2 opamp H-bridge

Thread Starter

Jacopo D

Joined Jan 14, 2016
3
Hello,
i'm trying to design a H-bridge circuit using 2 operational amplifiers. I'd like to drive a DC motor, using a 0V/5V input (at 2.5V the motor should stop). This is my schematic:
schemaTensione.jpg
And this is the simulation result:
graficoTensione.jpg
The two probes measure the voltage on the two motor contacts. The DC motor has a 12.3 Ohm winding resistance. I'm finding some difficulties reading the results: will the motor run in both directions? I know, this is not a classic H-bridge scheme: i'd like to use a single supply (+28V), that's why the circuit is a little messy.
Thanks in advance for your help
 

bertus

Joined Apr 5, 2008
20,180
Hello,

@Brownout ,

The mentioned opamp is a poweropamp:

Description


The OPA541 device is a power-operational amplifier capable of operation from power supplies up to ±40 V, and delivering continuous output currents up to 5 A. Internal current-limit circuitry can be user-programmed with a single external resistor, protecting the amplifier and load from fault conditions. The OPA541 devices fabricated are using a proprietary bipolar and FET process.
Bertus
 

AnalogKid

Joined Aug 1, 2013
8,227
will the motor run in both directions? I know, this is not a classic H-bridge scheme:
Actually, it is a semi-classic scheme. If the motor has brushes, it probably will run in both directions. However, depending on its internal design, it might not be happy going "backwards". If it is a brushless type (common in small cooling fans), it will not run with the standard voltage input reversed, and the reversal probably wil damage the internal electronics. What is the motor make/part number, and what is the application?

ak
 

PeterCoxSmith

Joined Feb 23, 2015
148
This is not an H Bridge. If you apply 0V with the left hand opamp and 28V with the right hand opamp a DC brushed motor will spin, lets say clockwise. If you apply 28V left and 0V right the motor will spin anti-clockwise. If you apply 14V left and 14V right then the motor will not spin. And at voltages in between you'll apply Vright-Vleft to the motor, either positive or negative and you can vary the speed. However the opamp will be operating in linear mode rather than the switching mode as you'd expect from a H Bridge and the dissipation will be higher.
 

AnalogKid

Joined Aug 1, 2013
8,227
True. If you look at just the two opamp output stages and how they connect to the load, it has the same general structure as an H bridge, and the same intent. However, the output transistors are not intentionally saturating, the most common operation of a standard H-bridge.

From the audio world, this kind of non-saturating output structure is called BTL - Bridge-Tied Load. Although, of course, that structure has its own exception, a BTL class-D output.

ak
 

Thread Starter

Jacopo D

Joined Jan 14, 2016
3
Thank you for all your answers. This is the motor:

My motor is slightly different, the nominal voltage is 50 V, but it works very well at 1.5 V too. (!!)

This is not an H Bridge. If you apply 0V with the left hand opamp and 28V with the right hand opamp a DC brushed motor will spin, lets say clockwise. If you apply 28V left and 0V right the motor will spin anti-clockwise. If you apply 14V left and 14V right then the motor will not spin. And at voltages in between you'll apply Vright-Vleft to the motor, either positive or negative and you can vary the speed. However the opamp will be operating in linear mode rather than the switching mode as you'd expect from a H Bridge and the dissipation will be higher.
Yes, i want to vary the motor speed, in both directions. And i'd like to know if the following is correct:
From the simulation result, if the voltage input is 0V, the first opamp gives 5V, and the second opamp 23V, so the motor voltage should be 5-23=-18V.
If the voltage input is 5V, the first opamp gives 23V, and the second opamp 5V, so the motor voltage should be 23-5=18V (and the direction is the opposite).
If the input is 2.5V, the motor should stop.
(All the intermediate cases follow the same rules).
 

PeterCoxSmith

Joined Feb 23, 2015
148
It may work, I've never driven a motor with power opamps; good luck with your adventure. Watch out for thermal issues, inductive current flow and what happens when the motor acts as a generator/brake.
 
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