Re: Jerry/Solid state switches

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.....

--.- Dave



"Too_Many_Tools" <too_many_tools@xxxxxxxxx> wrote in message
news:1124416072.845471.276160@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> Why are non zero crossing types available?
>
> What applications are they suitable for?
>
> Thanks in advance for the info.
>
> TMT
>
>


.

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