Ye Ol' G115 and G120PD Scanners

[lasermaster1977]

Why not just build an A102 Back-EMF and try that. Adds damping so you can follow square waves.

Steve

See attached.

Thanks for posting your back-emf feedback solution. I posted a month or so ago a version I used for years, back during the 80's, that I still have and am getting ready to fire up again. However, there was a visual bonus when using no back-emf correction in drive amps when displaying slowly drifting square waves in the 1:1, 1:2 and 1.3 XY ratios. In the digital world, that's some hard "animation" to reproduce when using position feedback of any kind.

In the beginning (1977) I used the G115's in the analog bandwidth of 45-450 Hz (Harmonic 1-10) for Sine, Triangle and Square (Two mixable PLL/Free Running channels for X and 2 for Y) used for base Lissajous images (either PLL or free runing) and .5 - 10K Hz for modulation effects. . Ringing effects got naturally damped out around the 4th or 5th harmonic, but in the 1st to 3rd harmonic, square wave ringing was a great effect.

But don't misunderstand what I'm saying, position feedback scanners are the preferred and yield massively greater benefits.

[lasermaster1977]

Here's the variant of the 791 scanner amp with back-emf feedback I did around 1980-81. My initial post was the final version that performed the best.



The top schematic shows a simple variation I used for driving DC motors or gear-reduction, DC motor driven 10 turn pots.

[Draco]

a lot of the car amps i see are indeed dc coupled and can pass almost dc, .5 to 1 hz is about as low as i have tested, will something like that damage the galvo? If i remember right you did tell me that can magnetize the armature?

on amps that are dc coupled you should find there is voltage on both jacks with respect to ground, but from the positive and negative there should be 0 so the load never "sees" the dc voltage

[lasermaster1977]

a lot of the car amps i see are indeed dc coupled and can pass almost dc, .5 to 1 hz is about as low as i have tested, will something like that damage the galvo? If i remember right you did tell me that can magnetize the armature?

on amps that are dc coupled you should find there is voltage on both jacks with respect to ground, but from the positive and negative there should be 0 so the load never "sees" the dc voltage

Thanks Draco for the information on today's audio amps. Driving a galvo from an audio amp won't hurt the galvo unless you overdrive the galvo with too much current. Magnetization of a galvo usually happens when it is driven to a particular point of rotation and held there for long periods of time, but what is magnetized can easily be demagnetized by bipolar deflection of the armature.

[mixedgas]

~1.25 Amps is the demag current for GSI 1Xx series.
So conservatively if more then a half amp average current is needed for any length of time, something is very wrong.

Some audio amps are actually used down to DC in Galvo amps, with split supplies.

LM3886 works good as long as the loop gain is at least 15-20

Others will work at unity gain and up, ie LM675.

So does L165, L265, which you can get surplus.

Steve

[Draco]

I have been surprised how low an audio amp can do before shut down or doing into a protect mode, most are not linear say below 20-15hz but even cheap ones can go fairly low, I have one dual mono block that is rated to dc, common ground though. Generally it's the speakers that don't like dc, lol there are movies now that have content below 10hz, great deflection on my galvo's and vibrating the windows on my home.