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### Author Topic: Combining grounds of 12VDC logic and linear unregulated HV  (Read 3482 times)

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#### simpson36

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##### Combining grounds of 12VDC logic and linear unregulated HV
« on: May 17, 2010, 05:06:20 PM »
I'm sure this has been asked before and I have done searches and read what I could find, but still I am confused a bit by the options.

I my setup, 5VDC and 12VDC are supplied by a standard PC power supply which is grounded to the control box chassis.

HV is a toridal linear unregulated PS with 72VDC output which is not grounded to the chassis.

Voltmeter says no continuity and no voltage between 72V + or 72V- and chassis.

I now need to supply a drive with 12V and HV that have a common ground.

What are the options and what is the best way to do this?

#### Tweakie.CNC

• 7,947
• Super Kitty
##### Re: Combining grounds of 12VDC logic and linear unregulated HV
« Reply #1 on: May 18, 2010, 01:40:54 AM »
Like most things, there is always a difference between theory and practice. I think you just have to connect it up, try it and see (common connection at the drive end). From my experience most electrical noise issues have been ground or earth loop related so if you are unlucky enough to get noise problems that would be a good place to start.

Tweakie.
Success consists of going from failure to failure without loss of enthusiasm.  Winston Churchill.

#### simpson36

• 1,374
##### Re: Combining grounds of 12VDC logic and linear unregulated HV
« Reply #2 on: May 18, 2010, 04:28:02 AM »
Noise is not the issue.

Typically, PS schematics show DC+ and DC- output. So my assumption is that with a linear unregulated PS, the DC is split into half positive and half negative? i.e. with a 72V output, is it +36 and -36?

If that is correct, then it seems logical to me that connecting the -36VDC to the chassis ground may have some negative effect. Yet this is apparently doable, so I have to assume there is something about this that I don't understand.

I am going to purchase a 12VDC regulated module specifically for toroidal linear PS that has a jumper for common ground with the HV, so the problem is resolved, but I still would like to understand it.

#### Tweakie.CNC

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• Super Kitty
##### Re: Combining grounds of 12VDC logic and linear unregulated HV
« Reply #3 on: May 18, 2010, 04:38:07 AM »
Quote
Typically, PS schematics show DC+ and DC- output. So my assumption is that with a linear unregulated PS, the DC is split into half positive and half negative? i.e. with a 72V output, is it +36 and -36?

Do you have a center tap on the psu output then ?. (3 wires)

If it is just a 2 wire output - then connecting the minus end to ground will make it 0 Volts and the other end will be +72 Volts. There will be no negative voltage levels.

Tweakie.
Success consists of going from failure to failure without loss of enthusiasm.  Winston Churchill.

#### simpson36

• 1,374
##### Re: Combining grounds of 12VDC logic and linear unregulated HV
« Reply #4 on: May 18, 2010, 05:26:28 AM »
There are only two wires from the toroid coil to the bridge.

Using a digital meter, I find no continuity or voltage between the HV DC output + or - to earth ground, but 72VDC between them.

With both the HV linear PS and the PC power supply running, there is no voltage between the HV + or - output and the PC 12VDC output.

If I understand you correctly, the 'floating' voltage potential will go all positive if one side is grounded to earth? Is it that simple?

#### Tweakie.CNC

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• Super Kitty
##### Re: Combining grounds of 12VDC logic and linear unregulated HV
« Reply #5 on: May 18, 2010, 06:01:33 AM »
It is indeed that simple.

If you connect the minus end(-) of your HV supply to ground then the other wire will be +72 Volts with respect to ground.

If you were to connect the plus end(+) of your HV supply to ground then the other wire will be -72 Volts with respect to ground.

Tweakie.
Success consists of going from failure to failure without loss of enthusiasm.  Winston Churchill.

#### simpson36

• 1,374
##### Re: Combining grounds of 12VDC logic and linear unregulated HV
« Reply #6 on: May 18, 2010, 06:09:29 AM »
Thanks!

That's what I needed to know