Different voltages

Discussion in 'The Projects Forum' started by ChrisHelvey, Jul 8, 2013.

  1. ChrisHelvey

    Thread Starter Active Member

    May 22, 2004
    45
    1
    I am somewhat embarrassed to ask this question, but since I'm baffled, I'm going for it. I have built a circuit and programmed a PIC 16f877A to control the charging of a 48V LiFePO4 battery pack. (Please no comments on charging lithium batteries - I know what I'm doing there.) I have not powered it up yet because I don't understand something:
    I have 12VDC coming in to a 5V regulator to power the PIC. The PIC output has a PWM to a TC4420 MosFET driver, which has the same 12V powering it, which will drive the MosFET (logic level MosFET.) (The driver is just for efficiency, not terribly necessary.)

    The MosFET is a switch connecting the negative (-) of the battery to the negative (-) of a 60V power supply, completing the circuit (sinking current.)

    Here's my question: The MosFET driver (4420) is on the 12V ground and the 60VDC supply ground is not tied to the 12V ground. It seems to me, the driver will not see any ground through the MosFET gate and it will not work. Tying the grounds together seems like it will blow things up. I've never used two different source voltages in a circuit before like this. (Yeah, I'm a self-taught lifelong newbie.) Should I tie them together?

    The more I think about it, the more it seems tying together might not hurt anything - still 0V potential to the sources, right? Any clarification of this Electronics 101 would be appreciated.
     
  2. wayneh

    Expert

    Sep 9, 2010
    12,089
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    Tying grounds together may be fine - hard to tell from your description.

    But as a general rule, if you have any concern at all, start by using a 200Ω or so resistor to connect the grounds. Look for a puff of smoke, or missing that, check for an AC or DC voltage across it. If it's tiny, go to a lower Ω value and try again. Be sure to try this with loads on the different power sources. As long as currents are not flowing, you'll be OK to tie them together.
     
  3. GopherT

    AAC Fanatic!

    Nov 23, 2012
    5,984
    3,722
    48 volt pack means you have many cells in series. Do you have a cell balancing circuit?
     
  4. wayneh

    Expert

    Sep 9, 2010
    12,089
    3,027
    Another solution would be to get your 5V and 12V all from the single 60V supply.
     
  5. #12

    Expert

    Nov 30, 2010
    16,250
    6,746
    How you arrange the grounds is everything in this circuit. A schematic is necessary. Words just aren't good enough. For example, you never told whether you have an N-channel or a P-channel mosfet, or even gave a part number so we could look up a datasheet for you.

    From your words, I envision an n-channel mosfet with a battery on the ground side of it, and that won't work. Please tell, as my imagination isn't sufficient for this.
     
  6. ChrisHelvey

    Thread Starter Active Member

    May 22, 2004
    45
    1
    OK, thanks everyone. Yes, this is a many cell pack, and yes I have a battery management system. Onward:
    The schematic is in my head, sorry.

    Goes like this:
    (Supply 60V+)---(Bat+)|||(Bat-)----(IRL530PBF-ND)---(Supply Grnd) It's an N channel, 100V, 15A (but I only use 3A)...

    (Supply 12V+)---(TC4420 Driver)------->(Transistor Gate)----(Supply Grnd)

    The PIC pin operates the TC4420 driver and they are on the same 0V rail.

    And now another interesting thought - the 60V is from a 12V to 60V boost converter device. The same 12V source.

    (12V Supply)---(Boost to 60V)---(Bat+)|||(Bat-)----(IRL530PBF-ND)---(60V supply grnd)

    I don't think it's going to be an issue, but I will take the advice of putting a resistor between as the tie - that's a great idea.

    Thanks everyone. I'll let you know how it goes...if it goes.

    If there are any other comments, I welcome them for learning.
     
  7. ChrisHelvey

    Thread Starter Active Member

    May 22, 2004
    45
    1
    Just for the record, this is a simple 60V supply to charge the battery pack from a 12V source (solar panels/battery bank.) The PIC monitors voltage, current, and temp, then makes adjustments, reads out on an LCD, and shuts off when complete.
     
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