+/- 24volts for galvos

[blowfly]

Here's an updated wiring drawing.


For my projector I need 12 amps at 5 volts to run lasers. Using a DC/DC converter with 80% efficient I need 15 amps of 48v to achieve this. I need 2A of -24/+24v for the scanners, and another 1.5 amps of 24v for everything else. Does this mean I need two 24v power supplies that can output at least 6.5 amps each? Or is it 20.5 amps each? I'm not sure how the two 24v power supplies in series affect the amperage needed by the DC/DC converter.

Thanks!
Mike

[-bart-]

Nonono ! At first, drop the second image, don't put the outputs of dc converters parallel ! That's not the way, it'll wreck them.

If you need 12amps at 5 volts that's 12 x 5 = 60 watts output of the DC conv.
With an efficiency of 85% you'll need 60W / 0,85 = 70 W DC conv input power.
If you feed the DC conv with 48 (2x24V) volts, it will draw 70W/48V = 1,5 A on each rail.
If you add 2 amps per rail for the galvos, and 1,5 headroom for everything else, you'll need 5 amps per rail.
Then for example two pieces of traco TXL 120-24S will do fine.


Keep in mind that if you choose to supply your DC converter with 48V(2x24v) it should have a floating output.
If not, the common of your galvodriver and the common of your laserdriver(s) will have a voltage differential of 24V.
This will cause a problem, as you will be driving both drivers with one ilda port.

[blowfly]

Thanks for the explanation Bart. Your math makes sense.

I think I understand the problem without floating output. The input common to the DC/DC converter is -24v compared to the common in the coming in from the ILDA connector, so there would be a relative difference between the output common of the converter and the ILDA connector. Not sure I understand the solution though. Is floating output a feature of a DC/DC converter? How does it know where to float to?

The datasheet for the Murata 811-2202-ND that Thomas pointed out above says this:

Floating Outputs
Since these are isolated DC/DC converters, their outputs are “floating” with
respect to their input. Designers will normally use the –Output as the ground/
return of the load circuit. You can however, use the +Output as ground/return to
effectively reverse the output polarity.

Does this mean it will just work to make the common from the DC/DC converter match that of the rest of the system?

Keep in mind that if you choose to supply your DC converter with 48V(2x24v) it should have a floating output.
If not, the common of your galvodriver and the common of your laserdriver(s) will have a voltage differential of 24V.
This will cause a problem, as you will be driving both drivers with one ilda port.

My first projector didn't have any grounding issues but... i had 3 separate power supplies in the projector, 1 for -24v/+24, 1 for 12v, and 1 for 5v. This was the brute force approach I guess. I like the idea of having only 2.

Thanks,
Mike

[-bart-]

'Floating' means the input is isolated from the output, there is no conducting path from in to out. The power is converted 'wireless' from in to out. You can connect the common 'anywhere' without consequences.

[Badpip]

A picture says a thousand words.

It is important that the DC/DC is floating/isolated for this to work. The murata I linked to will work.
/Thomas

[-bart-]

Keep in mind to obey the central grounding topology.
Although worth 1000 words , this picture doesn't convey that message at all.

[Badpip]

Keep in mind to obey the central grounding topology.
Although worth 1000 words , this picture doesn't convey that message at all.

Haha, yes, just showing how the "floating" part fitted in on his drawing

More complete picture:


/Thomas

[blowfly]

Great info guys! Thomas in your drawing I assume star ground can also be connected to AC ground?

Haha, yes, just showing how the "floating" part fitted in on his drawing

More complete picture:


/Thomas

Thanks,
Mike

[taggalucci]

Thomas in your drawing I assume star ground can also be connected to AC ground?

I wouldn't (this is discussed in Bill Benner's projector wiring paper on the Pangolin website).

I would keep DC common/ground and AC ground isolated. That will require you to isolate all DC devices from the baseplate e.g. scanner mounting block, circuits boards that could connect their DC common to metal work through mounting bolts etc. If in doubt, use your multi-meter to perform continuity tests.

[blowfly]

Thanks Taggalucci. I remember reading that in Bill Benner's wiring paper. In my first projector I didn't make any effort to isolate AC and DC ground. I'm pretty sure they are all connected. Good idea about checking with a multimeter though, I may have just gotten lucky. I'll take a look.

Mike