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Old Jan 21, 2003, 06:45 PM
Just an average RC'er
Jim McPherson's Avatar
Laurel, Maryland USA
Joined Aug 2000
4,291 Posts
I thought the RFFS's ESC was good to at least 2 amps, and probably more. It is rated for 2-4 nicads or one li-ion cell. You can use more than one cell by making a bypass to have just one cell power the RX and have the other cell's + lead go straight to the motor. With the Bob Selman actuators and 2 Li-Ion cells I'd think you could fly a pretty large airplane (in context) with the RFFS.

-Jim
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Old Jan 22, 2003, 08:54 AM
in persuit of low wing loading
Gordon Johnson's Avatar
Boston, Mass
Joined May 2001
6,438 Posts
Here is how I believe it would be done. The only reason I know is Graham explained it to me a couple of days ago. If I've got this wrong, please correct me Jim or Graham.

As I understand it this works because the RFFS-100 switches or pulses through the negative wire to the motor. It is also likely that the two LiPolys may become discharged unequally because one cell is powering the motor *AND* the receiver and actuators. However, since the actuators and receiver don't consume that much it may not be a big deal. Still it would be worth measuring voltage for the two cells after flying and if they are different, charging them separately.

Note, I have not tried this yet. And, I have not verified with absolute certainty that the RFFS-100 switches through the negative wire. When I do use this technique I will probably use either the M20-HV or the N20 motor, also probably the HV. But, I've also used the N20-LV on two cells and since they are about $1 to $2 each burning them out might not be a problem. But, if your motor is not easily replaceable I'd suggest going with the HV variety.

Also, your actuators will not be getting the volts from two cells, so will develop the same torque they would from one cell.

Again, thanks go to Graham for explaining all this to me.

Edit: thanks to Jim I've corrected the schematic to be series.

Gordon
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Old Jan 26, 2003, 11:04 PM
Registered User
Eastern Mass.
Joined Sep 2002
53 Posts
I have it working

I retract my statement that the unit does not meet the 5.5 V specification. I finally figured out partly how the circuit works. I checked my other rcvr and could read the designation on the output switch. It is an IRF7105 and I downloaded a spec sheet. It is a dual Hexfet power mosfet but one FET is N-channel and one is P-channel. I believe that the only reason to have a
P-channel is if a brake circuit is used. I traced the terminal connections and that seems to be what is being done. I tried to detect if the brake makes a difference by using the rcvr and then a direct battery connection. I could not detect any difference in the time for the prop to stop.
What happens when we connect a second cell is that the reverse diode in the P-Channel FET prevents the motor terminal voltage from being higher than the primary battery voltage by the FET's internal diode voltage drop. That is why the motor keeps running with the second cell. If the P-channel FET is disconnected, the rcvr still works normally and then there is only the forward voltage limit of the FET that determines how much voltage can be applied. In the case of the 7105, that is 25 volts. The current limit is of course the same as specified. If the dual mosfet contained two N-channel units, the current capability would be almost doubled although we probably don't need that much current for the small models we have.
HOW TO MAKE IT WORK:
The p-channel FET must be disconnected so there is no brake. All that is needed is to lift off terminal 3 of the 7105. Using a fine needle point tool pry up terminal 3 while heating with a pointed soldering iron. The 7105 is on the under side of the board nearest the motor output pins. Pin 1 is located near the circle mark on the case and pin 3 is the third one to the right if you hold the rcvr with the motor output terminals to your left. You can check if this is pin 3 with an ohmmeter from that pin to the battery plus terminal on the rcvr. There should be very low resistance.

I made this change and then could easily apply +8 volts between the motor negative terminal and the receiver negative terminal.

I checked out the mod using two Lipoly cells as Gordon showed in his drawing and a 10 ohm resistor for a load. I measured the voltage across the resistor as the transmitter throttle control was moved. The voltage changed smoothly and reached a maximum of 7.3 volts (730 mA).

More than two cells could be used; just keep below the 25 volt FET limit. This would be a lot of power if anyone needs it. At some point the actuators would not do the job due to the heavier and faster model.

I am looking at possibly making a twin engine model such as the DC-3. It is just in the thought stage now.

Roger Carignan
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Old Jan 29, 2003, 08:47 AM
in persuit of low wing loading
Gordon Johnson's Avatar
Boston, Mass
Joined May 2001
6,438 Posts
If no one (Acer?) has any objections, I'm going to ask Graham to take the part of this thread that deals with powering the motor on an RFFS-100 setup and put it in its own thread. I'm going to suggest it be titled "RFFS-100, powering the motor with two LiPoly". And I'll also suggest that the link be added to the good links thread. I think this is one worth keeping, and "RFFS-100 Questions" would not give a clue what's inside.

Gordon
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Old Feb 11, 2003, 12:58 PM
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Pittsburgh, PA
Joined Jan 2002
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Updates ?

Did this thread get split as Gordon suggested : "RFFS-100, powering the motor with two LiPoly". ? If so, could someone cite the new thread, please. A search didn't reveal anything.
-Mike
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Old Feb 23, 2003, 12:06 PM
Team 30 Micro EDF
Mike Taylor's Avatar
Camarillo, California
Joined Apr 2002
4,516 Posts
Roger Reported: (to ake the RFFS power the motor w/2 LiPolys)
"All that is needed is to lift off terminal 3 of the 7105. Using a fine needle point tool pry up terminal 3 while heating with a pointed soldering iron. The 7105 is on the under side of the board nearest the motor output pins. Pin 1 is located near the circle mark on the case and pin 3 is the third one to the right if you hold the rcvr with the motor output terminals to your left. You can check if this is pin 3 with an ohmmeter from that pin to the battery plus terminal on the rcvr."

Has anybody else tried this yet? If so, have there been any other problems deevelop with the radio? I've been waiting for a second opinion, and maybe a picture with circles and arrows... The reason I bring this up is that I need more power, and the question of the 30mm EDF on high voltage has come up again...
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Last edited by Mike Taylor; Feb 23, 2003 at 12:08 PM.
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Old Feb 23, 2003, 01:57 PM
Sticky Shepherd
Graham Stabler's Avatar
Oxford/England
Joined May 2001
4,017 Posts
I mave moved the relevant posts, unfortuantely I had to move them rather than copy them but here they are.

It might be better for someone to spend 5min just doing the thread from scratch as a "how to"
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Old Feb 23, 2003, 04:18 PM
Just an average RC'er
Jim McPherson's Avatar
Laurel, Maryland USA
Joined Aug 2000
4,291 Posts
Quick recap of why and what we're doing:


The stock RFFS will not work on 2 lithium cells. So in order to run larger airplanes Rodgercar found a way to wire 2 (up to 6 if you really wanted to) cells to the RFFS. To do so you need to:

"lift off terminal 3 of the 7105. Using a fine needle point tool pry up terminal 3 while heating with a pointed soldering iron. The 7105 is on the under side of the board nearest the motor output pins. Pin 1 is located near the circle mark on the case and pin 3 is the third one to the right if you hold the rcvr with the motor output terminals to your left. You can check if this is pin 3 with an ohmmeter from that pin to the battery plus terminal on the rcvr."


After you do that you wire the RFFS like this:




Note that after doing this you will not have greater actuator power as the RFFS/Actuators only see the voltage from one battery. You will also need to charge each battery individually becuase of the imbalance in discharging using this method.

Ok, now somebody go make a 6 lithium cell, 4 KP-00 powered B-52!


-Jim
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Old Mar 22, 2003, 09:48 AM
Team 30 Micro EDF
Mike Taylor's Avatar
Camarillo, California
Joined Apr 2002
4,516 Posts
How has this mod to the the RFFS worked out for those of you that have done it? Have there been any other problems frrom this mod?
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Old Mar 22, 2003, 06:32 PM
Registered User
Pittsburgh, PA
Joined Jan 2002
1,271 Posts
Quote:
Originally posted by Mike Taylor
How has this mod to the the RFFS worked out for those of you that have done it? Have there been any other problems frrom this mod?
Well Mike,
I guess it's up to me and you to actually try this. I'm hesitant like you are -I can barely even see the traces much less unsolder them I probably could get one of the techs or EEs at work to do the surgery.
I think I want to do this to get my 17" Mulligan going. After following your effort I'm convinced it needs a 2 cell (LiPo) motor.
And I'm a frugal kinda guy and so I am compelled to wait 'til I "Can't live without it" kicks in.
-Mike
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Old Mar 22, 2003, 08:10 PM
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Chris3D's Avatar
New Paltz, NY, USA
Joined Apr 2001
198 Posts
I wonder what the feasibility is of breaking the trace from pin 3 with a knife and soldering a tiny surface mount switch or some jumpers across the break so the RX could be switched back to normal is desired. Or is this not necessary at all? Would it be safer to simply break the trace instead of trying to unsolder that particular pin? You could always repair the break with a small dab of solder.
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Old Mar 22, 2003, 11:11 PM
in persuit of low wing loading
Gordon Johnson's Avatar
Boston, Mass
Joined May 2001
6,438 Posts
Chris,
My understanding is that once the modification is made you can use either the two cell technique or the single cell. The mod would not be needed if a different chip had been chosen for the RFFS-100. So, no swich is needed (I think). I plan to do this mod, but need to get a plane ready that can use it first.

Gordon
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Old Mar 23, 2003, 06:36 AM
Sticky Shepherd
Graham Stabler's Avatar
Oxford/England
Joined May 2001
4,017 Posts
you would only need the switch if you really wanted a brake on one cell. It isn't needed so a quick dab with a scalpel might be the easiest way to do the mod. I have desoldered entire chips like that, cut all the pins off and remove the remnants afterwards.

Graham
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Old Mar 23, 2003, 09:40 AM
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Chris3D's Avatar
New Paltz, NY, USA
Joined Apr 2001
198 Posts
Ok then, I might have to give it a try, so I can build the B-25 Mitchell I've been planning
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Old Apr 10, 2003, 11:04 PM
Who Says Penguins Can't Fly?
aeropenguin's Avatar
Bay Area, California
Joined Jan 2003
2,713 Posts
Wow guys, GREAT job!

Thanx! It is these guys right here who make this hobby great!
-aeroP
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