Class D Amplifiers\Drivers.....

[Dr_Ju_ju]

I first apologise if this is too technical or not technical enough, or even in the wrong place, but...  

Does anyone know if they have been used, and or tested for driving mount motors, or even focuser, filter wheels etc, ??

I ask, as I've recently been repairing a Bose Companion 3, active speaker system that uses 2 class D, power amplifier chips TDA7492(50+50 W) & TDA7498(100+100 W).

They sometimes, erroneously, get called digital amplifiers, because of the 'D', but they have absolutely nothing to do with digits of any sort, except those that get singed, replacing chips....

Whereas I am an old school engineer, where the pinnacle of power amplifier design was class A\B (and maybe still is ?, I even used to make my own Quad 405's), but these require beefy power supplies, driver transistors\mosfets, heatsinks etc. wasting lots of power & generating wasted heat. There are clever power rail switching systems, resulting in class G\H amps, but they are still, relatively, very inefficient (best to ~ 70-85%), more complicated, costly  et al...

One of the major advantages of a class D amplifier is their efficiency, typically 98-99%, especially so, over a wide bandwidth and differing loadings, so most of the power available, gets delivered to the motors\loudspeakers, with very little wasted heat, so would be ideal for our mount drivers, especially those running on batteries. Even those people who use large linear power supplies, and I'm one, can now happily switch to more efficient, smaller switched mode supplies, as they are much more 'noise' tolerant.

Even the stepper motor drivers, that are currently used, are not as efficient, and a lot of the power we supply, is lost as waste heat.

They work on the basis of pulse width, high frequency switching of the power rails, hence the greater efficiency, with the output being fed through a low pass feedback loop, which removes the switching frequencies.

For our purposes, the low pass section need not be over complicated, as we do not require the full audio frequency spectrum, unless you want to make your mount sing....

I don't doubt, that there would have to be a re-thinking of the control system\signals being fed to the amplifiers, and even the types of motors being used, but they could comprise a part of the feedback loop, which together with absolute encoders, could form the basis for a system that would know exactly where it was pointing, and be easier to guide ??

 

[andrew s]

I am not aware of any such use. I have two class D audio amps and they work just fine. Although one has a DC offset as part of the design!

I am not sure they are ideal for the use you propose as they are not intended to have a DC output. It depends on exactly what you want to dive and the input it requires.

Regards Andrew 

[Dr_Ju_ju]

That's the point, mount motors are not driven by a simple variable dc, but are usually 'pulsed' with current i.e. the same as an audio pulse\step, but at a much lower frequency. The same could be said for the stepper motors\drivers, that are used in 3D printing, CNC machines etc.

[symmetal]

I made my own Quad 405s too from the circuit published in Wireless World many decades ago. Made PCBs for them using tracing paper to work out a layout and 'Letraset' type transfers to 'print' it on transparent film for UV production. Made about 30 boards for work colleagues as well 

I can't see class D being more efficient than the usual stepper driver modules like DRV8825 which use power FET output drivers switching to the power rails anyway. They also handle the output bridge switching for the dual coil outputs and microstepping and cost next to nothing.

Were you intending to replace the stepper motor driver with your class D driver? Most audio amps are dual rail so half-bridge outputs will work. Off a single rail you would need a full bridge output for each coil.

Alan

[Dr_Ju_ju]

Yes basically, & FYI the TDA chips are single rail devices, not dual rail, plus a single 'stereo' amp chip could drive each individual coil of a stepper motor... 

the outputs from the TDA chips are labelled as +\- but the '-' negative pole does not relate to 0v\ground, but forms part of the feedback loop. (ref. attached PDF section 4)

tda7498.pdf

[symmetal]

Ah! Single rail operation is OK. You'll have to stretch the 'Step' pulse to a duration long enough to step the motor or to a continuous DC output if you want the stepper motor to hold a heavy load. Class D amps will only output a DC signal if the input signal exceeds the PWM modulator's reference triangle wave, (ie. clipped)  In this case it's not really a Class D amplifier any more. The main features of a Class D amplifier aren't really used to drive a stepper motor as you're only dealing with a two state signal. The 'Step Dir' signal will have to be processed too.

Here's the DRV8825 data sheet and here it is with the Carrier Module used for Arduino etc. to make using them easier (£2.50 on ebay). It can drive up to a 100W stepper motor with a little heatsink. And you get micro-stepping thrown in too.  For mounts and focusers you have much lower power steppers. The common high torque Nema 17 Stepper only uses about 10W.

Alan