Building a high voltage servo ~ h-bridge help?

Let me start by warning everyone that I'm an electronics newb. :blush: I am trying to modify a Hitec 7955 titanium gear servo for use with unregulated three cell lipo. I will still be controlling the servo from a hobby rc remote. My first thought was to replace the servo board with one that can handle the ~12v. I have been unsuccessful in finding a suitable replacement board. A few standard size 12v servos are available from Chinese manufactures but I cant seem to find just a 12v board available.

My second thought was to add a relay circuit between the board and the motor, thus isolating the board from the high voltage. I believe that I need a h-bridge mosfet circuit between the board and the motor. Space and weight are at a premium so an integrated circuit would be necessary. Can I control the h-bridge with what were the motor feed terminals off the original board? The board would be sending -6v to +6 volts depending on the desired rotation. I'm not clear if I can use this -6v to +6v output as the switching input for the mosfet h-bridge.

I really appreciate any help that I can get to move this project forward.

http://forum.arduino.cc/index.php?topic=169717.msg1262798#msg1262798

Plan A ~ Plan F.

BTW, high voltage servo means voltage is few hundred volts and up to thousands.

I am trying to modify a Hitec 7955 titanium gear servo for use with unregulated three cell lipo.

What is the voltage output of the three cell lipo cells? Why do you want to modify the servo?

Can I control the h-bridge with what were the motor feed terminals off the original board?

Yes, probably. H bridges are pretty good at driving more powerful H-bridges or MOSFETs. If you look at the internal schematics of of a "MOSFET driver", you'll see something that looks like an H-bridge. Getting add-on power transistors to work right might be more complicated than it's worth. Have you looked the openServo project? Sparkfun's board looks like it does/can do 12V to the motor... https://www.sparkfun.com/products/9014

sonnyyu: http://forum.arduino.cc/index.php?topic=169717.msg1262798#msg1262798

Plan A ~ Plan F.

BTW, high voltage servo means voltage is few hundred volts and up to thousands.

Thanks for the link. It looks to me like d, e and f wouldn't work since i wouldn't have the necessary inputs. A, b and c I'm not as clear about. These could be signaled by the original motor terminals on the servo board?

I didn't mean to confuse about the "high voltage". In the radio control hobby crowd high voltage servo means 7.4v and greater.

zoomkat:

I am trying to modify a Hitec 7955 titanium gear servo for use with unregulated three cell lipo.

What is the voltage output of the three cell lipo cells? Why do you want to modify the servo?

12.6v peak 11.1v nominal and 9v at lvc. I am pursuing this project to build a faster more powerful servo than what is available in the “standard size”. I am space and weight restricted in the final application but need more speed and torque.

westfw:

Can I control the h-bridge with what were the motor feed terminals off the original board?

Yes, probably. H bridges are pretty good at driving more powerful H-bridges or MOSFETs. If you look at the internal schematics of of a "MOSFET driver", you'll see something that looks like an H-bridge. Getting add-on power transistors to work right might be more complicated than it's worth. Have you looked the openServo project? Sparkfun's board looks like it does/can do 12V to the motor... https://www.sparkfun.com/products/9014

So do you think that anything that sonyyu linked above would work for me?

Back when first found the openservo board I thought my prayers had been answered. Unfortunately as far as I can tell it won't work with the futaba tx/rx that I will be using to control the servo. I believe the rx sends a pwm signal to the board and I don't believe that the openservo board will accept this.

The output of a small servo can be used to drive an h-bridge for a large motor. Some examples below.

http://www.youtube.com/watch?v=VCbM8qvdXxU

http://www.youtube.com/watch?v=Rpqsvw2iTIo

http://www.youtube.com/watch?v=x6yNfTb9LEo

http://www.youtube.com/watch?v=FoblOUokfJY

http://www.youtube.com/watch?v=sAC2Wkc6RLM

In one of those videos they are grabbing the pwm signal off the original board before it goes to the original h-bridge. I hadn't thought about that. That makes the stuff that sonyyu posted more straight forward since i could feed the pwm signal that has been altered by the board/pot to the new h-bridge. How do I determine where I can grab the signal off my board?

If you plan to have a large 12v motor and a large h-bridge, not sure why you would want to hack an expensive servo when an inexpensive one like below would probably do. Get a servo like below, remove the motor, and drive the large h-bridge using the former motor leads and ground.

http://www.hobbyking.com/hobbyking/store/__22928__Turnigy_TG9e_9g_1_5kg_0_10sec_Eco_Micro_Servo_USA_Warehouse_.html

zoomkat:
If you plan to have a large 12v motor and a large h-bridge, not sure why you would want to hack an expensive servo when an inexpensive one like below would probably do. Get a servo like below, remove the motor, and drive the large h-bridge using the former motor leads and ground.

http://www.hobbyking.com/hobbyking/store/_22928__Turnigy_TG9e_9g_1_5kg_0_10sec_Eco_Micro_Servo_USA_Warehouse.html

I’m not going to be driving a big motor. One of the reasons I’m doing this is to keep the package small. I will be powering the original motor (or a similar sized unit from a mini gear reduction motor installed in my hitec case) with the new h-bridge. I will still be utilizing the titanium gear set as well. Adding 1v to the motor in a hitec 7955 adds almost 100oz-in of torque. I’ll be adding slightly more than 1v. I should end up with a nice fast and strong “standard size” servo.

Can anyone explain how I find the final point of pwm signal on the hitec board? I guess it would be right before the original h-bridge but I wouldn’t have any idea what I’m looking at. I really appreciate all the help. This community is great!