't very useful
because:

- that is above the shower, so it is height
- I would like to have a better temperature control. The climate in my
region is very volatile.

thanks you,
PH