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Re: [PSUBS-MAILIST] link
Hi Bill,
>From what I remember here is a description of how PWM
works.
PWM turns the voltage fully on and fully off at a set
rate for a set amount of time. By fully on I mean the
voltage is at your supply voltage level. If you have a
12VDC supply then the fully on voltage will be 12VDC.
If a 48VDC the fully on voltage will be 48VDC. The
fully off voltage will be 0VDC or ground.
The on and off transistions are square wavish more
then sine wavish, sharp trasistions. Assume you have a
48VDC supply the wave shape is as follows with the
voltage either fully on, at 48VDC or fully off, at
0VDC.
48VDC ----- ----- -----
| | | |
0VDC ----- -----
The on and off trasistions occur at a set frequecy,
for purposes of discussion lets use 1000 hertz (1000
clock cycles or one clock cycle per 1 milli-second).
This is the Clock Cycle.
The Duty Cycle is how much of the time between clock
cycles the circuit on on. A potentiometer in the PWM
curcuit will set the duty cycle of some where between
0% and 100%. The percentage is the amount of time the
DC voltage is fully on. The off duty cycle is the time
the voltage is fully off, or 100% - duty cycle. Below
is a representation of what you see.
Clock Cycle (assume 1000 hertz clock)
|_________|_________|_________|
10% duty cycle
-_________-_________-_________-
20% duty cycle
--________--________--________-
30% duty cycle
---_______---_______---_______-
40% duty cycle
----______----______----______-
50% duty cycle
-----_____-----_____-----_____-
60% duty cycle
------____------____------____-
70% duty cycle
-------___-------___-------___-
80% duty cycle
--------__--------__--------__-
90% duty cycle
---------_---------_----------_
100% duty cycle
-------------------------------
For the 80% duty cycle, for example, the voltage is on
for 0.8 milli-seconds and off for 0.2 milli-seconds.
Now with the voltage being on part of the time and off
part of the time the effect to electromachanical
devices, a.k.a electric motors, is that the motors see
the the equivelant voltage of some portion of the
supply voltage. Say for the 80% duty cycle and a 48VDC
supply the motors see an equivalent of 38.4VDC.
Now you can do the on and off transistions with a
switch by manually flipping it on and off. You could
get a 80% duty cycle by turning on a switch for 8
seconds, count 1 - 2 - 3 - 4 - 5 - 6 - 7 - 8, then
turn the switch off for 2 seconds, count 1 - 2 then
repeat.
But that would get boring and tiring. Most important
is that the motor would really see on for 8 seconds
and off for 2 seconds. For the PWM to really work
clock cycles need to happen up in the 1000 hertz (1k
hertz) and faster ranges. That way the motor is not
fully on or off but some where in between with the
intertia of the motor parts keeping things moving at a
less then full speed rotation.
One last note. A motor will not run over the full 0%
to 100% duty cycle range. I think a motor will still
be stalled up to 80% range. Above that the speed can
be varied by PWM.
Regards,
Ray
--- Akins <lakins1@tampabay.rr.com> wrote:
> Hi Ray.
>
> Your e mail confirmed a nagging thought I had about
> the operating differences between DC and AC voltage.
> I was wondering if the ac and dc voltage would both
> work in the dimmer switch, but wasn't sure.
> Your e mail confirmed for me that it doesn't. Rats!
> Back to square one. So if I don't find a ready
> made, available for sale variable speed waterproof
> and pressurproof switch, it now appears my only
> choice is to make one using those circuit boards in
> conjunction with a magnetic proximity switch
> and waterproof and pressureproof it by encasing it
> in pvc. Ray, how does the pulse wave modulation
> broken into duty cycles for a percentage of time
> work? Does that mean I could only run it at one
> particular speed for just a certain amount of time?
> Or does that mean it would work as a variable
> speed controller like you know I want across the
> entire speed range? I've still got to check on the
> manufacturer NP sent me wherein he said they have
> ready made exactly what I need. Hope so.
> Hate to have to make this switch.
> I had some doubts about the ac and dc working in the
> dimmer switch but was going to try and
> experiment with it anyway, but now you have saved me
> time and effort by confirming those doubts.
> Thanks for your help and info and saving me from
> wasting my time and effort Ray. I'm great with
> ballast,
> hands on forming and a lot of other things, but the
> electrics are my weak point.
>
> Bill Akins.
>
>
>
> ----- Original Message -----
> From: Ray Keefer
> To: personal_submersibles@psubs.org
> Sent: Tuesday, February 07, 2006 2:35 PM
> Subject: Re: [PSUBS-MAILIST] link
>
>
> Hi Bill,
>
> I believe that household rotary light dimmer
> switches
> rely on the fact that the voltage is AC.
> Alternating
> between 0VAC and 120VAC as a sine wave. The SCR in
> the
> switch turns on when the voltage gets to a level
> set
> by the rotary switch then turns off when the sine
> wave
> peaks and drops below that point.
>
> With a DC circuit you do not get that automatic
> voltage change that the the SCR can trigger on.
>
> The way to control DC is either on/off, or using
> PWM
> where the DC is electronically broken up into duty
> cycles of being on for a percentage of time and
> off
> the other percentage of time.
>
> Regards,
> Ray
>
> --- Akins <lakins1@tampabay.rr.com> wrote:
>
> > Thanks again for thinking of me Steve. But it
> > wouldn't help it to stay dry even if it was
> inside
> > my sub. My sub is a completely wetsub that I use
> > scuba gear
> > to operate. No dry cockpit in my boat. It
> wouldn't
> > be practical for me to install it in a pod
> outside
> > the sub when I need it in the cockpit. It's not
> > locating the electronics
> > that is the problem. I can and have found them
> at
> > many sites. All kinds of circuit boards to
> operate a
> > DC variable speed motor. Just none of
> > them I have found thus far are already
> waterproof
> > and pressureproof with an exposed to the water
> > switch knob, handle or twistgrip. For my
> > application I don't need a fancy circuit board.
> I am
> > using between the voltage spectrum of 12 volts
> for
> > half speed and 24 volts for full speed.
> > Since a household rotary light dimmer switch is
> made
> > to handle 115 volts, it will more than handle my
> max
> > of 24 volts. So you see I don't
> > need a circuit board. If I wanted to make the
> switch
> > myself, it would be simplest and cheapest to
> just
> > use the rotary dimmer switch and
> > encase it in pvc and use a magnet to actuate it
> > without using a circuit board for the switch at
> all.
> > The problem is then installing the dimmer
> > switch in pvc to water and pressure proof it and
> > then to make a magnet actuate the shaft to the
> > switch. I could probably do it by taking the
> > time and trouble to experiment but it would just
> be
> > so much easier to be able to buy one like I
> need. If
> > I can find it.
> > Bill Akins.
> >
> >
> > ----- Original Message -----
> > From: steve
> > To: personal_submersibles@psubs.org
> > Sent: Monday, February 06, 2006 6:22 PM
> > Subject: RE: [PSUBS-MAILIST] link
> >
> >
> > considering they make them for land vehicles,
> i'm
> > not suprised!
> > surely it would be contained within the sub or
> a
> > custom pod outside?
> >
> > steve
> > -----Original Message-----
> > From: owner-personal_submersibles@psubs.org
> > [mailto:owner-personal_submersibles@psubs.org]On
> > Behalf Of Akins
> > Sent: 06 February 2006 23:33
> > To: personal_submersibles@psubs.org
> > Subject: Re: [PSUBS-MAILIST] link
> >
> >
> > Thanks for the links Steve.
> >
> > However I didn't see any waterproof and
> > pressureproof variable speed motor controllers
> ready
> > made at the sites.
> > Bill Akins.
> >
> >
> > ----- Original Message -----
> > From: steve
> > To: personal_submersibles@psubs.org
> > Sent: Saturday, February 04, 2006 6:37 PM
> > Subject: [PSUBS-MAILIST] link
> >
> >
> >
> > just in case it's useful to anyone...
> >
> > www.4qd.co.uk
> >
> > make electric vehicle speed controllers
> and
> > have a sister site www.4qdtec.com with lots of
> good
> > info.
> > pay the small subscription fee its worth
> it!
> >
> > steve
>
>
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