• If you are new to GiantScaleNews.com, please register, introduce yourself, and make yourself at home.

    We're 1st in Giant Scale RC because we've got the best membership on the internet! Take a look around and don't forget to register to get all of the benefits of GSN membership!

    Welcome!

Separate capacitors on receiver? To use or not to use?

econpatric

econpatric

70cc twin V2
Hey all, I have been doing a little digging around on whether or not to hook up a capacitor to an open channel of any of my Specktrum 6210's. There seems to be a lively debate on the legitimacy of its capabilities in the event of a brownout. I personally have not had any issues, but thought it would be an in inexpensive piece of insurance if it works for any reason.

The debates I've read seem to stem from the ability of the cap to maintain voltage to servos in the event of a sudden drop in current. We know that a small cap will not be able to supply the needed voltage of four servos for more than a split second, but my question is will a, lets say, 4800mf cap be able to protect from brown out, acting to aid the system before it (the cap) would be the only power source?

In 3D applications we're always looking to shed weight, so I don't know if its worth it ounce for ounce? I have all of the materials so I wouldn't be out anything for trying, but I am hoping for an educated discussion that will help me and others determine if this concept is fact or fiction.

This guy seems to think that they work: http://www.youtube.com/watch?v=FpPzbAfksH8&feature=player_embedded

Here are a couple of links to some of these devices:


What do you think?
 
njswede

njswede

150cc
Ah! An opportunityto geek out!

Well, it's actually pretty easy to settle this debate with a little bit of math. A brownout is typically caused by a spike in current that's higher than what the BEC can handle. The BECs we use for a small to medium sized plane is typically rated around 5A, so let's use that in our example. Let's say we're running the system on 6V. Let's also say we have a 3F capacitor (they seem to come in 1-3F sizes). Furthermore, let's assume we're toast if the receiver supply voltage goes below 3.5V.

We can then plug this into the capacitor discharge equation:

V = V[SUB]0[/SUB]e[SUP]-t/RC[/SUP]

V is the end voltage, V[SUB]0[/SUB] is the initial voltage, t is the time, R is the total resistance of the system and C is the capacitance.

The only variable we don't know is the resistance, but we can use Ohm's law for that R=V/I, so R=5/5 = 1ohm, if we assume the supply voltage drops to 5V if we pull 5A.

Solving the discharge equation for t (time), we get:

t = -ln(V/V[SUB]0[/SUB])RC = -ln(3.5/6)*1*3 = 1.6s

So, it looks like the capacitor would hold charge for 1.6s. This assumes the resistance of the system stays constant and a bunch of other things, but I'd say with some confidence that you should have around 1-2s before your receiver shuts down.

Is this enough? Maybe. Remember, whatever is pulling all that current has to stop doing so after 1-2s, so if it's a servo that's gone bonkers or binding, you may not be so lucky.
 
njswede

njswede

150cc
...also this assumes that a brownout happens right above the current the BEC is rated for. That's probably not true, since they build a little bit of margin into the system. But again, it will at least get you a rough idea of what would happen.
 
Last edited by a moderator:
Joe's Dad

Joe's Dad

70cc twin V2
O this will be fun. Farads and Mfarads are huge. They are used as starting caps on big Electrical motors.
Brownouts can be eliminated by correct battery sizing.

A good 330uf 25 volt tant cap hooked to the receiver makes the power going into the receiver clean/er.

But if you want to protect against brownouts, don't go the cap route.
 
econpatric

econpatric

70cc twin V2
"t = -ln(V/V0)RC = -ln(3.5/6)*1*3 = 1.6s,"

Some great info there! NJSwede I am following the process by which you have come to the time determination. With all of that said, we are assuming that the cap is in a position to act as a small battery for protection for that 1.6s.

What I wonder is will that same cap also eliminate the possibility of needing to be used as a small battery in the first place? Outside of giving us a second and a half of power, does it offer any protection from the brownout before it occurs?

Jim do you typically use caps in any of your applications for that cleaner power mentioned? That is, is it worth the weight?
 
Last edited by a moderator:
njswede

njswede

150cc
econpatric said:
"t = -ln(V/V0)RC = -ln(3.5/6)*1*3 = 1.6s,"

Some great info there! NJSwede I am following the process by which you have come to the time determination. With all of that said, we are assuming that the cap is in a position to act as a small battery for protection for that 1.6s. What I wonder is will that same cap also eliminate the possibility of needing to be used as a small battery in the first place?"
Click to expand...

Well, yes, it does. Sort of. But only for a very short time. As Jim says, a capacitor is fine for smoothing out spikes on the supply voltage. So if you have a sudden high amp draw of a few tens of milliseconds causing the BEC voltage to sag, the capacitor will help keep the voltage up. But you can't rely on the capacitor for anything longer than fractions of a second. So if you really need a bullet proof (well, nothing really is) receiver supply, a fresh, high quality, fully charged separate battery is the way to go.
 
cwojcik

cwojcik

70cc twin V2
NJSwede said:
Ah! An opportunityto geek out!

Well, it's actually pretty easy to settle this debate with a little bit of math. A brownout is typically caused by a spike in current that's higher than what the BEC can handle. The BECs we use for a small to medium sized plane is typically rated around 5A, so let's use that in our example. Let's say we're running the system on 6V. Let's also say we have a 3F capacitor (they seem to come in 1-3F sizes). Furthermore, let's assume we're toast if the receiver supply voltage goes below 3.5V.

We can then plug this into the capacitor discharge equation:

V = V[SUB]0[/SUB]e[SUP]-t/RC[/SUP]

V is the end voltage, V[SUB]0[/SUB] is the initial voltage, t is the time, R is the total resistance of the system and C is the capacitance.

The only variable we don't know is the resistance, but we can use Ohm's law for that R=V/I, so R=5/5 = 1ohm, if we assume the supply voltage drops to 5V if we pull 5A.

Solving the discharge equation for t (time), we get:

t = -ln(V/V[SUB]0[/SUB])RC = -ln(3.5/6)*1*3 = 1.6s

So, it looks like the capacitor would hold charge for 1.6s. This assumes the resistance of the system stays constant and a bunch of other things, but I'd say with some confidence that you should have around 1-2s before your receiver shuts down.

Is this enough? Maybe. Remember, whatever is pulling all that current has to stop doing so after 1-2s, so if it's a servo that's gone bonkers or binding, you may not be so lucky.
Click to expand...

Great work Pontus.

If you are having brownout issues, you need to upgrade your batteries, switches, connectors, or BEC. A properly operating radio power system should never drop more than .5 volts during hard use.
 
You must log in or register to reply here.
Top