# Low Voltage DC Power Supplies - Why?

Hi guys, first post here. I am an Electrical Engineering student (2nd year) and I have a part time job doing qualification and FMEA type work on power supplies. It's been a great learning experience and it's really helped me in my classes.

Right now, I am doing a qual on a low voltage dc (LVDC) power supply. The input for the power supply ranges from -36Vdc to -72Vdc and the output is two 12V where load current can range from 0 to 36A.

My question is, why are low voltage DC supplies used? Where is LVDC provided (i.e., what types of buildings provide LVDC)?

My understanding of other supplies is that they convert a 120Vac (or 220-240, etc.) to a smaller DC voltage. I just don't understand where a LVDC supply is useful.

I have asked at work, but there is so much bureaucratic catch-22 (very large company) that I can't seem to get a clear answer.

I've googled quite a bit but can't seem to even find anything remotely relevant, so any understanding you can provide will be greatly appreciated.

Regards,
Mike

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berkeman
Mentor
Hi guys, first post here. I am an Electrical Engineering student (2nd year) and I have a part time job doing qualification and FMEA type work on power supplies. It's been a great learning experience and it's really helped me in my classes.

Right now, I am doing a qual on a low voltage dc (LVDC) power supply. The input for the power supply ranges from -36Vdc to -72Vdc and the output is two 12V where load current can range from 0 to 36A.

My question is, why are low voltage DC supplies used? Where is LVDC provided (i.e., what types of buildings provide LVDC)?

My understanding of other supplies is that they convert a 120Vac (or 220-240, etc.) to a smaller DC voltage. I just don't understand where a LVDC supply is useful.

I have asked at work, but there is so much bureaucratic catch-22 (very large company) that I can't seem to get a clear answer.

I've googled quite a bit but can't seem to even find anything remotely relevant, so any understanding you can provide will be greatly appreciated.

Regards,
Mike
Welcome to the PF!

What's FMEA? To try to start answering your question, I can think of two applications for LVDC input power supplies. The first is in the telcom industry, where -48Vdc power supplies are ubiquitous. The second is in battery-based applications.

Airplanes?

a use for LVDC that i am familiar with is controlling servo systems of robotics and automation. where you use a dc signal to send a specific wave-form to a servo to get a desired type of motion, mainly in speed and direction.

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Zryn
Gold Member
I cant recall what it stands for but a FMEA is a design risk assessment where different people get together to assess the schematics for a device (e.g. 1MVA portable substation in an underground mine) in the hope of picking up any potential safety issues, design faults, problems with a lack of redundancy etc, before the device is actually built, and to make sure it complies with all the appropriate standards for the location it will be used in.

Most of the time LVDC supply is used in regards to either Intrinsically Safe (IS; not enough energy to create a spark in a hazardous environment) items, or for general safety.

For example, there is a big push at the moment to develop more stringent standard for pushbuttons in underground mines in Australia (including higher IP ratings; dust and water seals), as everything gets washed down, and when the seals on pushbuttons deteriorate they allow water to bridge the front button with the 240V supply, zapping people who try to use them. The push is to make the supplies 24V instead, and to have the FMEA recognize this problem and correct it before construction ;)