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Re: [PSUBS-MAILIST] variable speed switch.
Hi Bill,
Potentiaometers are center tapped variable resistors.
Schematically they are:
A B
______/\ /\ /\ /\ /\ /\ /\ /\ /\_______
\/ \/ \/ \/ \/ \/ \/ \/
^
/ \
|
| C
|
>From terminals A to B there is a resistance. Terminal
C is a center tap point that changes as the
potentiaomenter is rotated. If C is in the middle then
between terminals A to C would be half the resistance
and terminals from C to B would be half the
resistance. If C is 1/3 of the way from terminal A
then the resistance from terminals A to C would be 1/3
the total resistance and from terminals C to B would
be 2/3 the resistance between terminals A and B.
Assuming of course you have a linear potentiameter.
There are two kinds of potentiameter: linear and
audio. In linear the resistance from terminals A to B
changes are a fix rate.
But the human ear does not hear linear changes. Half
the voltage to a speaker does not sound like half the
volume. So the audio potentiaometer has a resistive
material that becomes more resistive the closer you
get to either termainal A or B in some logrithmic
progression.
A potentiometer is rated in resistance and wattage.
Not voltage. You can put 12VDC, 48VDC, 10,000VAC
across any potentiaometer. The question is, can it
take it. For instance take a linear 1k ohm (ohm is
measure of resistance), 1/2 watt potentiameter with a
12VDC power source across terminals A and B. 12VDC /
1k ohms = 0.012 amp current. The power disapated
through the resistance would be 12VDC x 0.012 amps =
0.144 watts. We are fine.
Now take the same potentiameter and put 1000VDC over
it. 1000VDC / 1k ohms = 1 amp. 1000VDC x 1 Amp = 1000
watts. You just let the smoke out of the
potentiaometer. In other words you exceeded its rated
limit and burnt it out.
However a 2mega (2,000,000) ohm, 1/2 watt
potentiaometer on a 1000VDC power source would work
fine. 1000 VDC / 2mega ohms = 0.0005 Amps. 1000VDC x
0.0005 Amps = 0.5 watts
There is more. The center tap is actually a voltage
divider and current draw effects things as well. I
could go on but I need to do some work for my
employer.
In short my advice is that unless you are ready to
learn electronics, purchase a ready made PWN circuit
board that will handle the voltage and current you
need and figure a water tight way to rotate the shaft
to the potentiaometer.
Regards,
Ray
--- Akins <lakins1@tampabay.rr.com> wrote:
> Hi Ray.
>
> I had read so many e mails last evening Ray, that I
> forgot who told me about the pwm for dc.
> I realize now it was you and not Ian who told me
> about the pwm. Thanks.
> Ray, do you know if 24 volt pots are made and even
> exist? I know 12 volt does, or else
> there wouldn't be any volume control pots on a car
> radio, but I don't know if 24 volt pots exist.
> If they do exist, since you said a dc pot acts as a
> variable resistor, would that mean the dc pot
> would wastefully eat up some of my battery current
> by heating up a resistance coil or heat sink?
> Or could I be lucky enough that the dc pot just
> allows the current thru that I need, without wasting
> any of it thru heat loss? If the 24 volt dc pots do
> exist, and do not waste battery electricity by coil
> or heat
> sink burnoff, then it sounds like all I would need
> would be a 24 volt pot, a waterproof housing and the
> port with O rings and attached stem that Chris told
> me about. Does this sound correct to you Ray?
> Bill Akins.
>
>
>
> ----- Original Message -----
> From: Ray Keefer
> To: personal_submersibles@psubs.org
> Sent: Wednesday, February 08, 2006 12:17 PM
> Subject: Re: [PSUBS-MAILIST] variable speed
> switch.
>
>
> Hi Bill,
>
> A car's radio potentialometer is a variable
> resistor.
> Changing resistance in the circuit changes the
> amount
> of current or voltage that is passed onto to later
> stages in the circuit. Thus varying the volume. No
> fancy dc to ac to dc conversion.
>
> Regards,
> Ray
>
> --- Akins <lakins1@tampabay.rr.com> wrote:
>
> > A thought just occured to me. Wasn't it you Ian
> who
> > told me this evening that dc voltage uses pulse
> wave
> > modulation to enable a variance in the speed
> switch?
> > If that is true then how does a 12 volt dc car
> > radio's volume control potentiomenter work? Does
> it
> > somehow convert the dc to ac for it's volume pot
> to
> > work and then
> > convert it back to dc again? From what you told
> me
> > earlier about the pwm it is making me wonder now
> if
> > my idea to find a 24 volt dc potentiometer would
> > even work.
> > Does a dc 24 volt potentiometer even exist or am
> I
> > missing something? A little help?
> > Bill Akins.
>
>
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