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Jun 30, 2004, 10:00 AM
Registered User

Help a Newbie Girl Understand OPTO ESC & S-BEC? Pics Attached

Hello All,

Aircraft World is recommending an OPTO ESC and an S-BEC for my:

1. 104" Flying Wing
2. 12-cell Sub-C pack
3. AXI 2820-12
4. 4 Standard Full-Sized Servos for Flight Control, plus 1 for Steerable Nose Gear, plus 1 for triggering Camera Shutter.

Why go OPTO and S-BEC?


How do OPTO ESC's and S-BEC's connect into the Radio/Power System?

Attached are the specs of my Flying Wing and a pic of Ming with a similar Electric setup on the same Flying Wing.

Thank you for the benefit of your experience to this very INexperienced, but very enthusiastic Newbie,

Julie ;o)
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Jun 30, 2004, 10:45 AM
(aka Cliff Lawson)
Wright Flyer's Avatar
Opto means that the ESC circuit is electrically isolated from the receiver/sevo circuit by optical isolators. So, in effect, the throttle channel of your ESC does nothing more inside the ESC than flash an LED on/off then an opto sensitive receiver on the ESC heavy power side of things "reads" the signal and uses it to modulate the power.

If the Rx+servos are optically isolated from the ESC then the ESC does not feed power back into the Rx to power the servos (like an ESC fitted with a BEC would). So the Rx+servos have to get their power from somewhere else. You can do this one of two ways. Either with a separate "receiver battery" which is normally four or five NiCd or NiMh cells (4.8V or 6V) or you can take your main flight battery (your 12 cells will be 14.4V) and somehow get that voltage dropped to 4.8V..6V (probably 5V in fact) to feed the Rx/servos. The magic piece of electronics that drops the voltage is the SBEC which is basically a voltage regulator.

The reason for using an SBEC is two-fold. The BEC you often found built into amn ESC is usually only good for supply 1-2A to the Rx+servos. It sounds like you've got too many servos for an average BEC to be able to cope - the current may be 3A+ when all servos are in heavy use. The other thing is that because a BEC has to drop from whatever the pack voltage is to 5V then for planes flying on 8.4V or 9.6V battery packs it only has to drop about 4-5V but as you're using almost 15V it would need to drop 10V and, again, the BEC in most ESC just can't drop that amount of voltage without producing a LOT of heat (enough to fry the ESC). The SBEC (like UBEC) regulate the voltage in a different way and have no problem dropping 15V to 5V

Jun 30, 2004, 10:51 AM
Registered User
Why? Because your flight pack has more than 10 cells in it. The built in BECs in most speed controls can't handle more than 10-cell packs because the voltage difference is too great. BECs reduce the pack voltage to 5 Volts for the receover and servos by turning the excess energy into heat. As the difference between pack voltage and 5 Volts gets greater, the BEC has to generate more heat. Something's got to give at some point, and the point is 10 cells on most ESCs.

Since you're running 12 cells, there's no point in buying a speed control with a built-in BEC, so you may as well get an OPTO speed control and an SBEC.

An OPTO electronic speed control is optically isolated, meaning that there is no direct electrical connection from the receiver to the ESC for the control signal. It passes through a gizmo that translates the electrical signal from the receiver into light, shoots it across the gizmo, then translates it back into an electrical signal on the other side. Electronic interference cannot pass down the signal wire to the receiver. Isolating the receiver from the speed control this way can be important on higher-power applications.

The SBEC is a "switching" battery elminator circuit. Built-in BECs are of the "linear" type, meaning they expend excess energy as heat. A switching BEC simply switches the power on and off very rapidly at regular intervals to simulate a lower voltage. This is the exact same way an electronic speed control works to simulate lower throttle levels. A switching BEC can theoretically handle any number of cells. You don't see switching BECs built into speed controls because they're relatively expensive (raises the price of the speed control and makes it less attractive to the typical R/C consumer who is only looking for the cheapest price), and they're relatively bulky (compared to the tiny chip linear BECs that you'd be hard-pressed to locate on a speed control).
Jul 01, 2004, 09:04 AM
Registered User

Thank You So Much . . .

. . . for the benefit of your experience and knowledge. I now have a very good understanding of WHY I need an OPTO ESC and S-BEC as well as how they work and how they connect into they system.

Thanks again!

Julie ;o)

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