Originally Posted by Carl Petersen
Lucien,
From time to time the subject of paralleling BEC outputs from two speed controllers, twin motor setup, to increase the current available comes up. What's your take on that?
Scorpion has done testing on their 4-cell ESC's with linear BEC circuits, and found that they can be put together in parallel with no ill effects. When you stop and think about it, the BEC's inside the ESC already have several voltage regulator chips in parallel to begin with. The BEC Circuits in the 4-cell ESC's are comprised of 2, 3 or 4 seperate voltage regulator IC's that put out one amp of current each. These chips are all connected in parallel with one another right on the PC board, and they all work well together. That is because Scorpion uses high quality voltage regulator chips with very tight output tolerances.
If you take 2 ESC's, and put them in parallel through a Y-Harness, you are basically connecting one group of voltage regulators to another. If they are all at the same voltage, then the output of one will not fight the other, since both are at the same voltage. Where you would run into problems is if you took two different ESC's and hooked them together, you could have the potential for a difference in voltage, and this could lead to a high amount of current flowing from one BEC into the other, and that would not be good.
The only down side to this is that if one of the ESC's fails, and the BEC circuit goes down with it, it can pull the remaining good BEC down as well. This situation can be avioded if you place a Schottky rectifier diode in line in the output between each of the BEC circuits. Schottky diodes are preferred, because they only have a 0.4 volt drop across them, as opposed to silicon diodes that have a 0.7 volt drop. All of the new Scorpion ESC's use 6.0 volt regulators in them now, so even with a Schottky diode in line, you still get 5.6 volts to the receiver, which is enough to ensure that you do not get a brown-out in a Spektrum receiver. Just make sure that the diodes are rated for at least 3 amps of continuous current and you will be fine. This way, if one BEC were to fail, the diodes would prevent current from flowing back into the dead BEC and allow the other one to continue functioning.
Talking about ESC's that have switching type BEC circuits, you should NEVER put these types in parallel with one another. The Switching type BEC's rely on a closed loop feedback system, where the output voltage is constantly monitored and pulse width of the switching circuit is constantly changing to adapt to the changing load on the system. This happens thousands of times per second, so that it can rapidly rrespond to a change in current when a servo moves or the load changes.
If you hook two switching type BEC's together, if one of them increases it's output voltage, the other one can see this as too much voltage and reduce it's output. Since the two outputs are tied together, you end up with a tug-of-war between the two BEC circuits where one adjusts, then the other one adjusts to compensate, then the first one re-adjusts, and this goes back and forth, throwing both of them out of whack.
If you intend on using a switching type BEC, you always want to use a single unit that is capable of handling all the required current demands.
That is my take on the situation, and hopefully you agree with it.
Thanks!
Lucien
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