pjsmith

Hi,

I bought an Astra Spitfire. It's almost finished. I did buy a 7.2V Graupner speed 400 for it at the same time. I've used the one with the built in speed controller before, and I'm thinking of using it on the Spit so it's lighter/neater. I read through the docs that come with the speed 400 and it says the controller is only good for 3 servos. The Spit uses 4. I'm using HS55's. The question is, will it work OK, or should I get a seperate speed controller?

Thanks.

Matt Kirsch

My Feedback: (21)

It depends. Do you use rudder a lot? Most people don't, so the rudder servo sits idle. You can then generally "get away" with four servos on a BEC that is rated for three.

This, IMHO, is one of those grey areas in the hobby. People run more servos off their BECs than they're rated for all the time, and you don't see many, if any, reports of BEC failures. I personally think the BEC is rated for full size servos, and that full size servos require more current than micros. They have to; more torque requires more power. Voltage stays constant, so the current has to go up. Some people claim that micro servos actually consume more power, but I don't see how...

Is it safe? Is it smart? That's a question you have to answer for yourself.

whanderson

You are dealing with 2 problems here. The BECs in most controllers are rated at 5V and 1A for the receiver and all servos. Larger servos draw more current than smaller ones. If you can use 4 small servos that do not exceed the 1A limit everything will work fine. The voltage regulator in the BEC pritects itself by not allowing more than 1A to be drawn from the circuit. The other problem, which is far more serious, is the difference between the battery voltage and the BECs 5V output. The higher the battery input voltage the hotter the BEC gets. If the BEC gets too hot, it will shut down and your receiver and batteries will not operate. If you can afford the weight use a receiver battery and avoid the BEC.

Matt Kirsch

My Feedback: (21)

This is a fairly low-power setup, probably a maximum of 8 cells on a Speed 400. Hardly welder material...

One possible solution is to get a higher-capacity ESC, like a 30-Amp. The heavier ESC is still lighter than a separate receiver battery, and the heavier ESC has a larger BEC regulator that can withstand higher loads.

whanderson

An 8 cell pack is 9.6V. Subtracting the BEC 5V ouput from the battery input is (9.6-5) is 4.6V. The 1A current maximum of the BEC means it has to be capable of eating 4.6W (4.6V*1A). This is about one third the heat of a small soldering iron. The "larger" ESC does not generally have a higher capacity BEC. The linear regulators that support more than 1 amp are singnificantly larger and heavier devices. Most of the real estate on ESCs is for the MOSFET farm not BECs. In addition MOSFET's current capacity derates by 50% at 75 degrees C. A higher current BEC means the ESC needs more MOSFETs to deal with the added heat. This is why BECs don't supply more than 1A.