Do I need an external BEC?

[Red3]

cc bec pro is far to big for a 450 mate. just using it as an example. hope it helps you out

[rudi]

cc bec pro is far to big for a 450 mate. just using it as an example. hope it helps you out

Agree, I've decided to go with the smaller 10A version they do.

[cjcj1949]

If you put BECS in parallel there may be a problem, depending on the design of the BECS. If they use feedback to control their output voltages you would have the BECS interacting. This could end badly depending on the stability of the feedback circuits. Having separate feeds for critical areas like the radio is a good idea. Any dips in output volts on the high current side will not affect the rx and cause brownouts. Keep the voltages the same unless you know that control signals greater than supply are ok. This would destroy any inputs that are not protected. Our equipment does not usually have proper specs in the engineering sense.

[rudi]

I won't be putting the BECs in direct parallel. Only the ground will be connected. +ve1 will power the receiver/3gx/telemetry. +ve2 will power the servos. The only interaction I can see will be the signal wire from the 3gx to the servos.

According to the Castle wiring diagram, that seems to be OK with multiple Castle units, so I'm guessing it will also be ok with the Align BEC and Castle BEC, providing the signal voltage matches the servo voltage.

[Red3]

just out of interest if you are going to use a 10a bec why do you want use the align bec as well ?

[rudi]

just out of interest if you are going to use a 10a bec why do you want use the align bec as well ?

I want to experiment with the 3GX high "Swashplate Accelerate Setting". MKS don't publish max current draw on their servos. By running the receiver on a dedicated bus, I should avoid a brownout situation if I inadvertently overload the servo bus.

The 10A BEC is actually only a 7A BEC with 5mph airflow, and servo wire is only rated to 5A, so on the servo bus things will possibly peak very near the limit. I'd feel happier flying the heli with a separate dedicated receiver supply.

Plus, it isn't going to cost me anything other than a bit of thought in the wiring.

[Red3]

ah ok my worry would be the becs don't match and as we know align esc electronics are not that good.

[cjcj1949]

You might like to look at the MKS website and see about their regulators. They recommend one regulator for each servo. This allows some overload protection while still giving each servo enough current. Digital servos (most) are not current limited (Henge is an exception) and are quite capable of destroying themselves when stalled. I don't see how this can be definitely avoided in a crash.

The interaction is that the output of the rx is defined by one bec and the maximum input volts (may) be defined by the other bec. It depends on the input circuits and the instantaneous difference between the two rails. The servo BEC voltage will tend to drop during load, whereas the rx one will be pretty constant.

[rudi]

I looked at the individual regulators. The MKS ones only reduce from 8.4V to 5V, so I'd still need a master BEC. I can see the theoretical advantage of shutting down a single servo, but if it ever came to that in flight, I've already got bigger problems than a regulator can fix. I'd prefer not to have an additional 4 extra components that could fail. Maybe I'd use it on a retractable gear servo, where it wouldn't be an instant crash if that servo got shut down.


After further research, I've decided to get a Western Robotics Hercules Super Mini BEC G2. It seems a better design (2 output leads, remote switch option, no need for expensive programmer). It is a true 10A continuous with 20A burst. It can even provide 3A in an undervoltage supply situation, which is good for my Spektrum receiver. With this one I feel happy to run everything from it and no need to split the receiver/servo power to avoid brownouts.

I may use the internal Align ESC for nav lights, night lights and camera.

[Bronek]

Hm what about attaching NiMH (or LiFe) pack directly to Rx, parallel to existing ESC connection?

[rudi]

Weight on a 450. The NiMh would also need regulating. I'm not sure if applying a voltage on the output from a BEC would be harmful to it. Also, the BEC would end up charging the NiMh if the battery was flatter.

[Bronek]

Actually I found some kind of answer on Kontronik page Drehzahlsteller - Allgemein - see "Can I connect my receiver pack in parallel to the ESC?" at the end and "Which buffer pack should I use?" in the middle. I do wonder whether this is specific to Kontronik only or not.

(I'm running trex 450 Pro with Kontronik Koby 40 LV ... was, now it's broken )

[cjcj1949]

I've seen that advice from Kontronic and I don't think much of it. NimHs are not meant to be used that way. You could use a lipo though if the voltage range was ok. A lipo would be happy being kept at a constant voltage with a current limit. That's how we charge them. Nimhs are not charged by constant voltage as their volts drop when they are fully charged.

[Bronek]

Re: Do I need an external BEC?

I've seen that advice from Kontronic and I don't think much of it. NimHs are not meant to be used that way. You could use a lipo though if the voltage range was ok. A lipo would be happy being kept at a constant voltage with a current limit. That's how we charge them. Nimhs are not charged by constant voltage as their volts drop when they are fully charged.

What about LiFe receiver packs? I'd rather not exceed 7V, my servos aren't HV.

[rudi]

Actually I found some kind of answer on Kontronik page Drehzahlsteller - Allgemein - see "Can I connect my receiver pack in parallel to the ESC?" at the end and "Which buffer pack should I use?" in the middle. I do wonder whether this is specific to Kontronik only or not.)

This is interesting. The website also says:

According to our measurements a single standard sized brushless high speed servo can cause current peaks of up to 6A. Parallel usage of servos of this type may cause overload for the BEC. BEC overload can be decreased by lower BEC voltage. A redundant receiver battery is advisable.

So they don't just say it is OK, but say it is advisable.

It also says under "Which buffer pack should I use?":

4 cells NiMH/NiCd
BEC current: 5-6V
Caution: cheque the specification of the servos and transmitter first to guarantee the correct input voltage.

Before reading this I would have been very dubious about connecting a battery in parallel with a BEC, so I googled "Kontronik buffer pack" and found a lot of stuff on Helifreak. Copy and paste: (original source by bmclaurin at Btw ... - Page 2 - HeliFreak

What is a buffer pack?

For those that use a BEC, it's a redundant, backup power source for your RX and electronics in the event that your BEC dies in flight. It will give you enough power to control the model and (hopefully) land safely. It just runs in parallel with your BEC. If the voltages are chosen carefully, the buffer pack will continuously "charge" during flight (e.g., running your BEC at 5.6v with a 4-cell NiMH buffer pack), so as long as you fly often, you will rarely need to manually charge it.

Buffer packs can also help reduce/eliminate the shutdown/brownout problem that Jive BECs can have with "feedback" voltage from certain servos (e.g., Align, JR). However, from what I've read on these forums and elsewhere, this problem usually happens when running the BEC at 6v, not 5.6v.

There are many opinions about whether a buffer pack is needed. My own opinion FWIW is that it is an inexpensive (in terms of both money and weight) insurance policy that is well worth it, particularly on larger machines that could be potentially lethal if out of control. Even the best BEC's can and do fail from time to time.