Hi,
I have a question on H bridges.
I am needing to build an RC servo with a 12v high torque motor.

I will be using the circuit out of a servo to run it, What i think i am after is a H bridge that will accept the standard 6 volt output that goes to the original servo motor to drive the H bridge to control the 12v side for the new motor. I havent measured the current draw on the motor as yet, but estimate it to be 40A at max load.

Any help much appreciated.

 

Welcome to AAC!

I will be using the circuit out of a servo to run it

Analog servo or digital servo?
How do you plan to take into account the inertia of a high power motor? You can't just stop and reverse one instantly without risking damage.

 

You can roll your own with some high current MOSFETs but I would consider just buying an off the shelf turn key solution. A Google of 50 Amp H Bridge will give you several options. Subject to Alec's question as to control.

Ron

 

Thanks for the welcome.

I can use either digital or analog. Not sure if one is more suitable.

Inertia is something i never thought of.
More info may help.
The unit i am building is a remote proportional braking unit. It will actuate a brake pedal in a towed vehicle.

I have a 30amp unit similar to your link but unsure on wiring the input side with the original motor wires.

 

The unit i am building is a remote proportional braking unit. It will actuate a brake pedal in a towed vehicle.

Then why would you need a H-bridge? You're just wanting something to push the brake pedal, correct? A solenoid would do that, no H-bridge needed. Before settling on a design look at how they do it for "hand controlled driving".

 

I need proportional braking.
To be able to adjust braking force to keep it all balanced.

 

I need proportional braking.
To be able to adjust braking force to keep it all balanced.

Shortbus has a point. If your application does not require current reversing, then you don't need an H bridge. A single pwm'd mosfet could do the job.

 

Hmm , ideas!
That could work, i would need a spring return.

 

Actually that may not work. Motor is geared through a 20:1 reduction so it would need a huge spring. I think i would rather have the motor return it .

 

Without actually seeing what you have a few possible issues are when braking a trailer things happen fast. Then as mentioned if you use a geared motor to apply a force you need to back the motor off to reduce braking and you also need braking evenly on all the wheels. Have you looked at how electric braking works on a trailer as that would be my first suggestion depending on how any current braking is setup. Next and getting more expensive would be using an I to P (Current to Pressure) pneumatic which less an air supply gets expensive and I to P transducers are expensive. Electric braking is efficient and a good way to go. Depends on what you actually physically have in front of you.

Ron

 

Hi Ron ,
A trailer braking setup would be easy yes.
This is for a 4wd being flat towed behind a motorhome. Due to the weight involved it cannot have the easy all mechanical override setup with a cable pulling on the brake pedal.
Rules are braking that can be controlled by the driver.
So i thought easy , large servo to actuate brake pedal. Operated by a modded servo tester in the motorhome .

Driving the larve 12v servo motor is what held me up.

 

Actually that may not work. Motor is geared through a 20:1 reduction so it would need a huge spring. I think i would rather have the motor return it .

So what your saying is, "you want to make something with what you all ready have"? Are you the same guy that was trying to do this a while ago? From Sweden if I remember correctly. Here in the US I wouldn't think it is legal to flat tow a vehicle controlling the brakes electronically from the towing vehicle.

 

No sorry, im new here.
Im also from Australia.
I have all the rules and regulations under control for towing.
Just wanted to know how to adapt a large motor to an existing proven circuit that does exactly what i require.

 

With any sustained braking the motor will be stalled for a considerable time and will get HOT. I would be concerned about a fire risk and about long term reliability. Have you considered using hydraulic braking instead, with proportional valves controlled remotely?

 

This is for a 4wd being flat towed behind a motorhome. Due to the weight involved it cannot have the easy all mechanical override setup with a cable pulling on the brake pedal.

Ah, now I get it. This gets tricky as when braking the RV you are applying varying force to the brake pedal. Even if a motor were used to brake the load you need to accurately control the force. You do not want to gently tap the RV brakes to slow and lock up the brakes on the towed vehicle. I would also be curious as to if the towed vehicle had power or standard brakes. Anymore up here just about anything including small 4 WD Jeeps have power brakes so without engine running braking is difficult, you have to stand on the pedal. This presents an interesting problem. I can see a few possible solutions but finding a practical and moderate cost solution is a challenge. Heck, at least now I understand the picture.

Ron

 

Yes power assisted brakes. I have built a 12v vacuum pump to eliminate needing the engine running
The reason i decided on rc servo based setup is its easy to control brake pressure.
I do have plenty of experience with rc radio control. Potentiometer on motorhome brakes for control, easy to adjust throw lengths end points etc.

Motor heating is something i have considered and have a couple of options to sove that.

 

Yes power assisted brakes. I have built a 12v vacuum pump to eliminate needing the engine running

You have some serious thought in this. Pretty cool. I will think about this more knowing what you have posted.

Ron

 

Have you looked at these H Bridges?
https://www.ebay.com.au/itm/Semicon...320860&hash=item467d414e30:g:UMcAAOSwgcVbB1pX

Then an Arduino to drive it. There is this one. I've not looked through it all myself.
https://www.instructables.com/id/Make-your-own-low-cost-servo/

But you may be able to extract the drive signals from the small servo board to run this H Bridge.

 

Hehe, yes a bit of thought has been used.

dendad, that could be what im after! Thanks!
Next trick will be finding out if it will operate on the standard 1 to 2 ms pulse.
If not i will be after more help! Arduino isnt something ive had any experience with.

I checked the motor i have, draws 10A unloaded. It is a 5 to 1 gear reduction unit. i was going to add another 10 to 1 reduction (already have the gears) to make it easier for the motor, but it will end up with enough torque to pull start a car!
I will look for something a little smaller.

 

I think something like a windscreen motor would work ok.
have a look at this one...
https://www.instructables.com/id/Wiper-Motor-and-Arduino-Mega-Servo/