Re: [PSUBS-MAILIST] [Fwd: take some time and view my web sites...]

[Dave Everett <deverett@idx.com.au>]

At 06:50 AM 12/7/2004, you wrote:

> les here, For speed control why cant you use a dimmer switch?  house hold
> item. trolling motors have them.

In simple terms, a dimmer switch works on AC power, a trolling motor is DC. 
The dimmer uses a device called an SCR (silicon controlled rectifier) to do 
the chopping up of the power and it requires the zero crossings that occur 
in AC.

Geeze, I guess that is a confusing explanation :)

AC is a sine wave, zero volt is like a horizontal line halfway up the 
waveform. The current goes in an arc above the line, then in an arc below 
the line, it is ALTERNATING, hence alternating current. An SCR is turned 
on, then remains conducting current until the AC signal crosses that zero 
volts line (zero crossing). If I turn the SCR on when the signal is at 
90deg (the waveform is at the peak of it's arc) the SCR will remain on 
until the zero crossing. Now I do that every wave, the ones above and below 
the line, I have effectively halved the power because current is only 
flowing from the SCR half as long as the AC input signal.

I can change that SCR position anywhere from about 5% to 95% to change the 
power output. As the SCR requires this zero crossing to reset, I must have 
AC to operate it, it won't work on DC.

Dave Everett 
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