DC-DC regulator - isolation question

**stantale** · 02-10-2012, 07:57 AM

Hi,

I will be runnning a computer (beagleboard XM) + a few microprocessor boards (arduino) with a many sensors (12V, 5V, 3.3V)

I'll need several different DC-DC switching supplies to give me steady 12V and 5V outputs, sensors may be driven on 12V, 5V, and even driven directly off the main battery.

My question is this: how do I ensure they all operate on a common ground? it seems some regulators provide "isolated" outputs. Does that mean they are not common ground? Or only isolated in the sense that input bursts don't filter through?

If they are isolated, can I combine all the grounds or does that not work?

I plan on using this for a regulated 12V: http://www.jameco.com/webapp/wcs/sto...0001_155708_-1
Jameco (SKE15A-12) Module DC-DC 1 Output 12 Volt 1.25A 15W 5-Pin

The regulator needs to be step up/down since the equipment will be running with engine on or off.

Thanks for the help, just starting out...

**davekra** · 02-17-2012, 12:28 PM

I'm interested in the same thing. I've got an M2-ATX running the system and will be getting an SD-25a-12 from Jameco to run the backup camera and display.

From the Mean Well site I found this: http://www.meanwell.com/faq.html#A1

The output ground (GND) and frame ground (FG) is the same point in my system, can MEAN WELL's power supplies be used in such system?

Ans:

Yes. Since our products are designed based on isolation concept, it will be no problem that the output ground (GND) and frame ground (FG) is the same point in your system. But, EMI may be affect by this connection.

**OldSpark** · 02-18-2012, 09:50 PM

Agreed. Isolated systems means that either output can be connected to either input supply. Just connect the -ve (or 0V) output to the vehicle's ground.

Many dc-dc converters however are not isolated - they use a common ground. (They use a single inductor/coil instead of a two-coil "transformer".)
dc-ac converters should all be isolated for safety reasons, however some older units were not isolated (idiots!!).

**anngabby** · 03-20-2012, 07:55 AM

The few isolated DC/DC converters that I know are used in communication lines only. I've always thought that these converters do not work for stable voltages.

**OldSpark** · 03-21-2012, 02:09 AM

I don't know what you mean by not working for stable voltages. All should be quite stable.

And I'm not sure if you mean telcos wrt isolation, but telcos were a big pioneer (or pusher) of dc-dc converters and they often required isolation - ie, traditional exchanges were 48V +ve earth/GND whilst newer & mobile equipment is usually 24V -ve GND.
But isolation for comms is usually desirable as well.

Telcos also required exceptional input filtering mainly for their comms & signalling bandwidth of 300Hz - 4kHz. I know of a few exchange outages caused by unqualified people connecting ordinary dc-dc and dc-ac (and sometimes even ac-dc!) converters to exchange batteries. Not only would the .3-4kHz dial anywhere, but that same in-band signalling was used for exchange commands - like drop traffic or shut down etc.
I used to laugh when saving a few hundred dollars on converters cost $millions in outages! (Though that wasn't as bad as the saving a few buck$ by reducing air conditioning by 1°C etc.)

I think too that it was a telco that first specified local gnd/earth-neutral connection for inverters, and dual-pole switching for inverter-bypass AC supplies. Nice to see that both have caught on! (Though I did recently comment on contradictory advice on the Xantrex website. But I suspect that isn't unique.)

Bless the telcos - not that we need them anymore - we just use the web.
(That's a joke. It's akin to "If it wasn't for Edison, we'd be watching TV in the dark!)