DC Motor Drive?

[Tenson]

I've seen this schematic to get 3-pin regs working with higher voltage but I'm not sure how to apply it correctly in this design.

http://www.national.com/ms/LB/LB-47.pdf

[sreten]

Hi,

That type of regulator drops most of the voltage before it hits the regulator.

Its pretty much a fixed output, you can't adjust it to low voltages.

rgds, sreten.

[KatieandDad]

Why not Pulse Width Modulation.

Its easy to implement.

Do you want a circuit with feedback (ie self controlling) or are you happy to adjust the speed by hand?

If its the latter you can just use an NE555 driving a MOS-FET to provide HT voltage pulses to the motor.

The beauty of PWM is that you will get higher torque at low speeds than you would with simple DC voltage control.

[sreten]

Originally Posted by sawreyrw

sreten,

You don't regulate the output of a voltage regulator by having a regulator
with a negative output resistance. This is nonsense and could easily lead
to oscillations in a real system. The only reasonable way to do this is to
use a feedback loop that senses the output and applies a correction signal
to adjust for any changes in the output.

Hi,

The only person talking nonsense for this application is you.

Of course a voltage regulator with negative output impedance
no longer regulates voltage, that is the point. It regulates
the virtual voltage across a virtual winding with no series
resistance, giving constant speed characteristics.

Any real system with feedback (including normal regulators)
will oscillate if designed wrong, your stating the obvious.

As the virtual point is not accessible for feedback, a small
series resistor between the load and ground is required
to provide a voltage proportional to current for feedback.

The feedback needs to be mixed to get the negative impedance.

Variable Amplifier Impedance


Figure 3 - A Negative Impedance Amplifier
(the floating input looks very awkward, not the best example)

Your "only reasonable way" is simply a regulator and
won't maintain near constant speed as I've described.

What is needed is like a single rail power amplifier
with no coupling capacitor and a single ended output.
That sounds like one rail of regulated amplifiers power supply ....
But with a lot more heatsinking and devices depending on the
voltage and current range you want to support for the motor ....

rgds, sreten.

[Tenson]

Originally Posted by sreten

Hi,

That type of regulator drops most of the voltage before it hits the regulator.

Its pretty much a fixed output, you can't adjust it to low voltages.

rgds, sreten.

Really? I got the impression it was adjustable from 1.2V to 160V.

[djQUAN]

The output of the transistor pre-regulator tracks the output of the 317 reg so the output can be set at a wide range of output voltages. You just have to scale up the pass transistors as the app note used low power devices.

The circuit could work with the one I posted but you will have to adjust component values around the feedback paths to have enough range for the amount of torque boost needed.

[darkfenriz]

Sreten,
Although I can agree you have some merit here with a clever circuit, this is usually not needed and if it is, the speed control of some sort is used.