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Basic Electronics » How to measure floating (isolated) high DC voltage?

January 17, 2012
by jfbrink
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I want to use the ATMega168 to do some simple data gathering on an EV that I have built. I would like to run the chip off the car's 12VDC, but I am taking readings from the 156VDC pack. There is no shared ground.

Do you folks have any suggestions of good ways to take accurate VOLTAGE readings from this floating high-voltage pack, for input into the ADC on the chip?

The most tidy solution I've found on the Internet is a voltage transducer, LEM LV25-P. My only hesitation is that it is quite expensive ($63/piece), and I have not found anyone else using it in a similar application.

As always, thanks in advance for any ideas....

Jesse.

January 17, 2012
by Ralphxyz
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Would a voltage divider work? Maybe with two 1meg or 10meg resisters?

I do not "think" (so watch out) that a commmon ground would do any damage so make one up.

Ralph

January 17, 2012
by jfbrink
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Ralph,

For safety in an EV, you always, always keep your high-voltage pack isolated from the chassis and/or low-voltage (12vdc) harness.

And, since my ATMega board will be controlling stock vehicle equipment that has a chassis ground, it must be isolated from the high-voltage pack, too.

Best,

Jesse.

January 17, 2012
by Ralphxyz
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Well not if you wish to use the mcu to measure your high-voltage.

If you "want" to use the mcu then they have to be combined and that implies a common ground.

Of course I am speculating off the top of my head, hopefully someone who actually knows what they are talking about will jump in.

But again if you want to use the mcu ADC that seems to imply you are going to need a common ground.

What is the "safety" issue with having a common ground?

I have used different power supplies (not a high voltage DC power supply) with a common ground and did not have any problems.

I am very interested in Electric Vehicles (EV) so I'd really like to see an answer to your question.

Ralph

January 17, 2012
by jfbrink
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Hi, Ralph,

In fact, there are numerous methods available to measure voltage without creating a common circuit. I linked to the data sheet for one in my original post, a voltage transducer. There is a high-voltage "source" side of the transducer and a low-voltage "signal" side that is wholly electrically isolated. Other examples include inductance reading (which apparently only works with AC, not DC) and surface reading (which is usually used for static charge).

My goal in bringing the question to the group was to see if there were other, perhaps less esoteric and/or expensive solutions of which I was unaware.

The issue with isolating EV battery packs is that the "common ground" is the chassis of the car itself -- that is, the big metal cage you're sitting in. Too much risk of current flow to the wrong places and the shocks or fires that that flow would create.

Jesse.

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