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Feb 16, 2015, 01:29 PM
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Dual ESC's & Batteries connection question


Hi all,
I'm doing a twin EDF jet. I've tried to research the various diagrams for doing the dual hookup and I was curious: When y-connecting the two ESC's (w/ BEC's) to the receiver, why is is necessary to remove one of the ESC's power wires? Seems to me that having them both run in would be a backup, no? (as in two parallel batteries)? What am I missing?
Thanks,
Ron
Last edited by MarvinK9; Feb 16, 2015 at 01:34 PM. Reason: missed info
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Feb 16, 2015, 01:57 PM
Registered User
Due to manufacturing tolerances, the output voltage of the two bec:s is not exactly the same. So you would have one bec trying to push the voltage up to its target value and the other trying to pull it down to its own target. Lots of unnecessary current flowing and waste heat generated.
Feb 16, 2015, 03:38 PM
Registered User
You can use both BECs if you put a diode in line with each of their power leads (the middle wire in the lead that goes to the receiver) to prevent them back-feeding into each other. The disadvantage of doing this is that the diode will drop the voltage slightly, but I've been using that method in my helis for a while now with the BEC in the ESC and a separate stand-alone BEC as backup. Schottky diodes work best because they generally have lower voltage drop than regular ones.
Feb 16, 2015, 06:21 PM
Retired Electronics Specialist
vollrathd's Avatar

Parallel ESC uBEC's


Quote:
Originally Posted by MarvinK9
Hi all,
I'm doing a twin EDF jet. I've tried to research the various diagrams for doing the dual hookup and I was curious: When y-connecting the two ESC's (w/ BEC's) to the receiver, why is is necessary to remove one of the ESC's power wires? Seems to me that having them both run in would be a backup, no? (as in two parallel batteries)? What am I missing?
Thanks,
Ron
It is not a very good idea to directly parallel connect two switching power supply type uBEC's. They can interact with each other.

One way to isolate them from each other is by the use of a pair of 10 Amp Shottky diodes. These diodes have 1/2 the forward voltage drop of a comparable silicon type diode. The dual ESC uBEC was used in two different models. One is my 900 Watt total TwinStar. The second was in a club members dual 4000 Watt electric jet model. Each motor was pulling 170 Amps on 6S LiPos. The ESC uBEC setup worked well in both models.

The attached photos show how it's done. FYI, IMHO, steer away from salvaged Shottky diodes pulled from computer power supplies and similar. Those transistor looking diodes have fragile leads on them. Bend them back and forth a few times, they will break off flush with the case. Bending the leads on the Shottky diode below nearly requires a pair of needle pliers.

It happened at work, they broke off by simple shipping damage inside the shipping carton.

This diode works quite well:
http://www.digikey.com/product-detai...5TR-ND/2179146
Last edited by vollrathd; Feb 17, 2015 at 03:34 PM. Reason: Updated Part Number Specification
Feb 17, 2015, 02:43 PM
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Thanks guys! Great info!
Feb 17, 2015, 02:55 PM
Retired Electronics Specialist
vollrathd's Avatar
Quote:
Originally Posted by MarvinK9
Thanks guys! Great info!

Aw crap
I still got the wrong Shottky diode on one of the drawings. That diode has been discontinued. Both drawings now have the correct diode part number. Note that Digikey is down to their last 17,000 of them, so order soon!

Digikey has their SB1245TR-ND diode which is a replacement.

http://www.digikey.com/product-detai...5CT-ND/2179164
Last edited by vollrathd; Feb 17, 2015 at 03:28 PM.
Feb 17, 2015, 03:11 PM
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Thread OP
Vollrathd, question: in your drawing (with 2 ESC's), Do I have to plug into the Battery channel seperately? I'm using a Futaba R617FS receiver which (I understand) is a "buss bar" type system; meaning the battery can be plugged into any slot on the receiver. Can't I simply Y-harness them (the two ESC's BEC's) into the throttle slot?
Feb 17, 2015, 03:17 PM
Retired Electronics Specialist
vollrathd's Avatar

Yes you can


Quote:
Originally Posted by MarvinK9
Vollrathd, question: in your drawing (with 2 ESC's), Do I have to plug into the Battery channel seperately? I'm using a Futaba R617FS receiver which (I understand) is a "buss bar" type system; meaning the battery can be plugged into any slot on the receiver. Can't I simply Y-harness them (the two ESC's BEC's) into the throttle slot?
Yup, you can Y harness them. However, if you have the extra slot available for the battery connector itself, you now have dual DC inputs to your receiver. That would be the throttle connector AND the battery connector on the receiver.

With a Y harness, if anything goes wrong with the Y harness or anything similar, you haven't gained anything.
Feb 17, 2015, 03:20 PM
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Thread OP
Just ordered some; Digi-Key (using your old number) automatically took me to the correct part number SB1245CT-ND. Part number SB1245TR-ND is for a quantity of 1,250, yikes!
Thanks, please see previous question.
Feb 17, 2015, 03:28 PM
Retired Electronics Specialist
vollrathd's Avatar
Quote:
Originally Posted by MarvinK9
Just ordered some; Digi-Key (using your old number) automatically took me to the correct part number SB1245CT-ND. Part number SB1245TR-ND is for a quantity of 1,250, yikes!
Thanks, please see previous question.
Dang my computer!!!!

Thanks for pointing that out. Proper number is:
http://www.digikey.com/product-detai...5CT-ND/2179164

I've updated the drawings to show the proper Diode part number in quantities of one piece.

Note that this diode is good for a non repeating value of 150 Amps for eight milliseconds. Should be OK for our receiver use.
Last edited by vollrathd; Feb 17, 2015 at 03:39 PM.
Feb 17, 2015, 05:39 PM
Registered User
Im also going to set up a dual 90mm edf with 2 esc that have external becs, and i was going to have a battery for each edf. Can I just y harness both throttle signal into one single plug in?
Feb 17, 2015, 06:35 PM
Retired Electronics Specialist
vollrathd's Avatar
Quote:
Originally Posted by fishface5
Im also going to set up a dual 90mm edf with 2 esc that have external becs, and i was going to have a battery for each edf. Can I just y harness both throttle signal into one single plug in?
No, there won't be any problems. That's what I did for my club members dual 4KW EDF model.
Last edited by vollrathd; Feb 17, 2015 at 08:46 PM.
Feb 17, 2015, 08:06 PM
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Thread OP
Thank you vollrathd! You are greatly appreciated.
Feb 17, 2015, 08:44 PM
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Quote:
Originally Posted by MarvinK9
Thank you vollrathd! You are greatly appreciated.
You're very welcome.

As for compensation, just post some photos of your project for all of us to look at
Feb 18, 2015, 03:27 AM
Registered User
Quote:
Originally Posted by fishface5
Im also going to set up a dual 90mm edf with 2 esc that have external becs, and i was going to have a battery for each edf. Can I just y harness both throttle signal into one single plug in?
The ESCs should be Y-leaded together to the receiver's throttle channel, but you shouldn't Y-lead two BECs together, for that can cause problems.

If each BEC has enough amps capability to power the model's radio equipment, then you only need one BEC. But if I have retracts I like to use a separate BEC to power them, to avoid any possible radio problems if the retracts stall when going up or down. If you simply want two BECs for redundancy in the whole system, you can connect them together if you use a diode in each of their power wires so they can't back-feed power to each other. This won't make them share the amps load, so they still each need to be big enough to power the model; but it will allow the second one to kick in and supply voltage if the first one fails for any reason.

It's not generally good practice to have a separate battery for each motor. If you need two batteries to get enough mAh, it's best to Y-lead them in parallel and to then Y-lead the two ESCs together so they both get a feed from the same pair of batteries. The reason for that is so that they're both getting exactly the same volts throughout the flight and, thus, spin at the same speed. Obviously, you can also use one larger battery instead of two smaller ones.


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