Op Amp Problems

[metaphysics1221]

Ok, I've discovered the problem behind my H-bridge, I needed a higher voltage than the 4-5V from the micro controller on the MOSFET pins. My solution was to make an op amp circuit for each signal line and increase the signal to 18V.

However, nothing seems to be working. This is the circuit I've wired for a non-inverting amplifier:



This is the data sheet for that op amp:
http://www.datasheetcatalog.com/datasheets_pdf/M/C/4/5/MC4558.shtml

I've been wiring a multimeter between the output pin and the common ground to try and measure the voltage increase. However, my readings don't make sense; they usually hover around 6V and don't change when I take off the power supply, so the power seems to be reading something coming from the signal source.

What am I doing wrong???

[galannthegreat]

My suggestion for this would be a solid-state relay for this application (maybe multiple ones as needed).

[metaphysics1221]

I thought about that, but size is really a limitation. This is for an underwater robot, and my pressure hull is very small. In addition, I have a pretty short budget and 4-5 motors that would need individual control, both forward and back.

[galannthegreat]

What kind of Driver are you using for the motors?

[aludra_55]

It seems that your op-amp circuit is okay, but op-amp integration can be tricky.  You might need a virtual ground.  But anyway, could you post a picture of your entire circuit or describe your whole setup so that we have a better idea? 

[metaphysics1221]

I've decided to go with a more direct control method for now, the circuitry required to make this work is just too space consuming and difficult to integrate into my robot.

While solid state relays are, I think, too expensive for this application (I'm thinking about future designs here) what do you think of regular old electro-mechanical relays? Could one be switched on and off with 5V but still regulate up to 10A ?

[Admin]

Could one be switched on and off with 5V but still regulate up to 10A ?

Yea, but you can forget about PWM . . .

[guru]

I don't understand, why not simply use some a transistor to drive the MOSFET input to 18v? You may need two to drive it "totem pole" style.

The opamp should have worked, albeit part-count overkill, if it was generating the proper voltage on the output. Can you confirm there is no short or something on the mosfet input circuit that is overriding any input?

If you use relays won't you have to add transistors to drive them anyway? ..and the post about PWM with relays is right, not possible. You'd basically only be able to drive the motor full forward or full reverse, no speed control.

[Soeren]

Hi,


Relays combined with PWM does make it simpler:

(Preliminary illo from an upcoming (e?)book)

Here it is without the annoying resize:
http://That.Homepage.dk/Img/Relay+PWM.png

[guru]

Well, that makes sense. But I got the impression "metaphysics" was going to use small relays to control the H-bridge mosfets.

Relays combined with PWM does make it simpler: