HV servo question???
 

I've got a Hitec 7954 and I would like to run it with a lipo. The problem I have is that my receiver is only rated to 6V, is there some way of getting around this without messing my receiver up??

Thanks
Glen

You could run an external BEC at 7.4V (I recommend Castle's BEC) to the + and - wires of the servo and just have the signal wire connected to the receiver. The internal BEC from your ESC (or RX pack if nitro) would still be powering the receiver with 6V

See this post: http://www.rctech.net/forum/7614876-post6.html

+1

Sounds like a plan...That actually make perfect sense, how come I didn't think of that.

Thanks

Sometimes we need to rub our collective brain cells together to make that lightbulb turn on above our heads!

I am actually set to do the same thing myself. My Spektrum receivers can handle up to 9V, but I'm switching radios and the new receivers are only rated for 6V. Since I already have a Castle BEC and HV servo installed it's just a matter of sending the BEC wires to the servo instead of directly into the RX.

I am tempted to see what happens if I run 7.4V to the new receivers, but only because they are the cheap FlySky ones, and I won't feel bad if I burn one out for $8. Wouldn't risk it with an expensive name-brand receiver.

Couple of questions though:

Can I have 7.4v going to the servo all the time?

HV servos such as the hitec 7954 can handle a 2s lipo without a regulator right?

Then all I would do is Y the battery connection--one end going to the 6V regulator which provides power to the RX and the other going to the servo(s) power wire(s) with the signal wire from each servo going to the RX. Is this correct??

Thanks again

Hey...I'm trying the FlySky TX out too...weird

So from what you describe I assume you are running nitro with an RX pack then? Yes, your setup sounds correct, and yes the servo can have 7.4V constantly. I would make sure to use a 3S lipo RX pack, since switching BECs have a 1-1.5 dropout voltage, meaning that if you want to run 7.4V constant you don't want the voltage of you battery pack to drop below 9V (1.5V above where you want the voltage at).

I am going to try running 7.4V directly to a FlySky RX tomorrow, maybe they can handle it?

[QUOTE=

I am going to try running 7.4V directly to a FlySky RX tomorrow, maybe they can handle it?[/QUOTE]

Keep me posted

Assuming it’s a nitro RC, correct. Be sure to remove the red-positive wire between the Rx and the servos. Probably need to keep the ground-negative-black wire between servos and Rx to complete the control signal circuit.

Seems another possible approach is to use a zener diode to drop the voltage for the receiver, as it’s fairly low power. Simpler, although would result in the receiver receiving slightly lower voltage as the Rx lipo runs down, but I don‘t think that would be a problem. Perhaps use two in parallel for redundancy. Something like this (2.4V zener, drop the full charge 2s of 8.4V to 6V):

http://www.mouser.com/ProductDetail/NXP-Semiconductors/BZX79-B2V4133/?qs=sGAEpiMZZMsnCtsnHQqDlaJO35jaRjnp4%2fs3amCV1pI% 3d

Believe you are mixing up a nitro and electric setup. The linked post was for a 4s electric setup, thus the regulator was needed to drop the 4s down to ~2s level for the HV servo. Connecting a lipo receiver pack directly to the receiver would be more than the 6V desired.

Don’t see the need for a 3s lipo for a nitro-lipo-HV servo setup. The 2s lipo output goes directly to the servo as the OP mentioned (no regulator), no need for a 3s. The regulator is needed to drop the 2s voltage for the receiver, not the HV servo.

The voltage drop for a regulator sounds like an older style linear regulator. In general switch mode devices do not have this limitation (can even raise voltage). Admit I’m not familiar with the CC unit, but they say it will go as high as the input voltage, which I would expect for a switch mode device. But either way not an issue for the nitro setup described.
　
Hope this helps some.

There is a dropout voltage on all the switching BECs I know of, about 1-1.5V, (linear regulator have lower dropout, closer to ~0.2-0.5V) but if he is running a 2S lipo direct to the servo then no worries about that. Only reason I suggested 3S lipo with a regulator is to give a constant 7.4V, whereas the 2S lipo will vary from 8.4 to 6V. I guess it's either a more complex system but steady voltage, or simpler system with varying voltage. I only have experience with the former, so can't really comment on how varying voltage will affect the servos performance and response. Probably not that much though as long as you keep the RX lipo topped up on the charger often enough.

Are you suggesting 2 regulators then, to cover the 6V limitation of the reciever that the thread started with? Or just assuming the receiver won't burn?

If you are that concerned about getting absolute servo performance, might as well run the regulator at the 2s full charge value of 8.4V or at least something approaching it. A regulator set to 7.4v would actually be lower than most of a useful 2s lipo cycle.

Well initially I assumed he was running 1/8 electric, and would run the receiver from the ESC internal BEC and the servo from an external BEC (see my first post) using his 4-6S main lipos. He agreed that was what needed to be done, as did others. But when it became clear his setup was nitro that solution became more complicated, requiring adding 2 regulators instead of 1 and didn't make much sense anymore.

I never suggested giving anything more than 6V to the RX.

If he were to give 8.4V regulated voltage to the servo it would need to definitely be a 3S lipo RX pack, in which case you'd need 2 regulators anyway. I run my HV servos to 7.4V because that's what the specs usually say (speed and torque ratings at 6V, and 7.4V) I think 7.4V is a good medium between 6V and 8.4V as far as performance and longevity. Plus the monster torque servos now are really starting to push current draw as it is, 8.4 would be more likely to overload BEC (not an issue with unregulated lipo RX pack I geuss) Anyway, I said that the varying voltage of using direct 2S lipo to the servo probably didn't matter much. I just never tried a 2S lipo RX pack because I run 1/8 electric and already have a 4S voltage source, for me it makes more sense to use a regulator.