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'] Design Challenge - Lowest cost PWM'
2004\07\09@063038 by hael Rigby-Jones

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>-----Original Message-----
>From: Russell McMahon [spam_OUTapptechTakeThisOuTspamPARADISE.NET.NZ]
>Sent: 09 July 2004 10:37
>To: .....PICLISTKILLspamspam@spam@MITVMA.MIT.EDU
>Subject: [EE:] Design Challenge - Lowest cost PWM

>A 555 MAY be able to do most of this by itself. Prove it :-)
>

Many years ago I designed a motor speed controller using 556, one timer
configured as an fixed fequency astable, triggering the other timer
configured as a monostable with variable pulse width (using the control
voltage input).  This drove a car starter motor via a bank of about 20
2N3055's (and a high power driver stage, suitable MOSFETS were very
expensive at the time, and 2N3055's were dirt cheap on the surplus market).

Linearity won't be good with the standard monostable due to the capacitor
being charged by a resistor, and compared to the reference.  Replacing the
resistor with a simple constant current source (maybe a jfet or constant
current "diode") should improve this considerably.  Getting 0% and 100% may
be trickier!

You might also look at the retriggerable monostables available in the cheap
logic families, perhaps using a simple voltage controlled current source to
charge the timing cap.

Regards

Mike

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2004\07\09@064113 by Alan B. Pearce

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>>A 555 MAY be able to do most of this by itself. Prove it :-)
>
>Many years ago I designed a motor speed controller using 556, one timer
>configured as an fixed fequency astable, triggering the other timer
>configured as a monostable with variable pulse width (using the control
>voltage input).

Russell
I worked at the DSIR in Gracefield, Wellington, for a couple of years, and
one of the projects there was a 5V 10A switch mode supply (primarily to get
experience with mains switchers) and it was done with a couple of 555's
exactly like this. We did it back in the late 1970's.

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2004\07\09@080318 by Lindy Mayfield

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Would a PIC12F675 using internal osc with code to control the PWM and AD's, with a linear or log pot as input, or with the PIC using it's A/D pin as input.  The PIC code would be minimal, and certainly offset if the volume produced is high.  
Would that work?
_____
Lindy

> Subject: [EE:] Design Challenge - Lowest cost PWM
>
> [EE:] Design Challenge - Lowest cost PWM
>
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2004\07\09@090426 by Russell McMahon

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> Would a PIC12F675 using internal osc with code to control the PWM and
AD's, with a linear or log pot as input, or with the PIC using it's A/D pin
as input.  The PIC code would be minimal, and certainly offset if the volume
produced is high.
Would that work?
>

No hardware PWM I think. And cost is rather high compared with alternatives.
Using Digikey as a guide (there MAY be cheaper but DK is easy to get
comparison prices from). 12F675 is about $US1.35/1000s

The tiny11 (also no hardware PWM) is $0.25 or less in that quantity! The
question as to suitability of low frequency PWM is the only issue.

_____________________________

> ATTiny13 would seem a good fit - internal RC osc, on-chip ADC,  internal
RC at up to 9.6MHZ
you could get about 0.1uS PWM resolution in software, so about 0.2%
resolution at 20KHz
>

A nice processor, but again, price is too high. About $US0.75/1000s
And still no hardware PWM AFAIK.
My PWM spec of at least 6 bits of resolution at 20 Khz means a PWM bit
changes at 1/(20,000 x 2^16) = 0.78 uS
At 20 Mhz you probably get about 15 MIPS (even though Atmel say 20 MIPS at
20 Mhz. If this sat in a tight loop doing nothing but PWm you would probably
make it. 15MIPs * .78uS = ~ 12 instructions per PWM bit! Not really enough
to run interupts. Reading an A2D in there would get tough. I've little doubt
that Scott D could coax out the magic needed, but it still costs more than
the analogue solution and does the PWM and nothing else.

______________________

Someone offlist suggested the Philips LPC901 8051 variant at under $US1 -
and it has got hardware PWM. No A2D. Sigma Delta would work.
________________________

At present the tiny15 at about $US1.20/1000's with A2Ds and hadware PWM is
attractive
I have a solutuion working with ease with a tiny26 at $US1.30/1000s. Big
brother to the 15 at about $US0.10 more.
_____________

The HIP 4080 addresses the FET driving aspect but is dearish compared to
other laternatives and doesn't seem to address the analog to PWM aspect
(maybe I missed something there).

____________

I'm going to have to try a 4060 version "just to see". And read the LPC901
spec sheet more closely

       http://www.acqura.com/datasheets.asp?ID=499



       RM

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2004\07\09@120132 by Mike Harrison

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On Sat, 10 Jul 2004 01:04:22 +1200, you wrote:

{Quote hidden}

You can get one-cycle software PWM resolution on a PIC or AVR, using either a short jump table to
trim the PWM period, or timer interrupts. e.g.
addwf pc
nop
nop
nop

will give you a 2 to 5 cycle trim for W values of 3 to 0

Timer int would normally be the best way as you'd have plenty of foreground time to play with. Any
trickiness dealing with the whole 0..100% range can be overcome, if necessary with more than one
piece of code.
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