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I have a pretty simple application but not sure where to start on selecting parts

I have a 28 volt variable output that needs to drive a 14v dc motor. The direction of rotation of the motor is controlled by reversing polarity. Speed is adjusted by regulating the 28v input. The motor runs at 300ma or so and stalls at 600 ma.

I can manufacture a bipolar 15v supply for an op amp. I was thinking of using a differential amp design using a high output drive current feedback opamp.

https://www.ti.com/lit/ds/symlink/ths3121.pdf?ts=1607705909112&ref_url=https%253A%252F%252Fwww.ti.com%252Famplifier-circuit%252Fop-amps%252Fpower%252Fproducts.html%253Fpqs%253Dpaqs%2526familyid%253D3523

The 28v source signal can push 500 ma if needed. The loop gain would be 0.5. This is as far as my design skills take me. Am I on the right path or is there a better approach? Even an off the shelf solution?

Thanks in advance

Tom

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  • \$\begingroup\$ What is the range of voltages needed at 14 V to control the motor speed over the desired range? \$\endgroup\$
    – The Photon
    Dec 12, 2020 at 2:45
  • \$\begingroup\$ Motor will start running at around 8v. Ie 16v on the input signal.. thanks \$\endgroup\$ Dec 12, 2020 at 4:02

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I am no expert at motor control, but I think you are on the wrong track in a few ways:

  • An op-amp is probably not the best way to do this, but if you want to do it that way, using a 120 MHz op-amp for a circuit that probably needs a few 10's of Hz of bandwidth is probably not the right choice. You could probably do this with a $0.05 LM2904B (with a transistor buffer on the output).

  • Current feedback op-amps were designed to obtain higher frequency performance from older manufacturing processes. They have some design quirks and require special design consideration. But since you don't need 100 MHz or higher bandwidth, you can avoid the whole problem by using an ordinary voltage-feedback op-amp.

  • Since your output voltage is always above 0 V, you don't need a negative supply.

But in any case, the op-amp design will be fairly inefficient, burning power as heat to reduce the supply voltage to whatever voltage you are delivering to the motor. A more efficient solution, if you don't need exceptional accuracy in the voltage divide ratio from the input to the motor, would just use a PWM circuit, with 50% duty cycle. If the motor can't handle a PWM drive directly (this is where you need someone who knows motors better than me), then you can filter the signal with an LC filter.

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ I think I would be tempted to put a DC regulator in front of all that - that way a fault condition, i.e. M1 being permanently ON isn't going to do any damage to the motor. This is going to need to drop up to 14V @ 0.5A - so a switcher seems a better choice than a linear regulator \$\endgroup\$ Dec 12, 2020 at 8:05
  • \$\begingroup\$ @YellowYeti, You could use a current limiter between the source and the switch (M1). Then you get fault protection but still keep the proportionality between the input voltage and the load voltage. \$\endgroup\$
    – The Photon
    Dec 12, 2020 at 16:45

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