so I have a lm339 and a TL082 and need a speed of like 100 KHz will any of these do ? I found the slew rates on the datasheets but am sure how to make this into frequenzy. if these won't do it what will ?

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Hi,
You have to look for the propagation time.
The LM339 is ok qs it is 0.5us.
I think there are many, to select one you need to check supply voltage, current, single, dual?
Then I can help you.

 

if you want a fast Comparator the LM319 is extreamly fast

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Presumably you're asking this about PWM again?, it would make a LOT more sense if you made that clear!.

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yes I am as you may have seen I finally managed to simulate a saw tooth generator and begin to wounder what is best simulation or real building when I tried to ad the comparator it gave an error. crappy software designer.
so yes now I need the comparator and scrap the flipping simulation, which do you recomend I would like to go up to 100-200 KHz if possible but at the moment want to start with a motor speed control.

__________________
I AM the exeption that disproves the rule in many ways but the rules still apply (unfortunately)

my site:www.simons-photography.com
http://rushdenrotaract.org.uk

 

Doesn't matter much for motor speed control, speed isn't an issue - but needs to be MUCH, MUCH, MUCH lower in frequency than that!

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Unless there is a current-loop in which a faster PWM is beneficial to reduce torque-ripple (cant go too high due to inductance of machine making it pointless)

another case of too little infomation me thinks

__________________
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Once a problem is realised, the rest is just details

 

yes you have a point there so what speeds should I use for a motor it is the internal air fan motor of a car so i suppose 5-8 amps
in any case how fast can the lm339 go, how do i work that out from datasheets for any comparator / op amp

__________________
I AM the exeption that disproves the rule in many ways but the rules still apply (unfortunately)

my site:www.simons-photography.com
http://rushdenrotaract.org.uk

 

there are factors envolved on a data sheet slew rate time from 10% to 90% and propagation time or time it takes to come out of saturation. unfortunatelly it depends some on the input but basicaly is typical. next thing to consider is bandwith of the amplifier for speed. And by the way an engine running draws about 6-8 amps from the generator not much from battery i think that unless you reduce some current someplace your battery will drain and even possibly destroy your generator look into it. battery is to start the car after the engine is started generator takes over and recharge the battery with an extra load of what you say 6-8 amp it may never get recharged think about that. after start you may remove the battery and the engine will still run.Me capisce signore buono. Stai bene per piacere.

 

oooh CAPISCO tu non capisci, perhaps er I was not specific enough ? the battery and fan are out of the car (the car is no more) and so the battery is charged from the mains and used to feed the fan or what ever else and then is recharged when flat. or I have an old computer power supply ! anyhow the fan would be part of the car any how and the pwm would have made it more efficient as it is resitors

__________________
I AM the exeption that disproves the rule in many ways but the rules still apply (unfortunately)

my site:www.simons-photography.com
http://rushdenrotaract.org.uk

 

I am having a similar problem with a 339. Running at 30 kHz and output is far from "square". I've not had enough time lately do work on the problem. I'll post more if/when I get time.

__________________
stevez

 

you should have posted a diagram or schematic so we all can see your application. by the specs i don't see amy problem if it is running at 100kc. it has a 5mv offset and 5 mv drive and looks like if the input drive is 100mv it streches to 1.5usec that relates to 666kc max operation with 5mv overdrive it is more the 1 Mkc. then basicly it is drive input as a function of overdrive.now at these max speed don't expect to look like a square wave not unless you provide some hysterisis which will make square and reduce freq. of operation.If you want speed you should have choosena lm119,219,319. same packgage but only 2 per.conclusion need overdrive to make it square need less drive to make fast take your pick.

 

Actually, the LM339 (and most other comparators) gets faster with more overdrive. See the graphs below, which are from the National datasheet.

Attached Images

LM339 response times.PNG (45.0 KB, 10 views)

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Ron

 

If your after very fast switching times, you should look at op-amps like these:

http://www.rapidonline.com/productin...moduleno=34442

The 339 isn't very fast, i'm pretty sure your better off with the 082 out of them both. However, you won't need, nor would it be advisable, to switch at frequencies as high as 100khz for ordinary PWM controlling. That sort of speed is only really required in class-D audio amplification.

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Angry!? I'm absolutely electrolytic!

Will have to make do with myspace now I guess...

 

or use a LM319 as I have said

__________________
Nothing is impossible.
Once a problem is realised, the rest is just details