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lentner3

Does anyone use capacitors in their rx?

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Just wandered if anyone has used these to help with excess loads from on the esc from servos and fans etc?

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I got a pair attatched to my brushless esc, what they do is store a small amount of power and when the esc askes for a load of power in one go it draws from the capacitor as well.

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I got a pair attatched to my brushless esc, what they do is store a small amount of power and when the esc askes for a load of power in one go it draws from the capacitor as well.

But this is capacitor use on ESC-side, not rx side.

If i understood him correct, he's talking about capacitors used on rx side. These are used to keep voltage peaks away from rx. Peaks are caused by strong digital servos when servomotor is being pushed ( sort of dynamo effect ). But this is more used in aircraft applications. I had these capacitors often when flying helicopters. But this is completely different application of capacitor.

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Some esc's, mainly the more powerful brushless ones, already have capacitators attached to them (as howell said). Even the very popular Hobbywing Quicrun 1060 brushed esc has one installed.

I just bought an external capacitator to hook up to the rx in my Mad Bull. It has a pretty powerfull servo to turn those big wheels (Savöx 12kg in 0.08 sec) which causes brown-outs. Haven't tried it out yet, but I'm hoping this will solve the problem.

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OK, I'll try and keep this simple..

A capacitor is a device that will hold a charge (voltage) and then discharge it at a very high rate, very quickly/fast.. This device when connected to the receiver is also known as a "Glitch Buster" (as marketed by Novak USA and maybe others)

A servo is essentially a motor (it has one inside) and all electric motors draw lots more current when they initially start to rotate, or when they are under load, than what they do free running under no load..

Now, what can happen with some ESC, receiver and high power servo combinations, is when the servo is just starting to move, or is under high load, it can initially draw more current than what BEC in the ESC can provide (a high current spike), and when this happens, the BEC voltage drops below the minimum operating voltage of the receiver, and the receiver will momentarily stop working, aka "a brown out"...

When you connect a suitable sized capacitor to the receiver bus, it will charge up to the BEC voltage when the cars radio gear is turned on, and then hold that voltage.. Then in the event that the servo draws more current than the BEC in the ESC can deliver (which would normally cause the BEC voltage to drop), the capacitor will then discharge some of its stored voltage, to keep the BEC's voltage at its rated voltage level, and hopefully stop the BEC's voltage from dropping below the minimum operating voltage for the receiver, and avoid having a brown out..

The other scenario where you would often see a brown out occur is when running a low turn (drive) motor and average batteries at take off (accelerating off from a standing start).. What would happen in this scenario was that the motor would try to draw more current from the battery than it was able to deliver, and the main batteries voltage would momentarily drop below the minimum operating voltage of the BEC in the ESC, which in turn would cause the BEC's voltage to drop below the minimum operating voltage of the receiver, and you would get a brown out.. (This is why a lot of modern (brushless) ESC's will have a series of external capacitors across the main battery bus..)

Understand??

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I understand how they work and thank you for explaining to everyone Backlash. But I have never had the need as never experienced clogging or brownouts. That said most of my setups are fairly low drain. Having said that my question is have any of you who are running more accessories in particularly via the rx creating a need for one. I have a few high torque servos and led kits which I am intending to install and wandered at what point the Bec is likely to get overloaded. I do have some external 5/6 volt 3amp Bec that I can use if needed with a micro rx.

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The need for, and experienced benefit from, a glitch buster cap connected to the Rx is highly situational. Variables would be: the servo you're using, the ESC (mostly its BEC), the motor, how you drive and the conditions under which you're driving.

If you have a high quality BEC in your ESC (for example: NOT a Castle), a high quality battery with strong discharge rate and capacity, an efficient servo (for example, NOT a Savox) then you may never see a benefit from a cap connected to the Rx. If you have a spare Rx channel, and a cap then I say use it. Better safe than sorry IMHO. If you skip it and experience brown outs, connect up a cap and see if it helps.

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On ‎22‎/‎04‎/‎2016 at 5:05 AM, lentner3 said:

I understand how they work and thank you for explaining to everyone Backlash. But I have never had the need as never experienced clogging or brownouts. That said most of my setups are fairly low drain. Having said that my question is have any of you who are running more accessories in particularly via the rx creating a need for one. I have a few high torque servos and led kits which I am intending to install and wandered at what point the Bec is likely to get overloaded. I do have some external 5/6 volt 3amp Bec that I can use if needed with a micro rx.

I was answering "steelo's" question, but its all good... ;-)

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13 hours ago, Backlash said:

I was answering "steelo's" question, but its all good... ;-)

Thank you, you explained it well. I guess the need for these don't really exist anymore as most modern esc have them built in.

and it's always all good at

Tamiya club 😄

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The primary function of large capacitors connected across the battery inputs to the ESC is to absorb high voltage spikes generated in long battery wires.  It is possible for voltages several times the main pack voltage to be generated if the inductance of the battery wires is high enough, i.e. if they are long.  These spikes can damage the electronics in the ESC.  These spikes are generated because the current from the battery is consumed in high frequency pulses by the ESC due to its PWM operation.  Switching off a high current very quickly causes a large voltage to be generated as the magnetic field around the wire collapses.

This is totally different to connecting a capacitor to the RX to prevent brown outs.

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I use rx capacitors ("glitch busters") because I make them myself (the readily made ones are 10x-100x overpriced) and, since I usually have free rx slots, why not - digital servos are power hungry and can generate large current spikes.

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20 hours ago, Backlash said:

LongRat,

Where did you get this information from?

It's not a subject that comes up too often amongst RC car electronics users, probably because of the fact that wires are generally always a similar length in every installation.  If you are building large planes where the battery and ESC could be a good distance apart then this problem can cause your ESC to fail.  Or, get extremely hot while operating well within what should be its rated power window.  Some ESC manufacturers give details of the capacitance that should be used as a function of wire length.  Of course voltage sag under burst load is also alleviated by these capacitors, but increasingly with low internal resistance batteries this problem becomes less of an issue.

A good thread on the subject can be found here: http://www.rcgroups.com/forums/showthread.php?t=952523

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5 hours ago, LongRat said:

It's not a subject that comes up too often amongst RC car electronics users, probably because of the fact that wires are generally always a similar length in every installation.  If you are building large planes where the battery and ESC could be a good distance apart then this problem can cause your ESC to fail.  Or, get extremely hot while operating well within what should be its rated power window.  Some ESC manufacturers give details of the capacitance that should be used as a function of wire length.  Of course voltage sag under burst load is also alleviated by these capacitors, but increasingly with low internal resistance batteries this problem becomes less of an issue.

A good thread on the subject can be found here: http://www.rcgroups.com/forums/showthread.php?t=952523

Arrr, OK.. So this is a problem more often seen in planes, and not so much with cars..

I had a quick look at the information in the link, and it appears that these "spikes" due to inductance in the battery wires is more of a problem with Brushless ESC's..

If you read the information on the Novak website about their Power Caps, it appears to me that they are marketing the power caps as a means of getting more punch... At the very bottom of the page, it says that they include transient suppressors in their power caps to stop the high voltage spikes... So what do you believe??
http://teamnovak.com/tech_info/view_article/17

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I don't think there is a contradiction here as such.  You need the large capacitor for the smoothing of the ripple currents that are produced by high drain loads, and because they store a large amount of energy for the reduction of voltage sag to the ESC input under very heavy drain.  Without the cap, the ripple currents would induce voltage spikes that would damage the FETs.  Novak don't elaborate on why the ripple currents could damage the ESC, but that is the reason.  Novak have clearly chosen to go one further with those Power Caps by putting the smaller low ESR caps in parallel for better response to high frequency and RF level oscillations.  I don't claim to be an expert but basically, smaller caps can usually absorb higher frequencies more effectively and you can quite possibly get oscillations of higher frequency than the switching freq when harmonics and PWM are considered.  They mention that the caps improve mid-throttle RPM, which would be because that's when ESC switching via PWM puts the most rippling onto the battery wires.  At full throttle on a brushed ESC, there is no switching so there's no ripple.  I'm not sure that statement holds true for brushless ESCs because they are always switching, thanks to having electronic commutation.

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I don't believe that I said there was a contradiction here..

I get what your saying..

What I am suggesting though is that irrespective of when the primary reason is for having these capacitors connected across the battery wires, many ESC manufacturers market them as being a means to get more punch from the ESC (i.e. to combat voltage sag) rather than as a device that is there to protect the FETs in the ESC...

In regard to a brushed ESC running at full throttle.. OK, the FETs will be hard on, so there wont be any PWM switching, but what about the "switching" of the brushes and the commutator..??..

OR the constantly varying load on the motor (thus varying current draw from the battery..)?

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I think you're right about the marketing of caps a performance enhancer.  Which they are, but I think what the manufacturers also want is for people not to suffer from sudden, unexplained failures.  You don't want the reputation of having the ESCs that blow up for 'no reason'.  Even if that reason is the FETs running hot and degrading over time due to not running caps.  That's a lot harder for them to explain than simly saying, "if you run caps your car will be faster".

On a brushed ESC at full throttle the comm does not switch the current in a sudden manner.  As the brush slides off one segment and onto the next, the current ramps down and up in rough proportion to changing areas of contact, plus the resistance and inductance of the windings are also resisting this change.  Because of this slower change in current over that delivered by a square-wave PWM voltage, I would expect far lower voltage spikes to be induced as the rate of magnetic field collapse would be a lot slower.  Throttling up and down by the driver is a whole boat load less again in terms of rate of change, so would probably not make any impact.

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10 hours ago, LongRat said:

I think you're right about the marketing of caps a performance enhancer.  Which they are, but I think what the manufacturers also want is for people not to suffer from sudden, unexplained failures.  You don't want the reputation of having the ESCs that blow up for 'no reason'.  Even if that reason is the FETs running hot and degrading over time due to not running caps.  That's a lot harder for them to explain than simly saying, "if you run caps your car will be faster".

On a brushed ESC at full throttle the comm does not switch the current in a sudden manner.  As the brush slides off one segment and onto the next, the current ramps down and up in rough proportion to changing areas of contact, plus the resistance and inductance of the windings are also resisting this change.  Because of this slower change in current over that delivered by a square-wave PWM voltage, I would expect far lower voltage spikes to be induced as the rate of magnetic field collapse would be a lot slower.  Throttling up and down by the driver is a whole boat load less again in terms of rate of change, so would probably not make any impact.

But then if their primary purpose is to protect the ESC, then it does make you wonder why most ESC manufacturers advertise these capacitors as a performance enhancer, when they clearly market Schottky Diodes as a device that helps protect the brake FET(s) in a brushed ESC..

 

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Not sure about that.  As far as I remember the performance aspects of these caps only started being touted once ESCs got small enough that the caps were almost bigger than them!  Thinking about Novak GTX, KO VFS-series etc.  Racing ESCs which are bought on performance above everything else, so that's what the message will concentrate on.  I guess it is hard to say what function - acceleration punch or ESC protection - is the dominant one in any particular application.  It's like trying to compare my blue car with someone else's red coffee cup.  Which is better?

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