Re: Jerry/Solid state switches

On Sun, 21 Aug 2005 00:23:47 GMT, "Dave August"
<august@xxxxxxxxxxxxxxxx> wrote:

>Non-ZeroCross switchs can be use to do PWM at waht ever you AC frecquency
>is... In effect that makes them a 'light dimmer'.
>
>Most all of these devices are made from either back to back silicon
>controled rectifiers or triacs. Both of these devices 'turn on' when given
>a control current and will stay 'on' with out the control current till the
>voltage they are passing goes to zero... If you synchronize a small pulse of
>control current with the 'zero crossing' then you can pick any place on the
>sine wave to turn on, letting it turn off when it crosses zero. so in effect
>you are using the sine wave period as a PWM time base....
>
>Simple examples... turn on at zero cross (well just a bit after) and it's
>full power, Turn on at mid sine wave and it's half power.. turn on 1/4 of
>the way through and its 70% power... (cos(45))
>
>Yes the spike from the turn on any where but zero can cause plenty of RF but
>ya just 'choke' the hell out of it.... this IS basically what those 'light
>dimers in a switch' do.. ever wondered why you hear the ligh bulb 'sing'
>some times, that's the practicaly straight up energy pulse hitting it.
>
>This is also what the cheap speed controllers for DC motors do, using just
>one SCR and is why there are 90VDC motors out there...
>
>I've built a bidirectional variable speed DC motor controller using a
>triac.. you choose which half of the sine wave for direction and then do the
>PWM thing for speed... with a sutibally rated triac you could even use
>reverse voltage as a brake... (I had a 1 HP Bodine gear motor moving a 1 ton
>piece of equipment and NEEDED the reverse power to stop the damn thing...
>BTW this is abusive to a PM DC motor and after a year or so ya tend to demag
>them, but work just fine on a field coil motor.
>
>The real cool thing on a SCR/Triac based controller is that with a PM DC
>motor you can sense the generated voltage when the SCR/Triac is off... it IS
>directly proportional to the speed the motor is turning (it's a generator)
>and adjust the PWM to keep you speed set.....

All true -- but zero-crossing loadswitches will not turn on until the
next zero crossing even if a drive signal is applied mid-cycle.
.

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