SMALL - espritmodel.com SMALL - Telemetry SMALL - Radio
Reply
Thread Tools
Old May 29, 2010, 01:53 PM
HAL... Open the damn doors!
jfetter's Avatar
Miramar, Florida
Joined Jul 2007
8,517 Posts
Quote:
Originally Posted by dag214 View Post
My son has been playing YYZ since he was 15, one of his favorites, Sub-Divisions is his next favorite RUSH song he plays. We have seen them live before.

Dag
That's about the same age I fist started playing it, an accomplishment for any drummer ;-)

Keep that date in mind though, seeing them do a full album, no doubt note-for-note as only Rush can will be a thrill!

Jack
jfetter is offline Find More Posts by jfetter
Reply With Quote
Sign up now
to remove ads between posts
Old May 30, 2010, 02:39 AM
I'd Rather Be Flying
RCBlackSheep's Avatar
Stockton CA
Joined Feb 2009
609 Posts
Quote:
Originally Posted by sneasle View Post
oldcoot and Deuce are correct, the voltage listed is their rating, it has nothing to do with what they will 'give' you.

A cap acts as a buffer, think of it as a water tower in a city. If the pressure in the mains drops due to high demand, water flows out of the tower to try and hold the mains at the proper pressure. If the pressure in the mains remains constant, the tower stays topped off.

Think of the 100V rating as the max pressure you could take the water tower to without risk of it leaking or blowing bolts/rivets.

edit:

A note on capacity of a cap.

Per the analogy above, a larger (the uf rating on a cap) water tower will 'hold' the pressure (voltage) for longer. Similarly, it will take longer to 'charge'. In a cap, the same hold true, but when charging a cap, a larger cap can possibly put more demand on your system as it tries to charge.

Caps are great because they can charge and discharge very quickly, this means you can have very high instantaneous currents (ie, you can make a cute little spot welder with a small array of caps the size of your computer mouse) that can be quite dangerous. These caps could pull upwards of 100's of amps on startup when you first plug in your batteries, possibly damaging the connector, so it may be advisable to add a current limiting resistor in parallel. If you are interested in doing it, let me know. I've got a nice picture stored that I found on the forum that explains it very well.
It seems sneasle is pretty much right on. If I may add a bit though. As I understand it, caps only charge up to the voltage that is coming from the battery. It would help if you posted a pic of the caps to be sure but the 220uf, 100v sounds like thier "ratings". If that is in deed the case then you should have no problems with them since your never going to apply more than the rated voltage. I'm not an engineer so get a second opinion. The current limiting resistor in parallel is a good idea when you get to higher voltages. I have seen an article in Model Aviation and I believe there is a thread on RCG about it. It mostly serves to keep from pulling too many AMPs upon hooking up batteries and getting that initial "oh my god, its a spark, I just got electricuted but I really didnt" reaction. lol I still laugh at myself every time I jump upon hooking up more than 3 cells. hehe I dont have the link to the current limiting resistor thread at the moment. BTW... I check this thread every time I turn on the puter and I am still awestruck by this project. I cant wait til I move into my new house with a nice size garage (in escrow now) to start some real projects too. Maybe not the size of this one but bigger than I can build on the dining room table! Who knows, I may even be lucky enough to start one of this magnatude. Truely awsume dude! Keep on keepin on! and thank you for sharing with us.
RCBlackSheep is offline Find More Posts by RCBlackSheep
Reply With Quote
Old May 30, 2010, 04:30 AM
Scratch Builder
AlexanderB's Avatar
Braunschweig, Germany
Joined Sep 2006
1,325 Posts
Dag, I agree with the others. Here is a picture of how easy loading of tha caps can be achieved in a smoothe way. The resistor may have 33Ohm and 0.5W (in the example I got it from). I'm not very much in electronics, but for my understanding the resistor limits the current and thus avoids the flash (the result of too much amperes being built up).

I got this from a German form and want to use that for my DC-6 as well.

http://www.rc-network.de/forum/showp...20&postcount=6


Usage: First connect the battery plus pole. Then touch the resistor with the minus pole of the battery and wait a second or two. The resistors get charged meanwhile and when you finally plug in minus (push through) no flash will happen and the battery will be saved.

Cheers Alexander
AlexanderB is offline Find More Posts by AlexanderB
Reply With Quote
Old May 30, 2010, 08:39 AM
Hooked on Heli's!
Chomper_Boom's Avatar
USA, NJ, Voorhees
Joined Aug 2005
303 Posts
I've seen people use a seperate lead with the resistor in it (parallel with the + terminal), but I like the simplicity of that method.

For selecting the resistor rating, it is simply Ohms Law: V= I x R
Where: V = The Voltage Across The Resistor
I = The Current Thru the Resistor
R = The Resistor Rating in Ohms

To limit the inrush current to approximately 1 Amp, the equation can be simplified to:

R = V

For Example using 6S Lipo, a resistance of 25ohm is appropriate.

Note: Sizing the resistor to limit the capacitor charging current to any value between 1A to 10A would more than likely be sufficient - Others with more capacitor experience may have a more specific recommendation.

The CONTINUOUS Watt rating required for a resistor is determined by the equation:
W = I^2 x R

Where: W = Watt rating
I^2 = I Squared (or I x I)

For short term operation of the resistor in the case of charging the caps on an ESC the continuous rating can most likely be reduced by a factor of 50 +/-

Therefore: W = 0.02 x I x I x R

In general a 0.5W to 1W resistor should suffice.
Chomper_Boom is offline Find More Posts by Chomper_Boom
Reply With Quote
Old May 30, 2010, 12:38 PM
Registered User
Joined Dec 2002
1,952 Posts
Quote:
Originally Posted by RCBlackSheep View Post
...The current limiting resistor in parallel is a good idea when you get to higher voltages.
Not to be picayunish, but the resistor is placed in series to charge up the cap before making the 'hard' connection.
oldcoot88 is offline Find More Posts by oldcoot88
Reply With Quote
Old May 30, 2010, 01:30 PM
HAL... Open the damn doors!
jfetter's Avatar
Miramar, Florida
Joined Jul 2007
8,517 Posts
Caps for the long cable runs yes, to prevent sparking though, nothing to worry about...

Jack
jfetter is offline Find More Posts by jfetter
Reply With Quote
Old May 30, 2010, 05:35 PM
I'd Rather Be Flying
RCBlackSheep's Avatar
Stockton CA
Joined Feb 2009
609 Posts
Quote:
Originally Posted by oldcoot88 View Post
Not to be picayunish, but the resistor is placed in series to charge up the cap before making the 'hard' connection.
Yea... what he said... hehehe Sorry my fingers didnt type in detail what I was thinkin. In the example it was refering to, the resistor is in series until the caps charge and then once the main connection is made then it will be in parrallel but but once the main connection is made then the resistor is no longer relevant to the curcuit since the electricty is now taking the path of least resistance. Alexander's pic is different than the example I was refering to and looks cleaner. I like it.
RCBlackSheep is offline Find More Posts by RCBlackSheep
Reply With Quote
Old May 30, 2010, 05:58 PM
I'd Rather Be Flying
RCBlackSheep's Avatar
Stockton CA
Joined Feb 2009
609 Posts
I found the pic of the example I was refering to. I still like Alexander's more.
RCBlackSheep is offline Find More Posts by RCBlackSheep
Reply With Quote
Old May 30, 2010, 06:27 PM
Registered User
Melbourne, Australia
Joined May 2006
6,407 Posts
Quote:
Originally Posted by Chomper_Boom View Post
I've seen people use a seperate lead with the resistor in it (parallel with the + terminal), but I like the simplicity of that method.

For selecting the resistor rating, it is simply Ohms Law: V= I x R
Where: V = The Voltage Across The Resistor
I = The Current Thru the Resistor
R = The Resistor Rating in Ohms

To limit the inrush current to approximately 1 Amp, the equation can be simplified to:

R = V

For Example using 6S Lipo, a resistance of 25ohm is appropriate.

Note: Sizing the resistor to limit the capacitor charging current to any value between 1A to 10A would more than likely be sufficient - Others with more capacitor experience may have a more specific recommendation.

The CONTINUOUS Watt rating required for a resistor is determined by the equation:
W = I^2 x R

Where: W = Watt rating
I^2 = I Squared (or I x I)

For short term operation of the resistor in the case of charging the caps on an ESC the continuous rating can most likely be reduced by a factor of 50 +/-

Therefore: W = 0.02 x I x I x R

In general a 0.5W to 1W resistor should suffice.
Using your example you would need a 25W resistor, or derated by 50%, a 12.5W resistor For 25V you would need to use a 1kohm resistor to get down to a 1W rating.

Your equation shows derating by a factor of 50 (i.e. 98%) rather than by 50%. Is that what you intended?
kgfly is offline Find More Posts by kgfly
Reply With Quote
Old May 30, 2010, 07:24 PM
Registered User
Joined Nov 2008
37 Posts
Hey Dag,

As always, I'm amazed by what a motivated individual can accomplish. I still follow all your threads in absolute amazement at your talent.

That being said, please take the rest of my post with a VERY light hearted approach.

I saw this post from you:

Quote:
Originally Posted by dag214 View Post
Attitude is for airspeed, and power for altitude.
And couldn't help but smile. YEARS ago, I LOVED for a new Flight Instructor Applicant to show up in my oral room and drop that little gem. The conversation goes something like this: So.... let me get this straight. The yoke controls airspeed and the throttle controls altitude? Just pull back on the yoke if I want to slow down?

You and I both know that pitch and power are absolutely linked. You can't change one without changing the other. And yes... there are 2 ways of explaining this concept, the Navy way and the Airline way. But on occasion I would have a student try to defend the simplicity of your statement to the death. It usually took about 30 minutes before the applicant was a quivering pile of goo infront of the chalkboard trying to defend his idea. I just always got a kick out watching someone fall on their sword trying to defend an idea they didn't understand. Maybe I'm just sick and twisted and enjoyed the suffering of others.

Anyhoo.... just a funny memory for me and wanted you to know you made me smile. Ok, back to lurking.
j32driver is offline Find More Posts by j32driver
Reply With Quote
Old May 30, 2010, 08:08 PM
I fly, therefore, I crash!!!
SteveT.'s Avatar
San Jose, CA
Joined Jan 2008
27,998 Posts
Quote:
Originally Posted by RCBlackSheep View Post
I found the pic of the example I was refering to. I still like Alexander's more.

The only problem with this picture, is that, (while many people do it as shown), it is really best to put a female on the positive side from the battery, and a male on the negative side from the battery. This will prevent accidentally plugging the power leads in with polarity reversed, which will immediately fry the FETs in an ESC.

SteveT
SteveT. is online now Find More Posts by SteveT.
RCG Plus Member
Latest blog entry: My hangar...
Reply With Quote
Old May 30, 2010, 08:39 PM
What's that button do?
Shpook's Avatar
Hollywood, FL
Joined Feb 2008
1,347 Posts
Quote:
Originally Posted by kgfly View Post
Using your example you would need a 25W resistor, or derated by 50%, a 12.5W resistor For 25V you would need to use a 1kohm resistor to get down to a 1W rating.

Your equation shows derating by a factor of 50 (i.e. 98%) rather than by 50%. Is that what you intended?
I agree, even a 12.5W resistor is one heck of a resistor.

Using a 3.3kW resistor will limit initial charging current to 6-7mA(on a 6s pack), and allows you to use a small 1/4W resistor.
Shpook is offline Find More Posts by Shpook
Reply With Quote
Old May 30, 2010, 10:18 PM
Hooked on Heli's!
Chomper_Boom's Avatar
USA, NJ, Voorhees
Joined Aug 2005
303 Posts
Quote:
Originally Posted by kgfly View Post
Using your example you would need a 25W resistor, or derated by 50%, a 12.5W resistor For 25V you would need to use a 1kohm resistor to get down to a 1W rating.

Your equation shows derating by a factor of 50 (i.e. 98%) rather than by 50%. Is that what you intended?
For all, here's a link to info on Caps that can be used to estimate the charging time using a specific resistor rating: http://www.bcae1.com/capacitr.htm

Based on a 25W resistor, it will take approximately 0.2 seconds to charge 1500uF worth of capacitance. The derating that I suggested was 'a factor of 50' or 98%, not 50% due to the short current duration. Note that this factor is somewhat arbitrary as I did not go thru the thermodynamic equations to calc the short time temp rise on the resistor for the momentary operation, however from a practical viewpoint - a 0.2 second 1A (25W) burst should not overheat a 1/2W or 1W resistor.

Also note that if the resistor is kept connected in the circuit (i.e. paralleled with the negative power connector as shown in one of the subsequent posts) that resistor should not be derated as much since it will see some continuous current even though small. In this situation a larger resistance value would be suggested. My example and charging currents (i.e. 1 to 10A) is suitable for the the setup shown by AlexanderB.

A larger resistor would be more suited for the connection method shown by RCBlackSheep. Keep in mind as the resistance value is increase so is the time that it takes to charge the capacitors. A 3000 ohm resistor will result in a charging time of around 22 seconds.

Hope this helps some - there really isn't any one rule of thumb and different methods would suggest different solutions. Maybe someone who is more familiar with the short time thermal ratings and performance of thin film resistors can shed some additional light...
Chomper_Boom is offline Find More Posts by Chomper_Boom
Reply With Quote
Old May 31, 2010, 02:06 AM
I'd Rather Be Flying
RCBlackSheep's Avatar
Stockton CA
Joined Feb 2009
609 Posts
Quote:
Originally Posted by Draknkep View Post
The only problem with this picture, is that, (while many people do it as shown), it is really best to put a female on the positive side from the battery, and a male on the negative side from the battery. This will prevent accidentally plugging the power leads in with polarity reversed, which will immediately fry the FETs in an ESC.

SteveT
I whole heartedly agree. I only posted the pic to show what I was talking about with the resistor. That is NOT an example of the connectors I use. I myself only use "female only" connectors on my lipo batteries for safety reasons. I have a RC buddy that had a lipo that smoked on him while he was driving home. It had a micro deans on it and the negetive pin of the micro plug touched the positive contact of the balancing plug of the same battery. Needless to say, that was exciting for him and not something he wants to repeat. The EC3 or the Ultra Deans work for my purposes very well. Plus they are made in a way that you cant plug them in reverse polarity. If I were to add the resistor to one of my setups, it would probably have another female connector on the battery as well and the resistor would be on the ESC side of the connection. I would go one further and incorporate Alexander's pic and make in a way that it would be easier to use and save weight.
RCBlackSheep is offline Find More Posts by RCBlackSheep
Reply With Quote
Old May 31, 2010, 03:32 AM
Scratch Builder
AlexanderB's Avatar
Braunschweig, Germany
Joined Sep 2006
1,325 Posts
Thank you all from my side.

I just have to make clear that I only posted the picture but I have stolen the idea

Plus I have learned a lot of how it works with the resistor.

Cheers Alexander
AlexanderB is offline Find More Posts by AlexanderB
Last edited by AlexanderB; May 31, 2010 at 08:26 AM.
Reply With Quote
Reply


Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Good B-36 site (drawing with cross sections, too!) Thomas B Scale Kit/Scratch Built 26 Oct 11, 2011 10:14 PM
Discussion B-36 B-52 kgmorris Scale Kit/Scratch Built 20 Jan 02, 2009 07:27 AM
b-36 dorysch1 Scale Kit/Scratch Built 32 Feb 01, 2004 09:33 PM
Questions about my wally B-36...I'm new to foam. Boba_Fett Foamies (Kits) 8 Jun 11, 2002 04:36 PM
B 36 Trobber Parkflyers 3 Dec 30, 2001 01:33 PM