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Old 2009.01.24, 08:12 PM   #1
ProfoxCG
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Capacitor Voltage question - EE help

I was reading about capacitors and how they work and such. From my reading I understood that the (uF) is the capacity value and that (x.xV) is the maximun operating voltage. Which was said that a caps voltage will be only as hight as the voltage of the circuit its connected to.

So, I have seen more mini-z that have a 5.5V cap running 4 AAA which equals 4.8V. So a 5.5V Capacitor would be sufficient. However at radio shack there are some vere small capacitor with a 20V, 50V rating. Can these be also used in leu of the hard to come by 5.5V capacitors?

Another thing I read was that the voltage lets say 5.5V was going to be the constant amount of voltage that the cap was going to release constanly thus eliminating any voltage fluctuations?

Can a EE please set things straight?
Thanks,
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Old 2009.01.24, 09:22 PM   #2
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Capacitor voltage ratings refer to the maximum voltage they should be charged to.

Remember capacitors are built from two metallic plate separated by a thin insulator. If you apply a high enough voltage, the insulating material will break down and conduct electricity.

To build a capacitor with a higher voltage rating, manufacturers typically use a thicker insulator - this makes the capacitors physically bigger and heavier.

So for your mini-z; use any cap you like that is rating above 5V.

Hope this helps.
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Old 2009.01.25, 03:54 PM   #3
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so i can use this for example?



why do people then buy the 5.5V caps which are berely over 5V or 4.8V for that matter. I have seen very small and light caps rated at 50V like the one above wich a capacity of 1uF.

I guess we would want a capacitor with a a very small capacity so that it can charger and discharge from the battries fairly fast?
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Last edited by ProfoxCG; 2009.01.25 at 07:32 PM.
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Old 2009.01.25, 04:50 PM   #4
atallfunguy
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Most of the racers at my club take the caps off our motors.

We all have moved to 2.4 and find no use for them.

We also have found a very slight speed increase without them. Approx 400 to 500 rps at 5.4 volts.
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Old 2009.01.25, 07:30 PM   #5
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I see, makes sense due to the fact that 2.4 doesnt get disturbed by anything. My question however is for caps connected at the baterry to get "constant" voltage to the board.

So if we have out batteries producing 4.8V then the car will fill up and constantly deliver 4.8V and charge itself from the battery. In essence the board (mini-z) will be running from the Capacitor rather then the 4 AAA.

EE's please confirm.

I was under the impression that a 5.5V capacitor was going to "flash out" 5.5V constantly and the voltage of the circuit. So for example a 50V cap like the one pictured about would deliver 50V - however if I understoood correctly, 50V is how much energy it takes for it to "burn" - it will only flash as much voltage as is in the circuit it is installed correct?
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Old 2009.01.25, 08:40 PM   #6
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Quote:
Originally Posted by ProfoxCG View Post
So if we have out batteries producing 4.8V then the car will fill up and constantly deliver 4.8V and charge itself from the battery. In essence the board (mini-z) will be running from the Capacitor rather then the 4 AAA.

EE's please confirm.
Well sort of. Keep in mind that your mini-z motor doesn't draw constant current. Motor current is switched on and off at a high frequency (that is what the FETs do). If the motor is turned on 50% of the time; you get a medium rpm; turn the motor on 75% of the time; you get higher rpms.

You are correct that the battery will "charge" the capacitor to 4.8V. The motor will draw current or "discharge" the battery AND the capacitor simultaneously. And when the motor current is off, the battery will re-charge the capacitor. This charge/discharge cycle all happens really fast - thousands of times a second.

So what is the advantage of putting a capacitor in parallel with the battery? Capacitors have less internal resistance than batteries so they can supply higher currents. But because they hold a very small amount of charge (energy) compared to a battery, they can only supply this large current for a very short period of time.

This extra spirt of high current from the capacitor is the "punch" some drivers look for.

PN Racing for example, offers a 5.5V 4700uF capacitor. Compared to the one in your picture (10V 47uF) it can hold 100 times more charge but can only be charged to half the voltage. A 4700uF 10V capacitor may be to large to put under your mini-z body.

I won't give a specific number that is best but.....a small 47uF capacitor may not be of much benefit because it can't hold enough charge to give a noticable "punch".

Again, hope this helps.
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Old 2009.01.25, 08:51 PM   #7
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Originally Posted by ProfoxCG View Post
I guess we would want a capacitor with a a very small capacity so that it can charger and discharge from the battries fairly fast?
Rate of charge is controlled the internal resistance (called equivalent series resistance or ESR) not the capacity.

In this case, bigger uF is better. Trade off is size and weight of the capacitor.
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Old 2009.01.26, 09:57 AM   #8
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But the real function of the caps is not to supply 'punch', but to protect the brushes, and they are needed whether you're running an AM car or 2.4, since both use DC current running a brushed motor.

When the current to the windings is momentarily interrupted due to the rotation of the comm against the brushes, voltage builds very rapidly at the brushes, and with no caps in the system, this can cause current to arc from the brushes to the comm, causing pitting. Take one of your motors to full throttle in a dark room and look at the arcing you'll probably see.

With a capacitor in parallel with the motor's power leads, voltage does not build as rapidly since the current is now going to charge the caps rather than trying to arc to the comm when current is interrupted. When current starts flowing again, the caps discharge, but the 'punch' effect is small, since the charge stored is very small compared to what the batteries are producing.

Think of the cap as a small current 'sponge' that soaks up current flow when it is interrupted,

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Old 2009.01.26, 11:00 AM   #9
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Originally Posted by ianc View Post
But the real function of the caps is not to supply 'punch', but to protect the brushes, and they are needed whether you're running an AM car or 2.4, since both use DC current running a brushed motor.

When the current to the windings is momentarily interrupted due to the rotation of the comm against the brushes, voltage builds very rapidly at the brushes, and with no caps in the system, this can cause current to arc from the brushes to the comm, causing pitting. Take one of your motors to full throttle in a dark room and look at the arcing you'll probably see.

With a capacitor in parallel with the motor's power leads, voltage does not build as rapidly since the current is now going to charge the caps rather than trying to arc to the comm when current is interrupted. When current starts flowing again, the caps discharge, but the 'punch' effect is small, since the charge stored is very small compared to what the batteries are producing.

Think of the cap as a small current 'sponge' that soaks up current flow when it is interrupted,

ianc
Ianc, great post. I think I will look into putting the caps back on my motors.

P
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Old 2009.01.26, 01:24 PM   #10
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Quote:
Originally Posted by ianc View Post
But the real function of the caps is not to supply 'punch', but to protect the brushes, and they are needed whether you're running an AM car or 2.4, since both use DC current running a brushed motor.

When the current to the windings is momentarily interrupted due to the rotation of the comm against the brushes, voltage builds very rapidly at the brushes, and with no caps in the system, this can cause current to arc from the brushes to the comm, causing pitting. Take one of your motors to full throttle in a dark room and look at the arcing you'll probably see.

With a capacitor in parallel with the motor's power leads, voltage does not build as rapidly since the current is now going to charge the caps rather than trying to arc to the comm when current is interrupted. When current starts flowing again, the caps discharge, but the 'punch' effect is small, since the charge stored is very small compared to what the batteries are producing.

Think of the cap as a small current 'sponge' that soaks up current flow when it is interrupted,

ianc
Yes I agree. Motor caps eliminate/reduce arcs which also reduce high frequency RF which is know to cause receiver problems, 2.4 AM FM doesn't matter. 0.1 or 0.01uF caps are commonly used.

I was referring to the installation of a large bulk capacitor (1000uF+) across the battery in case anyone got confused.

Last edited by kja812; 2009.01.26 at 01:30 PM.
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Old 2009.01.26, 02:06 PM   #11
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well i just received some 0.047f 5.5V caps to connect to the battery.
What benefit will this offer?
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Old 2009.01.26, 03:26 PM   #12
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well i just received some 0.047f 5.5V caps to connect to the battery.
What benefit will this offer?
Hook it up and take a drive. Let us know what you think. You may notice a little extra power, maybe not...depends on your driving stye.
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Old 2009.01.26, 04:36 PM   #13
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Hook it up and take a drive. Let us know what you think. You may notice a little extra power, maybe not...depends on your driving stye.
okay sounds like a plan.
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Old 2009.03.01, 05:52 PM   #14
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Quote:
Originally Posted by kja812 View Post
Yes I agree. Motor caps eliminate/reduce arcs which also reduce high frequency RF which is know to cause receiver problems, 2.4 AM FM doesn't matter. 0.1 or 0.01uF caps are commonly used.

I was referring to the installation of a large bulk capacitor (1000uF+) across the battery in case anyone got confused.
ianc's explanation is mostly correct, but RF interference (RFI) is the main reason for the caps. They filter out the RFI, and a byproduct of that is better brush life.
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Old 2009.03.02, 12:30 PM   #15
Darth_Hell
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Need advise...

Hi, I am running on stock 2.4 KO board and KO 2.4 motor, which comes with a capicitor originally.

Is it adviseable to remove the motor capacitor, will it burn my 2.4 board???
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