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Old Feb 18, 2002, 11:55 AM
Lifes 2 short, go sloping
colorado @ 5500feet
Joined Sep 2001
1,054 Posts
Voltage reducing diode??

I ordered a plane locator from tower hobbies that only works with 4.8volts, the problem is most of my gliders use a 5 cell 6v system (longer flights). Can I use a voltage reducer (diode) to put in between the locator and reciever to drop the volts between 5 and 10 volts. it doesn't need to be be that high of an amp rating as it is only for the reciever.

Or has anyone used the "Hobbico Air Alert Flight Pack Monitor " does it work with 6v ok or will it burn out?

http://www2.towerhobbies.com/cgi-bin...1&I=LXSZ87&P=0[/URL]

Can anyone help with this, I live in Colorado and the slopes are huge (average 1000-3000ft vert. drop) so it can be a huge pain to find my planes (lost one last sping )

Vince
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Old Feb 18, 2002, 03:56 PM
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West Dundee, IL, USA
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You could use a simple three leg regulator like a 7805. In, Out and Ground. It'll take up to 30V in.
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Old Feb 18, 2002, 04:20 PM
jlk
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I don't have the air alert but am thinking of buying some. We go to Piont of the Mountain in Salt Lake every year. I think the voltage will probably be fine but you will lose the low voltage function of the device. Since you are probably only using it for a plane finder it should be fine.

Jordan
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Old Feb 18, 2002, 05:32 PM
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Carver, MA USA
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Re: Voltage reducing diode??

Quote:
Originally posted by Vinnya42
..................., the problem is most of my gliders use a 5 cell 6v system (longer flights). Vince
5 cell 6 volt pack won't give you longer flights, only higher torque and faster actuation at the servo arm. Matter of fact if I have it correct this should actually give you less flight time. I know it works that way for DC motors, which is what servos are.

Ron
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Old Feb 18, 2002, 06:11 PM
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Re: Voltage reducing diode??

No, a diode will not reduce the voltage of a DC current. It's either full voltage or in reverse, 0V. A diode will cut AC about in 1/2 because it only lets 1/2 the current thru. Conducts on 1/2 the AC cycle and blocks the other 1/2 of the cycle.
hoppy


Quote:
Originally posted by Vinnya42
I ordered a plane locator from tower hobbies that only works with 4.8volts, the problem is most of my gliders use a 5 cell 6v system (longer flights). Can I use a voltage reducer (diode) to put in between the locator and reciever to drop the volts between 5 and 10 volts. it doesn't need to be be that high of an amp rating as it is only for the reciever.

Or has anyone used the "Hobbico Air Alert Flight Pack Monitor " does it work with 6v ok or will it burn out?

http://www2.towerhobbies.com/cgi-bin...1&I=LXSZ87&P=0[/URL]

Can anyone help with this, I live in Colorado and the slopes are huge (average 1000-3000ft vert. drop) so it can be a huge pain to find my planes (lost one last sping )

Vince
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Old Feb 18, 2002, 06:31 PM
DNA
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NE Ohio
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3,311 Posts
Any silicon diode put in series in the power lead of battery pack
will have a 0.6 volt voltage drop across it. So if use two diodes
in series you will get a 1.2v drop.
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Old Feb 18, 2002, 06:40 PM
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Colorado Front Range, USA, EARTH, Sol system, Milkyway Galaxy
Joined Nov 2001
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DNA is right.

A diode is like a check valve is for fluid or air.
It lets current through in one direction but
not the other.

The difference is that a diode drops .6 to .7 volts across
it in the forward biased mode. This is because it is not
a perfect check valve.

This is for a silicon diode, germanium is about .3 volts
but no power diode will be germanium.

The problem with this is that if you are pulling 1 amp
in your plane locator you will be dropping 1A * .7V = .7 Watts
of power in the diode. This is a fairly serious device
Somthing in a TO-220 or SMD-220 package would work well.

I don't know what your plane locator is pulling for current but
be carefull.
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Old Feb 18, 2002, 06:48 PM
jlk
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Yes, Ron was right, with the 5 cell pack you get less time, just like adding a cell to a motor with the same prop, more power less time! You might want to just go to 4 cells.

Jordan
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Old Feb 18, 2002, 11:47 PM
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Hi,

I also doubt that 5 cells = longer flight times.

Stronger faster servo movement yes, longer duration, I doubt it.

If you want longer times, use larger ( more mah) batteries. 4 larger ones may weigh the same as 5 small ones and will give you a lot greater flight time.

Info on .6 V drop of Silicon power diodes is correct. Thats what it takes to make them conduct.

I once built a 6 cell electric hydroplane boat.
I used four 5amp diodes in series right off the pack to run the reciever and servos.

Worked great! No problems for entire year I ran it.

Dave
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Old Feb 19, 2002, 02:28 AM
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Baden Baden, Germany
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We also use diodes for running two receiverbatteries in one plane. This is usually done in large planes having a big number of strong servos for safety and the currents there are sometimes higher then in a small Speed 400 plane. Choosing the right diodes you will have no problems. Especially as there is no further electronic stuff but the diodes this version is very reliable.


If you just want to reduce the voltage for the plane locator just solder the diode into the + cable going to the locator. You do not have to reduce the voltage of the whole RC-system.

Eike
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Old Feb 19, 2002, 10:55 AM
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USA
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Quote:
If you just want to reduce the voltage for the plane locator just solder the diode into the + cable going to the locator.
That is what I would do. I would be a little concerned about the signal line, since it could swing higher than the reduced VCC. But it is worth a try.

Quote:
I did not say 5 cells give longer flight times. I was saying the time reduction is hardly worth mentioning. BTW, have you really done any testing? It just might surprise you.
I have. The current consumption impact from a standard servo is only when the control arm is moving. If you are a lazy sport flyer, where the controls are not thrown around, then the time reduction is not worth mentioning. If you like to tear up the sky then YES, you will see a theoretical time reduction of up to 20%.

Quote:
This is like the wives tale that digital servos eat up batteries compared to analog. Not so! Rather exhaustive tests by MFI disprove this one also.
I was curious about that too, since JR reported the same thing in their marketing literature when the digitals first came out.

From what I can tell, they designed their tests to be biased towards the way a wild stunt pilot would fly. It is true, if you took a high torque standard servo and a similar digital, then flew like a crazed banshee, their servos would both consume similar current. From what I can see, both high torque designs draw about the same current when the control arms are moving rapidly.

However, if you fly like a typical sport or scale flyer, then there is a substantial difference in current draw. For example, my standard servos have an idle current (stick not moving) of under 15mA. However, my JR digital servos idle at about 180mA (their motors are always being driven). Slow stick movement current is still well under 100mA on the standard servo (still less than the idle current of my digitals).

A model with four digital servos could realize a current draw of more than a .6 amp just sitting on the runway (versus 0.05A for standard servos). In my brief test, the only time I saw that the two had similar currents is if I threw the sticks around like a madman. Being a sport heli flyer, the impact of using digitals is real. But, nothing a bigger NiCd pack can't fix -- the precision of a digital is worth the trouble.
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Old Feb 19, 2002, 11:26 AM
Lifes 2 short, go sloping
colorado @ 5500feet
Joined Sep 2001
1,054 Posts
6 volts

I also use the 6volts to balance (no dead weight) and to increase torque to the servos. But it is mostly for CG on my bigger gliders.

my flying abilities take away from any real time increases but I can imagine a day when I a get to fly for 5 hours straight and do the insane tricks back to back and not have to chase my dog down when she wanders off on the slope and my neck can handle the strain of 5 hours of looking up.
That will be the day!8-)
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Old Feb 19, 2002, 02:50 PM
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USA
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Quote:
Originally posted by Gary

You might want to check your system as there is no reason for the servo to draw that much current at idle nor should the motor always be driven. It should be about 15ma.
Gary, good idea. I was using my JR PPM radio (XP783). So, I just installed a JR DS8417 digital servo on my old Airtronics system (VG6). Nothing is connected to the servo's arm.

The servo still buzzes like a mad bee (that is the sound of the motor being driven). The control arm is static. The current varies slightly, but it is well above 100mA. I can sometimes see 200mA flash by. My wimpy standard Hitec was drawing 4mA under the same conditions.

From my observation, the digitals all seem to buzz while at "idle." The buzz is the sound of the motor being driven at a high rate, even though the servo's control arm position does not indicate any movement. Last summer I saw a CAP232 with Futaba digitals that sounded like a crazed hornets nest (yes, the engine was off). I wonder if other folks have noticed the same noise?

But, no doubt there are digital servos that are better behaved. Especially since you measured only 15mA on yours. What servo did you measure and what radio was it on? Can you please repeat the experiment to confirm your measurement?

I am sorry to have gone off-topic. Perhaps a new thread should be created if there is much interest in more digital servo dialog. I don't mind getting my arm chair science debunked, especially if others can learn something valuable from it.
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Old Feb 19, 2002, 06:29 PM
Dr John
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Lake Placid, Florida
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Back in the early days of electric cars (RC12E) before ESC's when speed control was with a servo driven reostat it was normal to use two diodes in series between the 6 cell power battery and the Rx to act as a BEC.
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