BEC bypass idea ?

[Bondonutz]

I'm setting up my SC truck with a MMP and a test motor from a unmentionable Co. to run on 2s lipo and wish to use a BEC but got a idea ?

Instead of soldering a adjustable Castle BEC onto the ESC battery wires couldn't I just use a female balance tab and wire up a JST plug to go directly into the Spektrum RX from the balance tab of the 2s lipo ? Of course I'd be pulling the ESC's RED wire.

The servo will work like gangbusters at first but even when Lipo hits LVC it still be getting 6volts.

I ask because I've never seen anyone do this befor ?

[trev3813]

I don't see why not, voltage is voltage from a battery that is.. Just gonna have an extra thing to unplug..

[Bondonutz]

Yeah, but it should be a neater install and less working parts, lighter weight. I think it's all good, just wasn't 100% ?

[bdebde]

I am setting up my new SCT with similar set up, running 7.4 to the receiver and pulling the red wire on the MMP plug.

[Bondonutz]

This is fairly common thing to do bypassing the MMP BEC but getting juice to the RX the way I'm thinking of doing it is the Question.

[bruce750i]

I run LVA's and leds out the balance tap often.
My mmp SCT's has lipo voltage running the servo(split the servo plug) and the bec runs the tx and pt. I figured after I took the servo load off of the bec it should run a PT and Tx pretty good.
Your way sounds good also. Your's will have two connections to make instead of one.

[Bondonutz]

Heres what I have to make what I need.

[suicideneil]

So long as the rx & servo are rated for 8.4v ( 2 lipo cells when fully charged ) it should work just fine. If 8.4v is too much though, then a little voltage regulator in-line between the lipo & rx to drop the voltage down a notch would be fine too.

[Bondonutz]

A peaked NiMh Rx pack has pretty close to 8v, I'd like to think a high qaulity servo (Hitec 7955) can handle it. Besides that 8.4 will drop off pretty quick after a few laps ?

Ultimately a Adjustable BEC wold be best and safest, but I'm going for it tho

[DrKnow65]

High voltage servo's runs just fine on 8.4v

[Bondonutz]

The 7955 is not HV, I did run one on 2s lipo straight into the RX in my 1/5 scale Savy for months w/no probs.

[Shonen]

I actually used a similar route when my ERBE was running on 4s. Discharge leads on the battery plugged into the ESC, and I used a BEC wired to plug into the balance tap.

Not sure how reliable that connection would be though. The BEC in my setup only ran auxilary fans and body shell LED's, so nothing bad would happen if it would stop working.

[Bondonutz]

Good point, I guess a piece of heat shrink over the balance plug connection wouldn't hurt then to ensure they stay together.

Thx

[radioman193]

Hitec 7955
Operating Voltage Range: 4.8-6.0 Volts.
Current Drain (6.0V): 300mA/idle and 4.2 amps at lock/stall

8v would Kill it in no time at all.

if you Dont set your end point =

AlmstEvil66503-02-2010, 06:05 PM
Don't run a 7955 at more then 6v!!!! I ran mine at 7 and fried it in about 5 minutes. I had the BEC wired directly to the servo just like you. I think you can get away with a little more than 6v if you run it to your reciever but down time in sending the servo back is not worth the risk. If you want more power buy a 7950 that is made to handle 7.4.


SACCO03-03-2010, 09:36 AM
I've got a dead 7955 that was run at 6.5 volts. If you want one too, turn up that bec"thumbsup"

over 6 you will eventually burn it up
Just leave it at 6v

Also get yourself a servo prgrammer. You can set the fail safe on it that will keep you from overloading the servo.


if you do just keep running it back and forth and see if the bottom of the servo is getting warm or hot
and Don't stall it and set the end points.

[Shonen]

^ well whatever, I've been running mine at 7v for the past month. Both of those quotes are from the same thread and even there the consensus is that it's fine as long as nothing's binding or stuck. Like it's been said before, a fully charged 6v receiver battery is over 7.5v. It's the extended stall currents that crawlers experience that kills them.