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Old 04-11-2002, 12:00 PM   #16
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David
Sorry but the maths says you are wrong in theory.

The only potential factor that could cause the difference in punch you claim you feel between using three diodes and 1 (assuming similar total capacitance) is the resistance of the wire between the capacitor and the speedo which will be reduced with three capcitors. I really do not believe this is significant enough to feel on the track. If however you had a bad solder joint to your large capacitor it could become a significant difference.

The other reason that i forgot about for running more caps with a low turn motor is impedance. Without knowing much about the circuits involved I don't know how significant motor frequency is relative to the speedo switching frequency. If we assume that the motor switching frequency is significant then lower turns will need more capacitance. The reason is that the impedance of a capacitor increases with frequency. In order to drop the impedance you need a bigger capacitor.
If you don't know what impedance is think of it as being like resistance to all intents and purposes.
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Old 04-11-2002, 12:14 PM   #17
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Not sure if this helps but I was running 3 16v 680uf (is uf the correct term?) 105degree, and they were the blue shrinked not the cheap browns. The large is a 16v 3,900uf 105degree.

Like I said I am sure I am wrong in theory, this is what I got out of on the track feel and it is how I run it at the track.
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Old 04-11-2002, 12:50 PM   #18
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uf is the wrong term but as you can't type greek letters on this board it'll have to do.
The 3 capacitors is equivalent to one large capacitor of 2040 uf.
This means your big capacitor is roughly equivalent to 6 small ones. So the larger capacitor should be better than 3 small ones.
The thing to look at is what are the relative weights of the various capacitor combinations. .
The size of the capacitor really isn't as significant as people would have you believe. If it was all the speedos would come with huge capacitors as standard. The manufacturers aren't that stupid.
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Old 04-11-2002, 04:00 PM   #19
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when you initially get on the throttle, there is a HUGE voltage drop (maybe up to 2v) accross the battery because the load is so high, the capacitor smooths this out so perhaps you only get a 1v drop.

the bigger the cap, the less this drop is however, if you repeatedly jab the throttle the cap may not fully charge up so you lose punch (this would only happen if you had a massive cap!)

1 cap of value 10,000uF is superior to 3,333uF because there is less ressitance in the circuit, if you have 3 caps, there are 3 lots of wire and 3 lots of dielectric material etc.

hope this clears the confusion.

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Old 04-11-2002, 08:14 PM   #20
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Quote:
1 cap of value 10,000uF is superior to 3,333uF because there is less ressitance in the circuit, if you have 3 caps, there are 3 lots of wire and 3 lots of dielectric material etc.
Rubbish! This would be true if they were wired in series but they are wired in parrallel so resistance gets less with more caps.

Resistors in parrallel:
R=1/(1/R1+1/R2......+1/R(n-1) +1/Rn) where Rn is the resistance of the nth resistor

How the dielectric is split up is irrelevant . It is the total surface area that matters unless the caps are wired in series..
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Old 04-12-2002, 02:34 AM   #21
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i assume yours is based on the assumption that the resistance of the 10,000 and 3,333uF is the same.

well realistically its not, the resistance of the 10,000 is less than the 3,333 wired in parallel. the main restriction is the diameter of the wire, its very small on most caps.

I am aware that it is the total area of the caps that matters, but the leakage also makes a difference, in reports i've read as part of my job research is done on caps and they've found leakage is higher in smaller caps than bigger ones.

don't get me wrong electrical engineering is not my forte, but i'm pretty sure what i say is right.

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Old 04-12-2002, 05:28 AM   #22
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I already promised myself earlier not to get involved in this again. BUT. I have suggested earlier twice to read the earlier topics related to this subject. Obviously you haven't.
As "dw" stated, most theories on this discussion are rubbish. One thing for sure is that a capasitor of some 1000s of uF can not provide any significant current to the motor for longer than some microseconds. This surtainly doesn't help the punch at all. The capacitance is however just large enough to provide current for the on period of the PWM. This helps keep the speedo's temperature down. Keeping the temperature down makes it function more efficiently. The capacitor also helps the batties as the current flow from the battery becomes more DC. This helps on the efficiency again. It's better to take for example constant 40A form the battery to the cap. And form there give 50% of the time 80A and 50% 0A. Taking the 80A and 0% directly from the battery would result in a lower battery voltage.
At full throttle (no PWM) the speedo does very little as the current waveform of the motor is closer to DC.
Also replacing the speedo's original 680 uF (or something) with a general purpose over 2000 uF capacitor might be a jump to worse. The impedance at higher frequencies is more important than the Farads.
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Old 04-12-2002, 08:32 AM   #23
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mine is not my own theory you bleeding muppet.

its from a text book, and if you go on the MRT website and e-mail them, they'll say the same thing, you need a cap to smooth out the voltage drop.

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Old 04-12-2002, 09:22 AM   #24
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JesseT - You can't get at the older topics because for some reason the site won't list anything in the archive older than 120 days. (yes I am setting the option to list topics older than this.)

Schumacher: Calm down, we all think our theories are right. We only learn one way or the other through disagreement. Nobody is out to offend you. If you actually read and understood what Jesse was saying you would realise that he too was saying that the capacitor is to smooth out the current flow. The more you smooth the current the closer you get to true dc rather than switched dc.
It's the difference between various capacitor sizes and combinations that we are not in agreement on and your opinion of the initial cause of the voltage drop.
You also look pretty stupid telling someone who is an electrical engineer (if he's who I think he is) that they are a bleeding muppet after just admitting that electrical engineering is not your forte!

Leakage current: The capacity ratings are only nominal. I really don't think that differences in leakage current and fringe effects are that much of an issue. You will not get the exact capacitance anyway, same as you wouldn't expect a nicad to have exactly 1.2 volts.

As for your wire diameter theory you would have to have a wire over 3 times the area fitted to the larger cap. I could be wrong but I'll think you'll find that until you get to much bigger differences in capacitor size that they all use the same wire gauge anyway.

Anyway that's my opinion. Feel free to disagree if you know better .
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Old 04-13-2002, 05:51 PM   #25
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has anyone tried the integy 8800 uf cap?
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Old 04-15-2002, 01:59 AM   #26
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dw:
Thanks for setting me straight. I didn't know that the older topics were out of reach. Just to summarize what I tried to say earlier. The capacitor improves the partial throttle accelerations considerable. Also saving battery energy and keeping the ESC cooler. However, the energy stored in the capacitor does not help improve the full throttle performance.
I'm making finishing touches to my home built dyno at the moment and perhaps I can provide some actual numbers to back up my claims.
Best Regards to you all, JesseT
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Old 04-15-2002, 12:20 PM   #27
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Hey guys mine is a 10,000uf 10V. Its probably doing just fine.
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Old 04-17-2002, 11:09 PM   #28
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where do you guys get your monstrous caps?
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Old 04-18-2002, 03:33 AM   #29
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radio shack,

i've got a 10,000uF 35v cap 105c, the 35v doesn't matter, anything over 7.2v will be ok. and the 105c, it never gets that hot in the uk!

have a nice day

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Old 04-18-2002, 10:10 PM   #30
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the hightest i found was a 6800uf at 25v and 85c
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