AVR-Chat

Robert Adsett wrote:

>At 11:18 PM 1/8/06 +0530, Anurag Chugh wrote:
>  
>
>>|why only PWM?.. why not use an R2R ladder instead with precision resistors...
>>|i know they would hog a whole PORT but arent they easier to implement ??
>>|and they can do DC output values too... a simple opam in front (voltage 
>>|follower) will boost current output too...
>>   Not quite in the same line of thought but no one has mentioned the other 
>>reason to prefer PWM over R2R in some situations and that is power.  Feed 
>>the PWM into a power device rather than an RC filter and with a slowly 
>>responding load like a heater tank or a motor the load itself acts as the 
>>low pass filter and you avoid the losses that a linear solution would 
>>entail.  At the cost of ending up with high frequency current transients of 
>>course.
>>    
>>
  Quite so.  In the  case of the purely resistive load (e.g., a heater 
element),, a complete win.  In the case of rreactive loads such as 
motoros or speakers, the high frewquency transients *CAN* generate a lot 
of heat.  One reason, among many, why it is best when choosing induction 
motors for PWM speed control to seelct motors rated for "inverter service.."

   They have internal insulation that is specoificed to handle both more 
heat, and higher voltages (the high frequency currents can also result 
in high voltage spiking withint the motor windings).

   When worlkgin with PWM for audipo (e.g., Class "D" a,ploifiers) one 
must either use specially designed class D driver chips which use 
techniques permitting filkterless output, or provide appropriate output 
low-pass filtering.  Low pass filtering for a 10 Amp AC circuit can get 
expensive, which is why filterless driver chips are all the rage with 
designers and chip vendors.

   Withoiut the lowpass filter, or a filterless driver chip, you run a 
sigificant risk of cooking the voicecoils in your speaker drivers.  
Particularly in the tweeters.  That can get expensive pretty doggoned fast!

tom