Rx Voltage

[Dwain Dibley.]

Having tested my new Radio link Tx and Rx, I got to the field to find that when I used all 4 servos (waggled the sticks) the Rx went into failsafe, which if unaltered, is last known position.

Now .........if I unplugged one servo, didn't matter which one, all was well.

When we used a 4.8v Rx battery, with ESC unplugged all was well with 4 servos working together.

so........ is the SBEC in the ESC DEAD ?

Or........ Just not supplying enough V's, RX range is 4.6 - 10v. ?

I can of course isolate the SBEC (take out the red wire from the ESC plug) and use a 5 cell battery at 6v, but, can I use a 2 cell 7.4v lipo without damaging the HS645MG servos, which operate on 4.8 - 6v.

Cheers in advance guys.........

D.D.

[Bob Cotsford]

It's tha As that your BEC is short of, under load it's not supplying enough current so it's voltage drops. You could use a 2S LiPo with a regulator, or a 2S LiFe (6.6v rated), or a 4 or 5 cell NiMh, but not the naked 2S LiPo as they are well over the 645MG's rating.

[Nigel R]

Waggling 4 servos without flying loads on the surfaces doesn't take much welly.

(1) yes, bec in ESC sounds like it is dodgy, at first read through your post

(2) your RX appears damn sensitive to low voltage, I would be a trifle concerned by this

(3) did it go into failsafe, or simply brownout? how was failsafe confirmed?

(4) just to eliminate it as a possibility, were you a few feet away from the model at the time (in case the RX doesn't like being swamped with RF and any extra noise from the SBEC tipped it over the edge)

[Peter Beeney]

The SBEC is probably not supplying enough current, Dwain, What is it’s current rating?

It’s a regulated supply source and when it tries to exceed it’s predetermined current flow cut off level it just shuts down. This is to protect the SBEC rather than the radio. The battery, on the other hand, is an unregulated supply source and is capable of supplying an instantaneous heavy current flow without the voltage dropping too far. That would explain the ability of the battery to drive the servos ok.

Not a good idea to use 2 cell lipo to power that flight pack. The servos may be a bit quick for a short while perhaps but then might just come to a permanent standstill! A 2 cell LiFe rx pack would be just the job though, they are used quite a lot nowadays.

For the above reasons I’ve always considered the battery to be the best option if possible …

PB

Edited By Peter Beeney on 30/04/2019 16:39:58

Edited By Peter Beeney on 30/04/2019 16:49:13

[Dwain Dibley.]

ESC SBEC is 6 Amps.

I was within a meter or two of the model.

D.D.

[Peter Beeney]

Looks as though you might need some fair test kit to check this out, Dwain. Although those servos are quite substantial as it happens and at the instant point of startup the motors will be causing a spike, enough to perhaps worry the SBEC. Waggling the sticks, motors constantly starting and stopping, spikes in quick succession perhaps… confused SEC… difficult to tell.

Just a thought, if you have an onboard led receiver battery voltage indicator you might try plugging that into the receiver and if that runs up and down and into the red as you waggle the sticks then that surely is indicating a serious voltage drop somewhere; most likely the SBEC.

A battery with reasonable capacity of your choice might be the easy answer. Generally speaking I’ve have thought that the servos are unlikely to be as active in the air anyway so I think I’d be happy with that.

Good luck.

PB

[Dwain Dibley.]

Thanks Peter and Everybody else, I will go with a 5 cell NiMh, as I have those in abundance.

D.D.

[Paul Marsh]

I had this before in a depron jet, with an orange rx, waggled all the sticks, it went into failsafe and then came back, did it again and the same.

Solved the problem by plugging a 2200 uf capacitor into the spare channel and not had a problem since. Even only a small model, the bec wasn't man enough even for the 4 tiddly servos. On my larger electric and edf's use a separate battery.

I bought the Hobby king Canadian Vampire and had two lock outs, luckily the Hitec Optima rx came back, landed, and found the bec on the esc weak. Strange, as the previous Vampire and Sea Vixen used seperate Ubec's but to save money, I assume removed it and use the insufficient Bec. Since then put a Ubec in it and make sure high performance models have so.

[PatMc]

What ESC is it & how many lipos in the battery ?

[Dwain Dibley.]

4 cells and its 4 Max's 70 amp ESC

This one 

Edited By Dwain Dibley. on 30/04/2019 20:40:10

[PatMc]

IMO the BEC is behaving like a linear type rather than a switching type that the SBEC title suggests. Also, looking at the "How to wire up a UBEC" user instruction for this ESC makes me suspect that it could actually be a linear BEC.

[Dwain Dibley.]

Ok...........PatMc, you have lost me there a little bit, I am interested to understand what you are saying so please elaborate.

Meanwhile, here is the simple solution for anyone who does not know how to do it. Apologies to those that do.

D.D.

Pull out the red power wire from the wire to the ESC, there is a little tab on the plug that you need to lift to free the connector, tape it back to insulate it and keep it out of harms way.

Plug a battery via a switch into an available channel.

The Rx is now independently powered, the ESC now only controls the motor, and does not supply power to the Rx

Edited By Dwain Dibley. on 30/04/2019 23:14:50

[J Moyler]

Just thought that I would add a comment. I would contact 4-Max, they are very helpful. With 4 servos the inbuilt BEC should cope.

JM

[Dwain Dibley.]

I agree totally J, it's rated for 4 - 6 digital servos, but the ESC is over a year old and I didn't think I could get a replacement. However, George and the guys might be interested from a technical point of view. I have to order the power train for my Cessna project soon, so I will mention it then.

I was just interested in finding out why the problem possibly occurred.

Another thing is, I could put the the spectrum Rx back in and see what happens.

D.D.

Edited By Dwain Dibley. on 01/05/2019 12:26:45

[Nigel R]

I concur with JM.

That's a decent sized ESC and its BEC should very easily cope with some stick waggling on the ground, with no complaint.

"IMO the BEC is behaving like a linear type rather than a switching type"

Pat, what do you mean by this?

Both types of BEC should hold up the output voltage just fine.

[Dwain Dibley.]

I tested the output of the Sbec this morning and it was 4.9 v.

I could not test under load as I didn't have enough arms, but that is very close to the min requirement of the Rx of 4.6. It should of course be a constant 5.5.

The 5 cell battery was supplying a healthy 6.8 v, So I am happy to continue using a separate Rx supply, as many of my other models use one too.

D.D.

[Nigel R]

Is it possible the ESC BEC is switchable between 4.8V and 6.0V output?

A 4.8V output would present no problem to a Spektrum RX which run down to around 3V (or thereabouts). Whereas the spec of the RadioLink RX suggests it is squarely aimed at a 2 cell lipo supply.

[Dwain Dibley.]

ESC is not switchable but I agree about the Rx Voltage. It would seem I will have to go with separate Power to Rx with the radiolink Gear.......Not the end of the world.....about a tenner per install.

Thank you to everyone who gave input, it's appreciated.

D.D.

[Chris Bott - Moderator]

This could well be the SBEC dropping in volts but it could possibly be a connection somewhere that has a little resistance. Do you have any extension leads or switches or anything in line with the SBEC/Throttle channel cable?

It could be the connection between servo plug and Rx, or even the crimp between servo lead and it's plug.

[Dwain Dibley.]

No Chris, But I do have an isolating/arming plug between battery and ESC ??? I wouldn't have thought that would be a problem tho, but Hey........I'm here to learn.

D.D.

[PatMc]

Posted by Dwain Dibley. on 30/04/2019 23:10:54:

Ok...........PatMc, you have lost me there a little bit, I am interested to understand what you are saying so please elaborate.

 

Dwain, a linear BEC uses a voltage regulator to reduce the input voltage to the desired output level but the total current drawn by the servos also passes through this regulator. This means that the difference in voltage between I/P & O/P times the current is dissipated in the regulator in heat.
For example let's say the I/P is 16.8v from 4s lipos with an O/P of 5v & 4 servos each drawing 1A . That's 11.8v * 4A = 47.2 Watts that has to be dissipated from the regulator. If the regulator isn't rated to this it will either burn out or shut down, either way power to the Rx is lost.
OTOH if there was only 3 servos the heat generated would be reduced to 35.4 Watts. If this is within the regulators rating all will be sweetness & light.

Further to this consider if the I/P was 12.6v from 3s lipos. That would give an I/P to O/P difference of 7.6v. With 4 servos at 1A each there would now be only 7.6v * 4A = 30.4 Watts to be dissipated.

Bottom line is that linear BECs are inefficient devices limited by a combination of the input voltage & number of servos they can run.

Switching BECs regulate the O/P voltage by switching the I/P on & off at a high rate with the the proportion of "on" to "off" time set to achieve the desired O/P voltage. They are very efficient (90% - 95%) so there's little heat generated in them & they hold a steady O/P voltage at full rated current demand over a much higher I/P voltage than linear BECs.
They are more complex & thus are costlier than linear BECs.

Sorry for the tardy reply, life got in the way of an earlier response.

PS there's a better explanation than mine of the difference between linear & switching BECs here.

Edited By PatMc on 02/05/2019 22:07:09

[PatMc]

Posted by Nigel R on 01/05/2019 12:33:43:

Pat, what do you mean by this?

Both types of BEC should hold up the output voltage just fine.

Nigel, see me reply to Dwain.

[PatMc]

Posted by Chris Bott - Moderator on 02/05/2019 17:19:57:

This could well be the SBEC dropping in volts but it could possibly be a connection somewhere that has a little resistance. Do you have any extension leads or switches or anything in line with the SBEC/Throttle channel cable?

It could be the connection between servo plug and Rx, or even the crimp between servo lead and it's plug.

Chris, as I read it Dwain got 4.9v when testing with no load (i.e. an open circuit) in which case a high resistance in line would have no effect.

[Chris Bott - Moderator]

Posted by PatMc on 02/05/2019 21:49:07:

Posted by Chris Bott - Moderator on 02/05/2019 17:19:57:

This could well be the SBEC dropping in volts but it could possibly be a connection somewhere that has a little resistance. Do you have any extension leads or switches or anything in line with the SBEC/Throttle channel cable?

It could be the connection between servo plug and Rx, or even the crimp between servo lead and it's plug.

Chris, as I read it Dwain got 4.9v when testing with no load (i.e. an open circuit) in which case a high resistance in line would have no effect.

I'm not sure I understand your point PatMc? A reading of 4.9v on a 5v UBEC doesn't seem out of order at all. That would feed the Rx fine but when servo current spikes as servos starts to move, the resistance in the wiring could cause the voltage to dip and the Rx to brown out. Just the same as the internal resistance of a poor battery or a BEC that can't supply the current.

[PatMc]

Sorry Chris, I misunderstood your post, I thought you were commenting on Dwain's reading of 4.9V when he was expecting 5.5v.