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Old 02-09-2015, 05:46 AM   #1
Virgil Kee
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Question Brushless RX voltage limits?

I'm setting up a new FMS J3 V2 (40") Cub. I have all the following EXCEPT the RX.

Batt: ................................... 3S 11.1V,
ESC: ................................... 20A BL,
RX(?): ................................. 4-ch, 2.4 mHz, voltage range 3.45-7.2V, DSM2 compatible.
Motor: ................................ 2408 BL, 1700KV

What specs will I need if the RX shown is not compatible?

My first visit to this forum - hoping for some advice,

Virg.
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Old 02-09-2015, 05:57 AM   #2
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That all looks perfectly fine. The Rx does not operate directly off the battery voltage, it runs off a stepped down (about 5V) supply that comes from the ESC (You ESC has a step down regulator called a 'BEC').
So it doesn't matter if you flight battery is higher voltage than the Rx can take, in fact they usually are.
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Old 02-09-2015, 05:57 AM   #3
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Your ESC should have a built in voltage output of about 5 v, to power the receiver and servos.

Welcome to Wattflyer, you bring the questions and we'll bring the answers.
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Old 02-09-2015, 06:52 AM   #4
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The ESC has a "BEC" built in. This will drop the relatively high voltage of the motor battery to something the RX can use.

https://en.wikipedia.org/wiki/Batter...inator_circuit

In some cases (or when it doesn't have one) we disable the BEC built into the ESC and use other means to power the RX and servos. There are times when this is the only way to safely provide adequate power to the RX. Generally this will apply to multi-engine or higher voltage motors being used or when there is an expectation of high load on the RX power supply.
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Old 02-09-2015, 07:21 AM   #5
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Originally Posted by Virgil Kee View Post
I'm setting up a new FMS J3 V2 (40") Cub. I have all the following EXCEPT the RX.

Batt: ................................... 3S 11.1V,
ESC: ................................... 20A BL,
RX(?): ................................. 4-ch, 2.4 mHz, voltage range 3.45-7.2V, DSM2 compatible.
Motor: ................................ 2408 BL, 1700KV

What specs will I need if the RX shown is not compatible?

My first visit to this forum - hoping for some advice,

Virg.
Okay Virg,

What these other fellows mean to say is that the ESC you have, has a single servo cable coming from it that connects to your receiver of choice. This cable from the ESC is different in that it supplies 5v from the ESC to your receiver to power the receiver and all of the servos connected to it.

There is no way to increase this voltage, nor is there a way to increase the amperage from the ESC to the receiver. It is what it is and usually that is sufficient for most applications. ESC's are rated in this way and for amperage they can supply to a motor.

Since you mention DSM2 compatibility there are a few exceptions. DSM2 can have a brown out when the supplied voltage falls below 4.2v. For this reason most people flying DSM2 or compatible Rx's tend to want to use a UBEC. A UBEC is a separate 5v power supply/regulator that is connected from a battery directly to the receiver. It normally supplies a steady voltage to the receiver in place of the ESC's BEC. (The ESC's built in BEC is disabled when a UBEC is used by removing the red power wire from the connector of the ESC. Power then is solely supplied by the UBEC to the Rx and all other components.)

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Old 02-09-2015, 07:43 AM   #6
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Not sure about the idea that most DSM2 flyers using a UBEC.
Sure some do, but in my experience the majority just use the BEC built into the ESC. For smaller models using mini/micro analog servos that's usually perfectly ok. Personally I'd not suggest complicating things with a UBEC, what he has should work just fine.
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Old 02-09-2015, 08:56 PM   #7
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I think that I am one of the dummies who suffered a brown out using a Spektrum DSM 2 AR- 7000, and the voltage coming off the ESC's built in BEC. And it wasn't a big rig either. Just a Turnigy 42 motor flying a Telemaster 42 on a 15 X 8 prop, 85 A ESC, and 4S, 3300 LiPo.

I'm a listenin ?

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Old 02-09-2015, 10:43 PM   #8
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Thanks to all who provided the reassuring information that my proposed setup should NOT give me any OVER-VOLTAGE grief. Your help was astonishingly swift (on the same calendar date!).

My original question came about due to my having gleaned from some forum (or other) that running a given motor from a battery of a much higher voltage was damaging. This was described as saying that even at half-throttle, the motor WOULD receive the power at a 50% rate, BUT, the pulses that DID get through, though fewer, were of the HIGHER VOLTAGE, hence damaging!

Your advice was that there should NOT be a problem. I plan to proceed accordingly.

Looks like this forum is a hangout for a bunch of EE's!

Thanks again, Virg.
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Old 02-09-2015, 11:11 PM   #9
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To test for adequate RX power supply for gentle flying and mild aerobatics... take prop off and "stir the sticks" moving them rapidly around into the corners for at least 20 seconds.

This will simulate some rapid maneuvering in flight and load the ESC's BEC long enough to heat it up. The first sign of a problem is if the motor stops responding indicating you hit a thermal safety (often the controls will still work) Second indication of a problem is all controls quit, possibly only very briefly, possibly intermittant and possibly for several seconds, indicating you pulled voltage below where the RX resets.
3rd indication of a problem is melting the wire insulation between ESC and RX indicating inadequate wire gauge for the BEC output. (seen that once...)

This does not test to meet maximum flight loads possible in aggressive aerobatics.

If it passes the test then its good at least until you want to try higher demand aerobatics.

Early ESC's usually had linear (based on resistance) BECs with low amp delivery capability. These types the BEC output creates heat proportional to voltage drop X amps of output and are only good for small voltage drops.
Modern ones tend to have higher amp ratings and use switching BECs which have a less direct relationship of voltage vs heat generated and thus can deal with larger voltage drops without overheating
But there are still some linear regulated BECs out there with low amp ratings...
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Old 02-10-2015, 07:22 AM   #10
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Originally Posted by Virgil Kee View Post
My original question came about due to my having gleaned from some forum (or other) that running a given motor from a battery of a much higher voltage was damaging. This was described as saying that even at half-throttle, the motor WOULD receive the power at a 50% rate, BUT, the pulses that DID get through, though fewer, were of the HIGHER VOLTAGE, hence damaging!
Actually they were right.

What they are talking about has nothing to do with the voltage that's passed to the receiver. They are talking avout the throttle and it's effect on the ESC itself. The receiver will be fine because it's not effected by throttle setting, it gets supplied the same voltage regardless of what throttle you apply, throttle only controls voltage to the motor, not the receiver.

What they are saying is that if you have a motor that wants to pull too many amps for the ESC to handle (more than 20A in your case) then you cant 'fix' it by running at low throttle setting. the ESC must be rated for the full amps that the motor can pull at full throttle.
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Old 02-10-2015, 08:22 AM   #11
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ESC is actually under more stress at partial throttle than full throttle.
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