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Capacitor and use with Rx's

Old 08-07-2016, 01:51 PM
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solentlife
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Default Capacitor and use with Rx's

OK - we have all heard of the capacitor as a voltage DIP protector. That is demand on BEC or flight pack to Rx gets high, voltage momentarily dips and Rx drops out.

Some add on units such as Stabiliser units even put a small capacitor in the box.

There are people that are not happy with a small cap and actually plug in large ones that take a significant charge.

So it leads me to start thinking.

With a NiMh, NiCd Rx Pack - the capacitor is like a small parallel battery and will not have any trouble.

With a Switch Mode BEC though - that relies on pulsing power. Would a capacitor cause some SBEC's to malfunction at times ? Would it upset the SBEC's ability to monitor and control its output to Rx.

Nigel
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Old 08-07-2016, 11:28 PM
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Panther
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Interesting thought Nigel and beyond my intelligence range as far as electronics go.
I see Go4it has responded to the first thread elsewhere and he seems to have some electronic knowledge.

You could have a very good point re Flight Pack as opposed to a fluctuating switching Bec which is what I have been using in those planes. I might change them out for some Turnigy Plush Linears that I have in other planes and check the results once I have my other problem managed.

Doug
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Old 08-07-2016, 11:48 PM
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fhhuber
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I'd have to ask my friend who is an electrical/electronics engineer to confirm... But what I expect:

The capacitor acts sort of as a "shock absorber", absorbing the voltage peaks and filling in the lows. Speed of this is based on the capacitance, resistance, time formula

The simplest RC circuit is a capacitor and a resistor in parallel. When a circuit consists of only a charged capacitor and a resistor, the capacitor will discharge its stored energy through the resistor. The voltage across the capacitor, which is time dependent, can be found by using Kirchhoff's current law, where the current charging the capacitor must equal the current through the resistor. This results in the linear differential equation
C d V d t + V R = 0 {\displaystyle C{\frac {dV}{dt}}+{\frac {V}{R}}=0} .
You look at the switching regulator BEC as an intermittent power supply and the RX system as the resistor. (with resistance not constant to complicate matters)

The capacitor is sometimes a current source and sometimes a load on the BEC. Result being a flatter voltage being seen by the RX system.

There may be a slight higher average power demand seen by the switching BEC, but I doubt its significant.
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Old 08-08-2016, 05:23 AM
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kyleservicetech
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Originally Posted by solentlife View Post
OK - we have all heard of the capacitor as a voltage DIP protector. That is demand on BEC or flight pack to Rx gets high, voltage momentarily dips and Rx drops out.

Some add on units such as Stabiliser units even put a small capacitor in the box.

There are people that are not happy with a small cap and actually plug in large ones that take a significant charge.

So it leads me to start thinking.
Nigel
I've checked my Castle Creations 10 Amp uBEC's with a capacitor connected across their output, using my 100 Mhz Owon 7102 oscilloscope. From what I saw, IMHO, it's not a good idea.

Other BEC Mfg's might be OK, or, they might not work with an extra cap across their output. All depends on the circuit design.

A capacitor connected across the BEC output in my opinion isn't a good substitute for a quality BEC, or a inadequate receiver battery system.

A little math here. Roughly, CV=IT, where C is capacitor value in Farads, V is voltage, I is Amps, and T is seconds.

My Spektrum receivers will reboot if the voltage dip lasts over 1 millisecond, and drops to below 3.2 Volts DC. With a typical peak current of say, 5 Amps, and a 470 uF cap, you get:

V = IT/C, = 5 Amps times 0.001 Seconds/470E-6 which equals 10 Volts. Since the DC input is 6 Volts DC or so, that cap won't do any good.

That puts you up around 10,000 uFd to provide useable results. And, perhaps 10,000 uFD across some China BEC DC outputs could affect their operation.
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Old 08-08-2016, 01:44 PM
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What kyleservicetech said; ++++
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Old 08-08-2016, 02:20 PM
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There has been some testing done my Mikado which does show some significant benefits from fitting of a capacitor on systems with both BEC's and Rx batteries.
The benefit is what fhhuber described in that the capacitor is able to act as a 'shock absorber' to dampen very short term momentary voltage transients that are caused by the servos.
They dont really function as a backup battery as such because the amount of charge they have is so small, so kyleservicetech is bang on when he said that they arent a substitute for an adequate BEC or battery. All the capacitor does is flatten out momentary voltage peaks and troughs, though in some systems this can be the difference between the Rx shutting down and it working normally. The damper or shock absorber is the best analogy.

Mikados testing of various BECs and battery systems with and without capacitors which shows pretty conclusive results: https://www.vstabi.info/en/node/1327

Also as protection from back EMF generated by the servo: https://www.vstabi.info/en/node/1422

PS Nigel.. The output from an SBEC isn't 'pulsing'. The SBEC does regulate voltage by pulsing power on and off but it has a smoothing circuit (using capacitors) on it's output side so that the final output from the BEC is actually a pretty smooth and steady voltage with almost all of the 'pulsation' removed.
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Old 08-09-2016, 02:10 AM
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kyleservicetech
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Default Capacitor assistance on uBEC's

I just ran some tests on one of my Castle Creations 10 Amp uBEC's, both with an 18,000 ufd capacitor connected across the uBEC output, and without a Cap on the uBEC output.

The voltage drop noted on the attached oscilloscope screen dumps shows a 0.4 Volt DC voltage drop at a load test of 5.8 Amps. It's interesting to note that the CC uBEC has a battery wire leading to the receiver that is 11 inches long. That wire is responsible for about 75% of the 0.4 Volt DC voltage drop at 5.8 Amps.

For the really high powered servos on giant scale model airplanes, it is hard to beat a pair of 2500 Mah A123 cells for receiver power. At 5.8 Amps, or even 15 Amps, the output voltage on these cells only drops a few percent. But, you have to have beefy wires from those cells. A standard receiver switch harness with those A123 cells can't do the job for the big models.

Attached is two JPG's I took, showing the difference between no cap, and an 18,000 uFD cap tied across the uBEC output.
Attached Images
File Type: jpg
CC ESC No Cap Text.jpg (192.3 KB, 365 views)
File Type: jpg
CC ESC 18000 uFD Cap Title.jpg (187.1 KB, 429 views)

Last edited by kyleservicetech; 08-09-2016 at 02:28 AM.
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Old 08-09-2016, 03:26 AM
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dereckbc
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Caps, even super caps, all have the same pro's and cons.

They can charge super fast. However they cannot hold any meaningful charge to cover a Brown-Out. Extremely poor energy density. A larger or seperate battery is a better use of weight and space.

Where they do have some benefit in electric vehicles is Regen Braking to absorb a large inrush of energy while braking, then release it on the next acceleration cycle.
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Old 08-09-2016, 07:12 AM
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Panther
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Well, it seems most folk seem to think the caps won't hurt no matter where they go, RX or A3 but preferably the Rx.
I flew 2 of them today and no glitches at all. I'm rather pleased about that as another new one arrived in the mail..... I have removed all the caps and will not be re-fitting them. (6 x cheap caps for sale)
As the old parson said... "Let us pray"
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Old 08-09-2016, 08:06 AM
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fhhuber
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Caps are very useful (necessary)for ESCs with long runs between battery and ESC. But you have to use large enough fast enough caps. The voltage spikes that can develop between battery and ESC, with extended wires, are impressive and can blow out the ESC input caps, then the ESC burns
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Old 08-10-2016, 02:11 PM
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AndyKunz
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The newest Spektrum receiver designs include a capacitor internally, separated from the input bus, which keeps the RF, micro, and gyros alive for a short duration. The servos will stop working, but the receiver will ride right on through.

Andy
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Old 01-11-2017, 10:48 PM
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UPDATE.
As mentioned in an earlier thread I had problems with some A3 Gyros fitted with caps.
Now, whether it is coincidence or not, I have had NO problems since I removed the caps.
I think enough time has elapsed to consider that it has been a fair test.
I removed one from my flying buddies Bipe as well and it hasn't reversed settings yet either. (as it did previously)
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