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'PWM speed control[OT]'
1999\11\24@065408 by soon lee

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1999\11\24@090847 by Wagner Lipnharski

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Measuring Voltage is not the ideal way to analyze a PWM waveform.

The old analog voltmeters could do it in someway because the needle
shows you the Voltage average by the mechanical inertia.

The actual electronic digital voltmeters can not. They are terrible to
measure square waves and all, even if set to AC mode. They use an ADC to
convert instantaneous signals to digital and then they show it at the
LCD display, so by doing this sometimes they got the positive side,
sometimes not, and it is not a regular thing, so they display crazy
levels.

Without the motor you are measuring right the digital switched voltage
(if your meter does it correctly).

WITH the motor you are measuring also the motor EMF when the PWM is off,
so in general terms you can not do it safely.  Try to scope the signal,
and you will find out how messy is the signal over the motor; there are
ringings, spurious signals and all.

Wagner

{Quote hidden}

1999\11\24@093140 by soon lee

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thanks for the infor
the wave that i see is not that noisy there are pulse that synchronics with
the pwm with the max at supply voltage and min at about  0.9 Vss
no matter the duty cycle min is about 0.9Vss

5v
___       ___
|      |      |      |    PWM
|      |___|      |
0v
Vss(output)
___        ___         ___
|      |___ |      |____|     |    0.9Vss
|
|
|
0v
so anyone have any idea what when wrong??
thanks

regards


{Original Message removed}

1999\11\24@101237 by Wagner Lipnharski

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If you are driving a transistor connected to ground, it means the motor
has one lead to VCC, than it seems to be an under driving  situation.
This should be the reason why your voltmeter shows the same voltage no
matter which pwm signal you use. The transistor is not switching
completely to ground.  Why don't you show a simple schematic about this
driving design?

Wagner

soon lee wrote:
{Quote hidden}

> {Original Message removed}

1999\11\24@103354 by soon lee

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Wagner wrote

> If you are driving a transistor connected to ground, it means the motor
> has one lead to VCC, than it seems to be an under driving  situation.
> This should be the reason why your voltmeter shows the same voltage no
> matter which pwm signal you use. The transistor is not switching
> completely to ground.  Why don't you show a simple schematic about this
> driving design?
>
> Wagner

the circuit that i have use is actually the circuit that is found in
appication note for L298/L293 (bi-direction)
the only thing thing i did not follow is tha used of diode at the output and
the others all the same


thanks for your help

regards


Related: :wonderland:clip_image002.jpg:00010010:38954A21:00000000:00000000

1999\11\24@114029 by Wagner Lipnharski

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If you followed exactly the bipolar SGS example, apparently everything
is ok.

a) You should use the diodes.

b) Check again all connections and possible "invisible short circuits"
at your circuit board. Don't assume everything is ok just because you
did it by yourself.  When I have a problem and can't find it, I go back
to the basics, checking grounds, VCC's, logic levels, every single
connection, check pin by pin in sequence, as if it was done by somebody
else with an error in purpose to test you, suspect everything, don't
assume things are correct. Changes of having a wrong connection, bad
assembly or short circuit somewhere is just 99.99%, with 1 in 10
thousand for a bad chip (except if you fried it).

c) You can also try to use a 100 Ohms 10 Watts resistor, or use a 100 to
200 Watts (115Vac) normal incandescent lamp in place of the motor, and
repeat the measurements.  The 200 Watts lamp should represent a
resistance around 60 Ohms when lit, something less when not complete
warm, mainly for DC supply, probably will drain around 1 Amp from the
30Vdc supply. If you are afraid to force too much the L298, use a 100 W,
or even a 60W lamp instead.

Wagner.

soon lee wrote:
>
> {Original Message removed}

1999\11\25@075541 by soon lee

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Hi Wagner
thanks for the advice : )
regards
----- Original Message -----
From: Wagner Lipnharski <spam_OUTwagnerTakeThisOuTspamUSTR.NET>
To: <.....PICLISTKILLspamspam@spam@MITVMA.MIT.EDU>
Sent: Thursday, November 25, 1999 12:36 AM
Subject: Re: PWM speed control[OT]


{Quote hidden}

motor
> > > has one lead to VCC, than it seems to be an under driving  situation.
> > > This should be the reason why your voltmeter shows the same voltage no
> > > matter which pwm signal you use. The transistor is not switching
> > > completely to ground.  Why don't you show a simple schematic about
this
> > > driving design?
> > >
> > > Wagner
> >
> > the circuit that i have use is actually the circuit that is found in
> > appication note for L298/L293 (bi-direction)
> > the only thing thing i did not follow is tha used of diode at the output
and
> > the others all the same
> >
> > thanks for your help
> >
> > regards

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