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PICList Thread
'pwroblems with pwm'
1998\01\04@181336 by Tom Sgouros

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After noticing just such discussion last month about using the PWM
module on a PIC to adjust LCD contrast, I designed a program to do
just that.  The problem is that the voltage output from the PWM
through my RC filter only works properly on the oscilloscope.  There,
it cycles nicely from 0 to 5v (although I get the impression that the
upper ranges are largely useless).  However, when I connect it to my
LCD module (run by a Hitachi whatever-it-is), it only goes from 2v to
5v, leaving the displayed characters largely invisible throughout the
range.

The filter I am using is just an RC filter.  I thought that since I
want 0-5v, I could leave the op-amp off my integrator.  Am I wrong?

I guess this is more an electronics question than a PIC question;
perhaps somebody could advise me whether this is [OT] or not.

-tom

p.s. I have consulted what I believe are all the relevant Microchip
PWM AN's, but I still wonder why I can't leave off the op-amp.

----------------------------------------------------------------------
tomss at ids.net - 401-861-2831 - 42 Forge Rd, Potowomut, RI 02818 USA

1998\01\05@013541 by Mike Keitz

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On Sun, 4 Jan 1998 18:17:32 -0500 Tom Sgouros
<spam_OUTtomfoolTakeThisOuTspamCIRCUS.GSO.URI.EDU> writes:
>After noticing just such discussion last month about using the PWM
>module on a PIC to adjust LCD contrast, I designed a program to do
>just that.  The problem is that the voltage output from the PWM
>through my RC filter only works properly on the oscilloscope.  There,
>it cycles nicely from 0 to 5v (although I get the impression that the
>upper ranges are largely useless).  However, when I connect it to my
>LCD module (run by a Hitachi whatever-it-is), it only goes from 2v to
>5v, leaving the displayed characters largely invisible throughout the
>range.

The LCD voltage input is connected to a resistor voltage divider in the
module (with the other end to Vdd).  It thus has a finite input
impedance, and will draw more current the lower the voltage is pulled.
Likely the output impedance of your filter is too high.  Either reduce
the resistor value (and increase the capacitor value) or place a buffer
amplifier inside or after the filter.

The need to occasionally reduce the voltage all the way to ground means
that a very low impedance is required.  This would cause the filter to
dissipate a (relative) lot of power in the resistor, and require a rather
large capacitor.  An amplifier is likely a better approach.  Most of the
single-supply op amps have no problem pulling the output voltage close to
ground, it's the range toward Vcc (which you don't care about) that
seperates the different models.  Having an op-amp makes it possible to
build a better filter too.

It's also a good idea to add some bias voltage so the PWM range goes only
from 0V maybe to 2.5V, then all the PWM settings produce a potentially
useful voltage for the display.

1998\01\05@173317 by Harold Hallikainen

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       Sounds like the LCD control line is loading your RC filter.  If
you can lower impedances (smaller resistor, larger capacitor) without
stuff getting out of hand, that might take care of it.  Otherwise, i'd
follow it with an op amp voltage follower.  That otta work!

Harold

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