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Old 11-28-2001, 10:16 AM   #46
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Yes that's right. Most ESC are fixed frequencies.... they dont actually vary according to speed. The are generally work on the PWM principle. They vary the width of the pulse which translate to the RMS power available to the motor, the wider the width, the smaller the gap and vice versa. Of course when the pulse width reaches maximum, it is a straight line ie the gates are all switched on, which means frequency ZERO, but dont forget that the timing that governs tha gates these wide-boys are still clocked are the same frequency to check for the spee controller input status.

thats my 2 pennies worth....
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Old 11-28-2001, 10:26 AM   #47
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Hey all, I've been reading this thread for quite some time and just had to sign up and put some info on this topic. I too am an EE BTW, and I don't want to step on anyones toes here but that cap has nothing to do with Fourier series or anything. Actually the 1 Farad cap would work just as fine as the one that came with the ESC except...Well, it's like killing a fly with a Bazooka. It's over kill. All that would happen with a larger cap is that you would have to wait for the cap to become fully charged in order to use the power. That is why you see on off road buggy's they have caps in parallel, parallel caps add thus creating a longer charge time, which makes the trigger delayed a bit. And for off road from what I have heard this is a good thing.
Basically, all that cap is used for is to make sure that the voltage coming off the battery doesn't have any spikes in it. The cap makes sure that the voltage going INTO the ESC is fully flat at 7.2V. The reason why it says 16V on the cap is because the people that made your ESC wanted to be safe and not have it explode on people.
I could understand about the Fourier series and things taking place inside the ESC but this cap is put on input side of the Bat.: + and - of the cap goes on the + and - of the Bat. It doesn't do anything but filter, and used as a "punch buffer".
Well I hope this helps and sorry if I offended anyone; it wasn't my intent.

Grav.

Last edited by Gravity; 11-28-2001 at 01:47 PM.
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Old 11-28-2001, 10:28 AM   #48
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At steady state where everything is constant the cap will most probably make no difference, but a car in a circuit will always have different current demands... and here is where the cap will help, because of the high internal resistance of the NiCad or NiMH cells....

About the sweet spot, it is not just about the ESR which is generally the lower the better. But its value is the important one.... too small will not be sufficient, and too large will actually be counter productive. I think unfortunately the sweet spot value is very dependent on the many variables, motor , car, circuit power band used etc.... so it should be taylored accordingly.

The main problem of the power source be it NiCad or NiMH cells are the fact that they have high internal resistance....... just try and use a lead-acid battery in place of the NiCd and see the difference!!! Lead Acid can give absolutely huge instantaneous currents which can fry almost anything.... it is just too bad they are heavy and can spill!!!
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Old 11-28-2001, 10:35 AM   #49
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If it is for reducing spikes then a small value is probably better than the large resevoir caps used.... the battery is in its own right a large cap.... so it will keep the volts steady.... while the small ones keep the spikes at bay.....

If the cap is for instant power punch, then a suitably large one will compensate the batteries' inability to give off large currents in an instant...
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Old 11-28-2001, 11:58 AM   #50
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Sonneteer, obviously you didn't understand me.
Gravity, I just don't agree with you.
One thing is for sure though, there are too many EE's in here Anyhow it's nice to have so science on this board also.

Jesse Tuominen
Metrology Research Institute
Helsinki University of Technology
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Old 11-28-2001, 04:18 PM   #51
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JT, maybe I did misunderstood you... but I was not only referring to you comments

All I am saying is that the sweet point is hard to define when not operating in steady state... its quite a trial and error job

For keeping spikes on the batter line, I am saying that electrolytic caps are not suitable, better to use a small low K type cap.

ESC generally are fixed freq, not varying....as regards to square wave output, it is if driven into a resistive only load, but when driven into an inductive load, the waveform is far more unpredictable....

My main experience is from designing from microwave amps to hi power amplifiers capable of driving 2 Ohm speaker loads (ie motor), the instantaneous current sometimes reach very high peaks..., and I "tune" power caps to achieve a broad range of peak current ability for all frequencies....6 Hz to 40kHz..... My latest project was a differential push pull Digital amplifier using +/- PWM, not disimilar to ESC....... check out my website and click on Bronte logo....

Thats all the techie stuff for now.... I am taking my car for a good hard drive.....
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Old 11-28-2001, 04:22 PM   #52
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Oh JT I forgot.....

R. Casadei
M.Eng (Electronic Systems) A.M.I.E.E AMPEE 1/2

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Old 11-28-2001, 04:22 PM   #53
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I think we all could use a hug. Oh, ohh, me, me first.
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Old 11-28-2001, 04:40 PM   #54
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there there.....

I just couldn't resist that one....
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Old 05-01-2003, 04:11 PM   #55
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Why do KO Propo use 35v caps?
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Old 05-05-2003, 05:42 AM   #56
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Nobody knows?
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Old 05-05-2003, 11:01 AM   #57
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Jesse,
If you look at the form of the fourier series then you will see that the coefficients get smaller as the frequencies of the sinusoids get higher. This means that the high frequency terms are relatively insignificant compared with the first few (low frequency) terms.

The result is the optimum capacitor size will be determined by the lowest frequency in the series which I think I'm right in saying matches the switching frequency of the speedo.
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Old 05-09-2003, 09:03 AM   #58
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I am NOT an EE but I do have a question that I hope fits this thread/topic ... I have the LRP quantum competition ESC and just can't bring myself to pay $25 for the two capacitors in their "Worlds Upgrade" kit ... LRP doesn't mark the caps with any specs so you can't just go out and buy them at Radio Shack. I've posted on other boards to see if anyone could provide the specs for the Worlds Kit capacitors, but nobody seems to know. If someone on this thread could solve that riddle I'd be much obliged ... as in I'd be happy to pay you a more reasonable amount if you can keep me from paying $12.50/cap! And I don't think I'm alone, you could probably sell quite a few online if the price is right.

-John
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Old 05-09-2003, 09:51 AM   #59
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Quote:
Originally posted by Luckyman4
I am NOT an EE but I do have a question that I hope fits this thread/topic ... I have the LRP quantum competition ESC and just can't bring myself to pay $25 for the two capacitors in their "Worlds Upgrade" kit ... LRP doesn't mark the caps with any specs so you can't just go out and buy them at Radio Shack. I've posted on other boards to see if anyone could provide the specs for the Worlds Kit capacitors, but nobody seems to know. If someone on this thread could solve that riddle I'd be much obliged ... as in I'd be happy to pay you a more reasonable amount if you can keep me from paying $12.50/cap! And I don't think I'm alone, you could probably sell quite a few online if the price is right.

-John
Try digikey.com ..

Only catch is you have to order minimum of $25.00 or you will be charged $5.00... They do sell some Schotty Diode.. Or any electronics components for that mater.

I got 10,000 uF 16VDC from them, I think I paid $3.00 for it and these were MIL spec, individule packed.
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Old 05-09-2003, 10:57 AM   #60
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Komkit,

Thanks for the tip ... I'm not quite sure I'm reading your reply right, does the LRP World's Kit contain a "10,000 uF 16VDC" cap and is that the "big one or the little one" [lol, showing my level of technical expertise with that question]. My problem is I don't know the cap ratings for the two caps in the World's kit, nor do I know how to figure it out since the caps are unmarked, "plain blue wrapper" caps. Is there a way to measure a cap that is unmarked?

Here's a link to a pic of what you get for $25 from LRP
http://www2.towerhobbies.com/cgi-bin...?&I=LXBDR9&P=0
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