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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