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Old 07-31-2007, 10:29 PM   #1
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ok im a first year mechanical engineering student currently have a class called electronic engineering, love it its tempting to change my major but it got me thinking.
on our esc we use capacitors to smooth out voltage spikes thus lowering the tempreture of the esc. now ideally we want the least resistance and highest ripple current. so what would be the advantage of having two capacitors in parralel?
pretty much any of you guys knw where i can discuss electronics in detail?
cheers.
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Old 07-31-2007, 10:58 PM   #2
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i am no EE but my guess is that more caps in parallel will first increase capacitance, and second, lower ESR (the resistance equivalence in a capacitor), so the capacitor is more efficient (that's just a hunch).
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Old 08-01-2007, 06:24 AM   #3
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Two caps would smooth out voltage spikes or voltage drains much better. This means that the ESC has higher voltage. Without the caps, the ESC would receive less voltage because there are no caps to store the energy.

Think of it this way, the ESC is like a shower head and the caps are like the water heater. There is more hot water (voltage) if there are two water heaters (capacitors). If you turn up the throttle on the ESC (crank up the heat on the shower head), the caps will drain until the voltage equals the battery (the water heaters drain and then you end up getting cold water).
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Old 08-01-2007, 01:34 PM   #4
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Hmm... I think there is something wrong in mattnin's post. Like I said in my previous posts, I am not engineer, but "the ESC has higher voltage" is incorrect.

Parallel capacitors will increase the capacitance in the typical case. Increasing the capacitance allows the amount of charge stored by the capacitor to increase.

The capacitor is used to filter ripple currents while the ESC is operating. So the capacitor removes voltage spikes, protects the ESC and allows you to be slightly faster before full throttle in comparison to not having a capacitor (full throttle speed is the same).

The best way to get good answers is to use the search function. There are plenty of good explanations here, but unfortunately, I believe this one is flawed. Maybe someone with better knowledge of electronics can confirm this.

I am not here to flame, but since some of mattnin's post seems to go against what I know (which may not be the truth), I want to post my 2 cents here too.
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Old 08-01-2007, 02:35 PM   #5
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No, what I said is correct. I have a bachelors in electronic engineering if you have any other questions as well. Maybe I can explain it a little better if you are confused.

There is no regular ripple current that occurs from a battery. Ripple current would happen from a DC power source, one that is created from rectified AC circuits.

Let me first say that yes, you are correct that the cap decouples the unwanted voltage transients or spikes that can occur due to the ESC feedback. Remember that spikes can be both positive and negative. A negative spike would reduce the total DC voltage the ESC sees, and a positive spike would increase the total DC voltage the ESC sees.

However, probably the most basic uses of a capacitor is as a backup voltage source. The storage cap is connected between the power supply (battery) and ground. When the ESC is operating from the battery, the capacitor remains fully charged to the battery. If the battery voltage drops due to the ESC pulling a lot of current, the storage capacitor temporarily becomes the power source for the ESC, therefore maintaining the voltage. Without a cap, the ESC would see lower voltages at this time.

Remember that the cap provides voltage and current as long as its charge remains sufficient which isn't a very long time for a ESC. As current is drawn by the ESC, charge is removed from the capacitor and voltage decreases. For this reason, the cap can only be used as a temporary power source. The length of time that the capacitor can provide sufficient power to the circuit depends on the capacitance and the amount of current drawn by the ESC. The smaller the current and the higher the capacitance, the longer the time.
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Old 08-01-2007, 02:49 PM   #6
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I apologize on the use of the word "ripple" current. Since the ESC regulates voltage to the motor through PWM. What I meant to say was that during the "off" sections, the capacitor helps maintain a constant voltage to the motor.

I apologize for not reading "Without the caps, the ESC would receive less voltage because there are no caps to store the energy."

That is what has caused my confusion.
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Old 08-01-2007, 05:59 PM   #7
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Quote:
Originally Posted by aus jd 2703 View Post
what would be the advantage of having two capacitors in parralel?
Having 2 caps in parallel halves the Equivalent Series Resistance (ESR) and doubles the capacitance.

But larger capacity capacitor have lower ESR than smaller capacity ones so just putting larger capacity capcitor reduces the ESR anyway. Lower voltage capacitors usually have lower ESR and are physically smaller for given capacitance.

The question is of course how low ESR do we need?
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Old 08-01-2007, 09:30 PM   #8
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So you would want enough Capacitor that it would not fully discharge when throttled. As in the longest straight. Enough charge to have some at the end of the straight that it could recharge in the infield. Off power is when it regenerates right.?

If you have to much it will never recharge so the recharge would have to be as quick as the bursts.

Fact is you can over do it on the capacitor. Unless you have a regenerating type of ESC.
That would mean a third wire right.? Or a battery in place of the capacitor.

Well am I close to understanding Esc's or should I just throw it on there for looks. Just in case.

Hopefully one of you electronic wizards can help.
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Old 08-01-2007, 09:39 PM   #9
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2 caps in SERIES increases voltage but capacitance remains the same. In PARALELL the capacitance is doubled but voltage remains the same.

The cap DOES not store energy to power the motor - those tiny little caps couldn't even get the motor to twitch.

The most common uses for capacitors is actually to decouple DC circuits, remove AC where not wanted, pass AC/block DC, and for tuning in RC typ circuits.

I BELIEVE the purpose of the cap on the ESC is to suppress transient voltages - back EMF - from the motor.

It is important to remember that the cap is not used to store energy in the ESC - it is far too small.
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Old 08-01-2007, 09:52 PM   #10
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Is it true then that a capacitor can reduce the effect of high battery resistence? Say if you have 2 packs and one has a resistence measured at 24mh at the end of the charge cycle on a CDC and the second pack has a resistence of 60mh. Would the capacitor help negate the resistence of the second pack?
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Old 08-01-2007, 10:02 PM   #11
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I am just curious, how will capacitors suppress back emf from the motor if it is located in parallel with the battery? (because the back emf is fed into the esc before reaching the capacitor?)
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Old 08-02-2007, 07:56 AM   #12
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Quote:
Originally Posted by rcnewb2004 View Post
I am just curious, how will capacitors suppress back emf from the motor if it is located in parallel with the battery? (because the back emf is fed into the esc before reaching the capacitor?)
Again, I am guessing as to the design here since the ESC docs don't really describe the purposes of the caps.... I believe the cap is closest to the motor and not close (elecrically) to the battery. if you put a cap in parallel across a DC source it will absorb or smooth out AC components. This is how they are used in linear DC power supplies. Any voltage coming FROM the motor is going to be AC therefore the having the cap close to the motor leads should result in the cap absorbing that voltage - in the form of back EMF or other "noise" from the motor.
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Old 08-02-2007, 10:36 AM   #13
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All I know is caps make the motor not glitch so I allways use one
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Old 08-02-2007, 12:14 PM   #14
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Are we talking about the same capacitors? there are some capacitors (usually 3) attached close to the motor to suppress motor noise, and there are capacitors attached in parallel to the battery. I am referring to the ones attached in parallel to the battery.
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Old 08-02-2007, 04:42 PM   #15
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http://www.teamnovak.com/tech_info/p...power_caps.htm
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