H bridge Circuit Question

Greetings fellow, I have a little question for those smarter then me, concerning a model dump truck I'm building.
I have a radio receiver and small servos taken from a trainer helicopter, to use in running the dump truck

The servo is clearly too small to handle the steering, so I have removed the small servo motor from the housing and am needing to attach the wires that went to the motor, to a bigger more powerful motor. The question is, will an H bridge do the trick? The motor is getting signals at an incredibly fast rate.

The servo decoder and pot remain attached to the steering linkage to send feedback to receiver.

The servo decoder and pot remain attached to the steering linkage to send feedback to receiver.

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Can you explain what you're talking about a little more, that doesn't sound like a hobby servo (you said this was from a trainer helicopter?) Can you post some pictures of what you're calling the 'servo decoder' and 'pot'? Hobby style RC servo systems don't work as you described I think you might have your nomenclature a little mixed up.

Ok, lemme try. The servo is attached to the steering linkage. Inside the servo there are three units:
A tiny motor
A potentiometer
A decoder
Pretty straight forward, this is the norm for most any servo.

I am using its decoder and pot. But the tiny motor has been removed, because it too small to turn the wheels.

Can I take those motor wires from the servo decoder, and use them to run an H bridge, which will in turn drive a more powerful motor, that I have also attached to the steering linkage?

You'd need interpretation logic before the second h-bridge it couldn't directly run it unless you tapped the servo's circuit board after decoding at it's hbridge (what you're referring to as a decoder is both a decoder and h-bridge driver) but that wouldn't be particularly easy.

This seems like a long way to go to avoid buying a stronger servo, they're not that expensive.

Ya, I was thinking I should just buy a stronger servo, was trying to use what I had. But now you got me thinking, I'm pretty sure on the back of the decoder chip I did see an H bridge, I should open it up again and tap in there.

I found an H bridge array from an old racecar somebody give me, but in looking up the transistor number, they dont appear to be MOSFETS. Is that important?

If you got the parts and don't mind hacking it go for it. I don't know what effect simply removing the load and tapping the other signals will do though, it might not be a bad idea to terminate the old motor terminals with a small resistor just to avoid those lines being totally floating. Wouldn't need to be much, maybe a few MA at full voltage, when you go hacking this much all kinds of weird complications can occurs cause you're basically putting two different circuits in a blender and hitting 'puree' without understanding everything that's going on in fine detail, it's pretty much flipping a coin as to what will happen, and it will always be a hack. But it may work too. Worth playing around with.

If the original h-bridge is designed to drive mosfets, there's nearly a virtual garuntee, that it will not be able to drive bipolar transistor h-bridges.

Thanks for the info, tapping into the decoder isnt looking too promising at the moment.....

Seeings what I'm up against, I have no problem getting a more powerful servo, the question is, will my radio receiver be able to power it, or do bigger servos have a dedicated input to allow for different power source to drive the motor?

The receiver does not power the servo, the battery does. All the receiver has in common with the servo is ground and the signal line (very low voltage very low current), the power supplying line (VCC) is a soldered bus (metal conductors) immediately at the output of the receiver, not actually part of the receivers circuitry.

Small or large it's trivial to separate the servo's power bus (VCC at least not ground) You just re-route the VCC line.

From that generality you get into some very detailed specifics from here on out.

It's common to run a NiMh hobby receiver from 4.8 volts where the servos are powered from a 6volt pack.

I would strongly suggest some more Googling on the topic, there is a HUGE amount of information out there on this subject already. Specific questions are welcomed in the forums, but some more research on your part would be good.

Not to say you're being lazy, but I'm not sure what you're having trouble with the most to best describe what your best course of action is, seeing as how every bit describing what you've talked about so far is available if you spend the time looking for it.

Best of luck, I'll comment as much more as I can, and don't be disheartened by complexity.. The more you don't understand the better you know you're looking in the direction that will teach you the most, even if you don't acomplish your goal.

Thanks for the reply, I did indeed google, and spend much time on google researching, but some fine detail is often missing.

Your info has helped me in my decision and what works and wont work. Very much appreciated my friend.

I hope I didn't come across as too harsh, just trying to inform from my perspective. This is not a common perspective, but if you've learned anything and are learning more on your own, then the posting time was well spent, than you for your appreciation.

The question is, will an H bridge do the trick? The motor is getting signals at an incredibly fast rate.

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Is it just two wires going to the small motor? Those are probably PWM drive voltages. You should be able use them to control an H-bridge and more poweful motor.

Hi Alec, I was also under that impression. Reason being, in a google search a while back I came across a guy building an oversized monster RC truck, and he was doing just that. Now he didnt go into detail wether he used the motor leads, or tapped in before the servo's H bridge.

In a test run I used an LB1641 motor driver chip I found off a circuit board, but it overheated. I dont know if it uses MOSFETS though.

Here's my design which uses CMOS Schmitt trigger inverters to provide H-bridge PWM control signals at logic level, assuming the input signal is a 5V PWM pulse train on two wires, the polarity being reversed to reverse the motor. I modelled the H-bridge using Darlington-connected bjts, but with a suitable interface chip between the inverters and bridge the Darlingtons could be replaced by FETs (the CMOS can't drive power FETs directly).