Servo Powered Straight Off Battery Question

[yoraish]

Originally this question was asked in another thread but I thought It is better placed here.

Anyway, I plan on buying a Hitec HS-7954SH servo. Now I want to power it (obviously), but I don'y quite know how.

Here comes my first question, on a receiver, the port that says b/c (battery right?) provides the voltage of the battery? Can I plug in any motor and it will run on 7.2 volts? (for example)

I was told I can power the servo straight off the battery this way:

Originally Posted by sweli

The stock servo has around 57 in-oz of torque at 6v.
So the one you ordered will be night and day difference!

You CANNOT run your new servo off the stock HPI ESC in your pics!!
It will brown out or possible smoke.
It probably only has 1.5-3 amps of BEC current at 5v.

You will need to wire it directly to the BATT or will need a separate BEC.
You can get a servo "Y" harness, pull the red wire out of the part that plugs into your Rx. Then cut one output leg of the "Y", and solder the black and red leads to the connector of your ESC that plugs into your BATT.

Does your LHS has a CC BEC? If so, usually they will program it for you while buying. Have them set it to 6v or maybe 6.2max. Wire in and power the Rx and servo off of it.

Please help me understand what he said. He means I cannot hook the servo power (red wire) to the receiver because I'll fry the ESC, so I can take a Y harness, hook one side to the servo and the other to the receiver. The connection on the receiver side should have its red wire cut. Leaving only the black and signal wires attached to the receiver. And now take the third output of the Y harness, trim it, and connect its red and black wires to the red and black wires that come out of the battery (after the connector of course).
That way I don't need a BEC right? But now the servo will receive 7.2 volts straight from the battery, is it OK for it?

Thank you very much for your help!

yorai

[spillproof]

Your RC has 2 power systems. A motor system, and a rx/servo system. The motor system might be 3s(12v), 4s(16v), 6s(24v), or higher. This voltage is to high for na RX/Servo. A BEC steals power from your motor system for the RX/Servo. This is done via either an internal BEC in your ESC (ESC is part of the motor power system) or is a seperate item (external bec) .

The BEC included in most ESC is going to operate at 5v, and maybe supply 3amps. This is plenty for general applications in RC. Crawlers are not general. The big wheels jam up and need more torque to turn them. That torque is not free, it costs either speed (gearing) or it costs wattage (amps x volts). An external BEC can help supply the increased voltage and amperage to actually used by the 7954 you are considering. It runs at 50% more voltage (7.4vs vs 5v) and can draw more amps.

[yoraish]

That's a good point, but considering I'm running a 7.2V Ni-Cd battery this is actually a good voltage to run the servo on (isn't it?).

[bob1961]

yes most voltage you should run through a servo is maxed at bout 7.4 for a safe voltage........bob

....

[stanmondz]

the nicd will be 8+ volts charged though. get a BEC or a HV servo and know it will be safe, or try it and see if the servo will handle the voltage

[spillproof]

Your servo might like it, if its rated, but most RX don't like anything above 5 or 6v. Its common to use the ESC BEC to power the RX, and then have a high torque servo run off a different voltage via BEC through a bypass. In theory you could run the bypass directly off the battery since its the right voltage. You would have to do some creative soldering/wiring from your battery lead.

With nitros, the rx/servo system is fed directly off a nicd battery.

[Trubble]

Originally Posted by yoraish

That's a good point, but considering I'm running a 7.2V Ni-Cd battery this is actually a good voltage to run the servo on (isn't it?).

Originally Posted by stanmondz

the nicd will be 8+ volts charged though.

Yup, a LiPo cell should be 4.2 volts when fully charged (8.4v for a 2S), and a NiMh/NiCad cell should be 1.45-1.50 volts per cell when fully charged (8.7-9.0v for a six cell pack).

Or just tie it in as you outlined in your original post and see how long it flies.

[indiana mudcat]

7954 is rated to run off 2s, mine has no issue with my CC BEC set at 8.4. A BEC is kinda a waste unless you switch to 3s Lipo, I'd say your plenty safe to power off the battery.

[SMR 510RR]

You are buying a $80+ servo and you are still running stick packs? Time for an upgrade!

[yoraish]

Thanks everyone for your great responds.
I think I will power it straight off the battery, as I heard some people even run it on 9 volts.

And another question, when you talked about the BEC placement, is it between the battery and the servo or between the receiver and the servo?

Thanks

[Olle P]

First the Rx:
- The "battery" connector on the Rx mentioned in the first post is there to attach battery power to the Rx (when the Rx is used for something that isn't battery powered). The rest of the connectors are then used for servos or similar stuff.
- The power feed pins on the Rx connectors are all internally connected in parallel, so it actually doesn't matter which "channel" connector is used to provide power to the Rx, and all other can be used to draw power.

The Servo:
I use the very same servo, happily feeding it off my 2S LiPo. It pulls 4A peak and 2A typical at that voltage.
For convenience I'm running that extra feed through a two-pole switch that use one pole for the +V battery voltage to the servo, and the other pole replaces the little switch on the ESC. This way the servo is powered on and off at the same time as the ESC.
The 0V line to the servo can go through the Rx, but there are some weak cables so I've added an extra cable straight to the battery here too, and it goes to the Rx as well to keep that voltage stable.
For even more convenience I'm using colour coded mini-Deans to hook all parts together.





When I switch to a higher voltage battery it's simple to add a separate BEC in serial with the brown mini-deans.

[yoraish]

That's a great setup!! Thanks for sharing!
But what's the white wire that comes out of the ESC?

[SwampDawg]

motor wires i assume

[spillproof]

He has it taped off. Thats the third wire used on brushless motors. the MMP, sidewinder, and their micro versions, are brushed and brushless type ESC's.

I never understood why people paid so much for Castle, until i got my sw micro's and castle link...

[Olle P]

Originally Posted by yoraish

But what's the white wire that comes out of the ESC?

Originally Posted by spillproof

Thats the third wire used on brushless motors.

Correct! I'll buy a fine brushless motor later on but use a cheap motor for now.

[MeadowMuffin]

Invest in one of these bad boys . Make life easier . And they are only 5$


Can be found here along with the info needed
http://www.creationfab.com/product/6...s-Adapter.aspx

His username on the forum is:
Chris_the_battery_man

"Thumbsup"

[MeadowMuffin]

No prob dude. I'm actually running a pair of these on my XR and my wraith. It just makes it so damn easy and keeps for a clean wire job. Really well built too, deff worth the 5$ to have your BEC ran directly to the servo and to have the BEC powered straight off the battery .
I just used a mini style dean off the batter leads and boom. Quick disconnect incase you change the voltage on the BEC

[yoraish]

Looking good! But isn't that cable like a mini Y harness with rewiring?

[MeadowMuffin]

Yeah it's similar to a Y harness , but its wired to run power from the BEC straight to the servo, without cutting any wires or soldering. From there the sensor wire (white) and the ground plug straight into the receiver . It's a plug and go system. Check out the website I listed and read the description might explain better than I can

[yoraish]

Originally Posted by MeadowMuffin

Yeah it's similar to a Y harness , but its wired to run power from the BEC straight to the servo, without cutting any wires or soldering. From there the sensor wire (white) and the ground plug straight into the receiver . It's a plug and go system. Check out the website I listed and read the description might explain better than I can

Got it My cables are already on their way so I'll be soldering a little bit, fun it will be!