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7955 with a Y-Harness?

Old 02-13-2010, 05:44 PM
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IFlyEm35
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Default 7955 with a Y-Harness?

Looking to run 7955's with a Y-harness. All the servo wire will be HD with regulated 6.0 volts. I would normally run a power expander or PowerBox but kicking this idea around. How much power loss can I expect with this set up?

It's going on a EF 110 Yak. I would be using a Y on both ailerons and Rudder.

Good? Not good?
Old 02-13-2010, 07:31 PM
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Default RE: 7955 with a Y-Harness?

Why?????????????/ Poor set up
Old 02-13-2010, 07:56 PM
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Default RE: 7955 with a Y-Harness?

I agree and nornally I wouldn't do it this way. However I have a 9 ch receiver and need to power 9 servos, smoke, and a optical cutoff

Old 02-13-2010, 07:57 PM
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Default RE: 7955 with a Y-Harness?

I agree and nornally I wouldn't do it this way. However I have a 9 ch receiver and need to power 9 servos, smoke, and a optical cutoff

Old 02-14-2010, 09:24 AM
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Default RE: 7955 with a Y-Harness?

When using a "Y", the current flow to each servo is cut in half.
I never use "Y"s.

Jim
Old 02-14-2010, 09:36 AM
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Default RE: 7955 with a Y-Harness?

One of those servos can very nearly exceed the amperage capablity of the servo lead connectors and that's if it's working normally. They can pull up to 50% more current if stalled or there is some other problem and you wanna put two of 'em on one lead????
Old 02-14-2010, 09:58 AM
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Default RE: 7955 with a Y-Harness?

What would be the difference if someone was using 8611's with a matchbox. I'm just looking at using a Y because the 7955's are programable.
Old 02-14-2010, 10:01 AM
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Default RE: 7955 with a Y-Harness?

The results would be the same with a matchbox (unless the matchbox has it own power supply). Current flow would be cut in half.
I never use matchboxes either.

Jim
Old 02-14-2010, 10:11 AM
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rcairflr
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Default RE: 7955 with a Y-Harness?

ORIGINAL: IFlyEm35

What would be the difference if someone was using 8611's with a matchbox. I'm just looking at using a Y because the 7955's are programable.
You are exactly right, there is no difference. If using a matchbox the load gets constricted down to 1 22AWG wire coming out of the matchbox. I have been running a Y harness on the ailerons of my QQ Yak for well over a 150 flights with no issues and on my Wildhare Extra 260 for 250-300 flights.

Now of course the best solution is to be able to run each servo to a separate channel, but I am in the same situation as you, my recevier/transmitter (JR 9303) is not capable of this.

There is no way to simply answer you question about current loss. The formula is E=IxR (where E is the voltage loss and I=the current draw from the servos and R=the wire resistance). So as you can see from the formula the voltage drop will be more as the current draw goes up, but to make things even more complicated, the resistance of the wire will be higher as the temperature of the wire goes up, then of course you have the connections, but to say the current will be cut in half is not necessarily true.

Old 02-14-2010, 10:16 AM
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Default RE: 7955 with a Y-Harness?

RC,

What servos are you running with the Y?
Old 02-14-2010, 10:22 AM
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Default RE: 7955 with a Y-Harness?

HS=7955 on the Yak and HS-5945 on the Extra

I use 20 awg Y harnesses.
Old 02-14-2010, 10:35 AM
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Default RE: 7955 with a Y-Harness?

Servos with limited current flow still work, they just have less strength (torque).

Jim
Old 02-14-2010, 10:45 AM
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Default RE: 7955 with a Y-Harness?

ORIGINAL: Hammbone

Servos with limited current flow still work, they just have less strength (torque).

Jim
The servos will never be even close to max'd out on the ailerons unless you are hanging a brick from them and until you hit the limit of the weakest point in your system, there will be no difference in operation.
Old 02-14-2010, 11:00 AM
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Default RE: 7955 with a Y-Harness?

ORIGINAL: rcairflr

ORIGINAL: Hammbone

Servos with limited current flow still work, they just have less strength (torque).

Jim
The servos will never be even close to max'd out on the ailerons unless you are hanging a brick from them and until you hit the limit of the weakest point in your system, there will be no difference in operation.
I agree with part of this statement. "Until you hit the limit of the weakest point in your system, there will be no difference in operation."
I believe that the limit is reached much sooner than you do, though.
These new high torque digital servos use a lot of amps, and I think when you are trying to power 2 of these servos through one standard connector, the servos will not get the current they need.
Even if you are able to force excess amps through the one standard connector, you are then going to get a voltage drop, which will also weaken the servo and cause less torque.
Jim
Old 02-14-2010, 11:29 AM
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Default RE: 7955 with a Y-Harness?

7965 servos have a stall current of 3A. 7955s probably draw the same.

If the Y harness is HD and can deal with 6A, then it will not be the bottle-neck. The same idea applies to the receiver as the battery provides the power through the bus on the receiver and then the Y-harness to the servos.





Old 02-14-2010, 11:36 AM
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Default RE: 7955 with a Y-Harness?

A standard connector is only rated at 3 amps. If you try to push 6 amps through a standard connector you will get a huge voltage drop.

Jim
Old 02-14-2010, 01:17 PM
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Default RE: 7955 with a Y-Harness?

Let's assume your statement is true. What maneuver is someone going to perform on a 35% aircraft that would cause a load from each aileron of 6 amps. Let's assume someone pulled a maneuver that caused 6 amp load from each aileron. How long would that max load last, maybe a fraction of a second.

I don't think in our usage that you will see a huge voltage drop (I'm not sure what is meant by a huge voltage drop) with a 6 amp load through our connectors applied for a short period of time.

I am going to try and come up with a couple of 1 ohm resistors to try this using my A123 batteries and perform a little experiment. The only sure way to know what kind of voltage drop is happening, is to actually do it and measure the results.
Old 02-14-2010, 01:57 PM
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Default RE: 7955 with a Y-Harness?


ORIGINAL: Hammbone

A standard connector is only rated at 3 amps. If you try to push 6 amps through a standard connector you will get a huge voltage drop.

Jim
what standard connector? Even if it is rated at 3A, can one use a better one?

My point is Y-harness can be chosen so it is not a bottle-neck. The same argument goes to other components.

The other point is of course the chance that the 6A load can happen.
Old 02-14-2010, 02:01 PM
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Default RE: 7955 with a Y-Harness?

This is from the Smart Fly website:

We have measured a digital servo, unloaded, drawing 0.6 Amp when kept in continuous motion.

Now this is one digital servo with NO load. If you have 2 servos on a "Y", you are feeding those 2 servos through one standard connector. You have just cut your rated capacity in half, and you now have 1.5 amps rated capacity per servo (because a standard connector is only rated at 3 amps).
With a working servo and NO load, you are already at almost 1/2 of your rated capacity (.6 amps). Imagine how much more current you might use when you start applying an in flight load to the servos. You will easily be over the rated capacity. Will 2 aileron servos be at 6 amps all the time........ No, but during high load maneuvers when you are using a lot of amps momentarily, you will get a big voltage drop and your servos will have less torque. So, right at the time when you need torque the most, you will get voltage drops and have the least amount of torque available.

Here is more info from the Smart Fly website:

We have measured a half volt drop over eighteen inches of 22 gauge wire and one connector at 5.0 amps.

This is the law of physics in play here guys. Voltage drops are always present in our set ups, the trick is to keep these voltage drops to a minimum. You keep the voltage drops to a minimum by using components that are up to the task. A standard connector is capable of supplying current flow to one high torque digital servo. It is not up to the task of supplying current flow to more than one high torque digital servo. You will get excessive voltage drops.

Does this mean that your plane will fall out of the sky...... No, it just means that your servos will have reduced torque. It just means that you will not have the optimum set up.
Many guys are flying planes with matchboxes, "Y"s, and other components and set ups which limit their current flow. The plane will fly this way, just understand that you don't have an optimum set up. You can choose not to have an optimum set up.

Compare this to engine selection. Many guys use cheaper engines that weigh more and don't produce as much power. They saved money though. They could have purchased an engine that weighs less and produces more power, but it costs more. They chose to buy the cheaper engine. Their plane will still fly around with the cheaper engine, but it won't fly as good because it weighs more, and it won't have as much power. They won't have the optimum set up because of the engine they chose to buy.

It's the same thing with how you set up your radio system. There is an optimum way to provide the best current flow, but other ways will work. Other ways just may not be as good. You may have limited current flow. Your plane will still fly, and you can still enjoy the hobby and have fun, but you won't have the optimum set up.

All I have done in this thread is present the facts which should help someone decide how they want to set up their radio system.
You can use "Y"s if you want to, but understand that you will get voltage drops. You will save money this way though, because you won't be purchasing a Power Expander or similar.
If you want to spend the money for an optimum set up, you have that choice as well.

Jim
Old 02-14-2010, 02:05 PM
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Default RE: 7955 with a Y-Harness?

ORIGINAL: nonstoprc


ORIGINAL: Hammbone

A standard connector is only rated at 3 amps. If you try to push 6 amps through a standard connector you will get a huge voltage drop.

Jim
what standard connector? Even if it is rated at 3A, can one use a better one?

My point is Y-harness can be chosen so it is not a bottle-neck. The same argument goes to other components.

The other point is of course the chance that the 6A load can happen.
A standard connector is what comes on a servo when you buy it. A standard connector is what you plug into the output ports on a rx. We can not upgrade them because the upgraded plug can not be plugged into the rx. This is why many people use Power Expanders. A Power Expander allows you to plug each servo into it's own port. Each servo now has it's own dedicated current flow. You don't use "Y"s which cut the supplied current flow in half.

Jim
Old 02-14-2010, 02:13 PM
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Default RE: 7955 with a Y-Harness?

Another problem is that most rx's have standard ports for power input into the rx. Now you are trying to provide current to all of your servos through only one or two standard plugs on the input side of the rx. This is a huge bottleneck.
This is why Power Expanders have power input directly into the Power Expander through HD plugs with much higher ratings.

I'm a big fan of the JR Powersafe rx's (the 922, 1222, and 9100). They have HD inputs for your batteries, and they come with a failsafe switch that is not an inline switch and does not limit your current flow at all. Plug only one servo into each output port on the rx, and you have a very simple, inexpensive, and robust system that will provide all the current your servos will ever need.

Jim
Old 02-14-2010, 02:19 PM
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Default RE: 7955 with a Y-Harness?


ORIGINAL: Hammbone

ORIGINAL: nonstoprc


ORIGINAL: Hammbone

A standard connector is only rated at 3 amps. If you try to push 6 amps through a standard connector you will get a huge voltage drop.

Jim
what standard connector? Even if it is rated at 3A, can one use a better one?

My point is Y-harness can be chosen so it is not a bottle-neck. The same argument goes to other components.

The other point is of course the chance that the 6A load can happen.
A standard connector is what comes on a servo when you buy it. A standard connector is what you plug into the output ports on a rx. We can not upgrade them because the upgraded plug can not be plugged into the rx. This is why many people use Power Expanders. A Power Expander allows you to plug each servo into it's own port. Each servo now has it's own dedicated current flow. You don't use ''Y''s which cut the supplied current flow in half.

Jim
The servo connector is designed to handle the stall current. It should not present a problem.
Old 02-14-2010, 02:22 PM
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Default RE: 7955 with a Y-Harness?


ORIGINAL: nonstoprc


ORIGINAL: Hammbone

ORIGINAL: nonstoprc


ORIGINAL: Hammbone

A standard connector is only rated at 3 amps. If you try to push 6 amps through a standard connector you will get a huge voltage drop.

Jim
what standard connector? Even if it is rated at 3A, can one use a better one?

My point is Y-harness can be chosen so it is not a bottle-neck. The same argument goes to other components.

The other point is of course the chance that the 6A load can happen.
A standard connector is what comes on a servo when you buy it. A standard connector is what you plug into the output ports on a rx. We can not upgrade them because the upgraded plug can not be plugged into the rx. This is why many people use Power Expanders. A Power Expander allows you to plug each servo into it's own port. Each servo now has it's own dedicated current flow. You don't use ''Y''s which cut the supplied current flow in half.

Jim
The servo connector is designed to handle the stall current. It should not present a problem.
It doesn't present a problem if you are only powering ONE servo through it, but if you are powering TWO servos through it (like when using a "Y"), then it IS a problem.

Jim
Old 02-14-2010, 02:43 PM
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Default RE: 7955 with a Y-Harness?

In Theory, it is a problem, in practice, I'm running 7955s on Ys on my 110 with no ill effects. In fact, I backed off my aileron throws because they were too much. I do use two channels for the rudder servos to get all I can there. putting a short Y at the rx helps with line-loss also. Thor.
Old 02-14-2010, 03:42 PM
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Default RE: 7955 with a Y-Harness?

Aileron throws were too much? Too much for what?
I guess it all depends on how you fly. I've never owned a plane in over 21 years that had too much throw on anything. I'll take all the throw I can get, and I don't want any blowback.

Jim

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