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I have recently used a H bridge to control a small stepper motor obtained from a DVD drive and now want to look into controlling something larger, for example large Nema 23 steppers. From reading online I found that H bridges cannot be used for controlling such a motor and instead a "chopper circuit" is to be used such as the EasyDriver. This raises the first question:

Why can ordinary H bridge circuits not be used for larger steppers?

I kept reading about this to find an answer and then saw that many people are using drivers such as TB6600 instead of an EasyDriver. From what I can see, EasyDrivers cost around £1 whereas the TB6600 controllers are almost 10 times that! Why is there such a big difference, do they not just do the same thing? Why do so many people use these when there is such a large price difference?

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Plain H bridges can be used to control large steppers, provided that they have the current/thermal capacity. But it's not efficient to do so.

The problem with a stepper motor is that the windings have lots of reactive impedance, and a motor with fine steps, rotating at or above a moderate speed, will be trying to switch the current flowing through that inductance very quickly. Doing this requires a quite high voltage - eventually many times the voltage necessary to push rated current through a stationary coil which shows only resistive impedance.

The designer of a simple driver has a choice: they can size the voltage for the stationary case, and lose torque (and soon miss steps) as the step rate increases. Or they can size the voltage to overcome the inductance of the high speed case, and overdrive (and overheat) the motor when it is not turning.

An early solution was to use a very high voltage, and huge power resistors in series with the coils - in effect reducing the ratio between the total impedance in stationary and rotating cases. This was actually done on some early CNC conversions of full size bridgeport milling machines, but effectively means there's a resistive heater strapped to the back of the cabinet.

The modern, efficient solution is a chopping current drive. This is effectively an additional circuit which rapidly enables/disables an H bridge. When a step occurs, the winding is energized at a high voltage. A comparator then monitors the rise of current though the winding inductance over time (typically by sampling the voltage on a high power fractional-ohm sense resistor). When the current has risen to a set point level, the driver is disabled and the current falls. It's then re-enabled and the cycle repeats - as long as a given winding is commanded to be energized, it will be "chopped" on and off to achieve the specified current.

Ultimately a chopping drive is an H bridge - but one with an extra current regulator inserted between the step generator and the control signals to the FET's comprising the bridge.

NEMA23 is about at the dividing point for H bridge construction - anything much larger and you want an assembly of discrete power FET's, while for limited applications at that size and lower (desktop 3d printers, etc), you can probably use an integrated circuit bridge or complete driver circuit with chopper included.

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  • \$\begingroup\$ Is there also the potential for micro-stepping with the pricier controllers, something you wouldn't be able to do with simpler ones. I've heard of this - but don't know the details... \$\endgroup\$ – Danny Staple Oct 15 '14 at 23:00

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