-----Original Message-----
From: Russell McMahon <apptech@clear.net.nz>
To: Jon <PHXSYS@aol.com>; PICLIST <PICLIST>
Date: Thursday, 14 May 1998 18:07
Subject: Re: Re: Frequency for PWM


>Sorry if this is too laboured - I don't know how much you
>know so I'll try to spell it out well.
>
>If you are willing to follow the PWM output by a filter and
>a buffer (which can be as simple as a 1 transistor emitter
>follower) then you can make the PWM as  slow as you wish.
>
>PWM output to Resistor.
>Other end of resistor has a capacitor to ground.
>Resistor/capacitor junction goes to NPN transistor base.
>Transistor collector goes to V+
>Transistor emitter goes to gauge etc.
>(Real life may get a little more complex but this is a good
>start).
>
>The product of the Resistor value in ohms x the capacitor
>value in farads is the time constant Trc.
>Trc should be "several times" longer than the period of the
>longest PWM pulse (experimentation will teach you more
>faster here than theory).
>
>If you make Trc very long and the PWM slow to match then
the
>gauge may respond too slowly for your needs. A standard
>meter (with relatively fast response) can be used in place
>of the gauge here.
>
>In practice you will find that capacitor leakage and
>transistor base drive (amongst other things) (and you
>willingness to wait for the gauge to move :-) ) will place
>an upper limit on the time constant achievable.
>
>For R= 100K and C = 1uF then Trc = 0.1 seconds.
>PWM at 0.01s LONGEST pulse component would (off the top of
>my head) be somewhat jittery on a meter (but a gauge
>wouldn't be). 1 MOhm/1uF would be OK in this case but as
>Ibase is only about 5v/1MOhm = 5uA you can only supply
about
>1ma out (typical transistor current gains (beta) are 200
ish
>(more is possible)  200 x 5uA = 1mA).  Using a bigger
>capacitor instead would be OK (10uF, 100uF, ...) but watch
>capacitor leakage currents.
>
>Use of an op-map follower as a buffer would reduce these
>problems.
>
>-----Original Message-----
>From: G.Daniel Invent Design
><g.daniel.invent.design@xtra.co.nz>
>To: PICLIST@MITVMA.MIT.EDU <PICLIST@MITVMA.MIT.EDU>
>Date: Tuesday, 12 May 1998 22:37
>Subject: Re: Frequency for PWM
>
>
>>PHXSYS wrote:
>>>
>>> Hello
>>>
>>> I have an application where I want to PWM a FET to drive
>a gauge. Is there any
>>> rule for the frequency PWM should be applied. I don't
>want to dedicate my
>>> 16F84 to a PWM task. Can I run at i.e. 50HZ and complete
>other tasks in
>>> between, kind of like a R/C servo? I suppose it really
>depends on the
>>> characteristics of the driven device, but I am looking
>for some general rules.
>>>
>>> Thanks
>>>
>>> Jon
>>
>>Fuel guage example:
>>turn the ignition on and time full scale deflection(if
tank
>full of
>>fuel). divide this time by minimum of 20 and PWM at this
>period or
>>smaller
>>
>>many car guages use bimetalic strips with nicrome heater
>winding,
>>thermal inertia of this would take next to zero time
>overhead for a PIC
>>
>>regards,
>>Graham Daniel.
>>
>
>