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Old 02-15-2002, 01:50 AM   #16
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Alvin

Well said. So how do you rate the right cap on your esc? Are there values to follow on the cap? Like 10,000uf, 20,000uf or 12V,16V? What matches the speedos better?
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Old 02-15-2002, 04:03 AM   #17
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I recommend you use a 16V version. 10V is fine but as mentioned it is always better to have extra headroom so not to stress the CAP, especially in high temperatures.

See if there is a temperature rating written onthe cap. I suggest to use a rating of 105 (degrees celcius, not F!) The reason is also for reliability.

Now the difficult bit is the ESR, the lower in theory is better. This relates to the quality of the Capacitor.

As to its value, I have seen people using fronm 1500 to 22000uF. What is the difference? Well, this is the sort of a black-art part. But lets see if we can decide this after knowing what is the CAP there for:

NICADS and NIMH generally has a high internal resistance, that means it is slow to react to small spikes ir instant current draws(whether from interference from high current fluctuations). Also one can consider that it will have a limited instantaneous current flow limited by its internal resistance.

I read many ESC instructions saying it needs a cap for the ESC to run better, this is normally because the new generation of ESC can only be so small without the cap in question. So they take it outside! Now no one is complaining about the size of their ESC! I mean this is really silly but it is obviously working for them. Who want s a brick sized ESC anymore??? Just think about it.... they used to have them inside, but not anymore??? why is that then??? I tellya, they can shrink IC's and transistors, but when it comes to storage caps, the size is always an issue..... it happens in all fields of electronics....

So what do we get out of it...?? Well, thing is, by using a different type of CAP does have different effects on the outcome in performance.... A large cap with a low ESR can actually supply a much higher instant peak current than any type of battery used in RC. But a large CAP is not so good at filtering small spikes of interference and it takes longer to replenish it self..... So some people noticed more punch with a large'ish cap, but they do try a few within a range and can notice there is a sweet spot value they prefer (and bigger is NOT always better!), this can be related to the operating frequency of the ESC and basically there can be a cap value that works best with the ESC freq. The sweet spot I supposed it is where it works best under the given operating conditions i.e. the cap can charge and discharge most effectively within the operating frequency of the ESC.


As for supression, the rule of thumb is if you want to eliminate as much intereference as poss, the supression device (CAP/Shottky) it has to be as near to the device you want to supress, like soldering the diode and cap on the motor is always better than somewhere down the line where the interference can radiate from the wires. The CAP should be as close to the ESC as possible so that the little distance of wire from the CAP and the ESC have little chance of picking up intereference. Naturally I hope there is a small cap inside the ESC to take care of small spikes which the large cap are unable to filter....

Anyway, that's my 1 penny worth!
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Old 02-15-2002, 04:06 AM   #18
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On a different note, I have having probs on sticking an image under my username.... any pointers?
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Old 02-15-2002, 04:43 AM   #19
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Sonneteer

I suggest you log in first then go to control panel. Then edit options. You go further down and press replace avatar. You need to have a gif or a jpeg pic file so can choose what to put there. It requires a certain size so it may fit as your avatar.






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Old 02-15-2002, 05:46 AM   #20
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Silly me... thanks!...
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Old 02-15-2002, 05:50 AM   #21
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Then let's not forget about the wiring too. The cells, connectors and the power wires might have a very low impedance at DC (resistance), but the current is not DC, but square-wave AC, and the impedance of the whole system is definately much higher at 2-20kHz than at DC. The point of the capacitor is also to keep the current more or less DC to the ESC/capacitor point and thus minimize the power lost.
Also if your motor is drawing at some point a time average of 30A with half throttle, this means without the capacitor that the current is half time 60A and half time 0A. With a large capacitor the current tends to be around 30A from the battery all the time and the power lost (P=RI^2) is smaller in the later case. Your batteries thus will last longer.
Also if you are totally lost, read the previous discussions on this topic also if you can find them. It's sometimes tiresome to explain things over and over again.
Isn't this explained anywhere in the net? Somebody should put up a RC-physics website ;-)
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Old 02-15-2002, 05:54 AM   #22
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Jesse T:

Who drives with "Half Throttle" ????
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Old 02-15-2002, 05:19 PM   #23
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The answer to this is easy. It's how to explain it that could be more difficult. You guys are being taken for a ride on this one, believe me. Some manufacturers make their ESC's as small as they can because they know that size matters to many folks out theres in 'racin land'. If it's smaller than the rest it's got to be better, right?, nope it's WRONG!!!

Here's the answer (for those that want to know the truth not the 'hype') if a ESC is designed properly it can have the correct capacitor inside the ESC on the circuit board, the same is true about a schottky diode. On a 'regular' size ESC with all the gizmo bits inside (where they should be) you don't have to worry about what to connect outside coz it ain't needed. It can run perfect without anything added on the outside. It runs proper and cool and efficient, it's just that to cheat to make the ESC smaller (and more 'neat/cool' for us racers to wanna buy) they need 'em strapped on the outside coz there's none inside. If you run without or with the wrong type the ESC gets hot and can go BANG!!!

It's true that using extra ones outside of the ESC can improve the ESC performance but ONLY coz there's none inside and with none/wrong ones outside the ESC is NOT running proper and gets hotter and is less efficient. With a good ESC with all the bits inside using extra caps/schottkies outside dow'n make nothin better it's already the best it's gonna be!!!

Do you get that? (or are you still beleivin the lies)

You'z guys can run 'em outside, it's not bad, a 1500uF or higher 'low ESR' 16V electrolytic cap soldered on the ESC, and a schottky soldered direct on the motor is best you're gonna get.
They'll reduce harmful voltage spikes but they ain't gonna give you massive more punch, believe me I know.

Some ESC's can run at higher frequencies (faster switching speeds for the FET's) the higher the freq the hotter the FET's get, so a capacitor is needed more, and a big one keeps it runnin cool At low freq it runs cooler and don't need a cap. But even a massive big cap don't store enough to add any more punch it'd need to be as big as a oil drum to notice. It's all hype to sell worlds kits etc and get more $'s outa ya!!!

And,
the FET's are worse on the small size Quantum's etc, that's why they been havin all the trouble we hear about with blue smokin and a load of trouble!!!
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Old 02-16-2002, 12:34 AM   #24
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ProMax,

Amen.

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Old 02-16-2002, 06:47 AM   #25
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Default capacitors and shottkey diodes

Promax, your partially right. For the shottkey diodes it is avantageous to have them right at your motor, although they can be build into the ESC. Here is the explination why:

Any magnetic device that has electricity flowing through it will want to continue its flow of electricity even after the source is removed. What the diode does is allows this electricity to dissipate before reaching the power FET's of the speed control. The diode dissipates the power because it has a lower forward voltage drop than the FET's. Since electricity will flow through the path of least resistance it dissipated through the diode. IF you place the diode in the ESC now the voltage drop of the diode must be added to the voltage drop from the wires back to the ESC. An you have effictively used a diode with greater forward voltage drop. So the less voltage drop there is before you get to the diode the better it will be able to protect your FET's. The stronger the brakes of an ESC the more likely it will need a shottkey diode. When I talk about voltage drop Im taking from the motor to the diode and ESC btw.
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