Isolating a Voltage Divider Leg?

Discussion in 'General Electronics Chat' started by ScooterDude, Aug 11, 2006.

  1. ScooterDude

    Thread Starter New Member

    Aug 11, 2006
    2
    0
    Hi there,

    I plan to use a resistor based voltage divider to drive a AD converter input circuit. The input can take 3.5VAC max.

    The divider works just fine as it doesn't fluctuate after many hours hooked up to a multimeter. The problem arose when I tried to drive the input circuit. Since it's based on 5vdc, one of the coupling capacitors on the 7805 did not like seeing the neutral coming from the original AC load at all and showed its displeasure by self destructing.

    Does anyone know of a way to isolate the neutral from the divider?

    Unfortunately, I can't provide any info about the input circuit since the demo boards we have are all potted.

    Thanks for any help you can provide. Let me know if I'm omitting any details.
     
  2. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    Forgive me if this is a really stupid question, but what does "neutral" mean in this context?
     
  3. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Hi,

    It sounds like you might be using the divider to drop line voltage down to a useable size. Doing this without isolation in the form of a transformer is always likely to blow up your input circuitry, and presents a lethal shock hazard to yourself.

    Get a step down transformer to drop the voltage and revove the shock hazard. Make sure the voltage you are monitoring floats - that it is only referenced to your A to D input ground.
     
  4. ScooterDude

    Thread Starter New Member

    Aug 11, 2006
    2
    0
    Beenthere,

    That's basically what I'm trying to do, but I was hoping that there may be some workaround out there in the ether. (I should've mentioned that we were trying to avoid transformers in the original post, but I was exhausted by that point, probably would've clarified things quite a bit.) We would like to be able to use a resistor divider network to drop the voltage since it's about a million times cheaper to use .1% tolerance resistors than having custom transformers wound for us since it's a relatively small project.

    So, it should be easy to hook an isolation transformer between the output of the divider network and the input of the converter, right?

    Can you get relatively cheap, low loss isolation transformers? I've never looked into them before now, but I'm about to jump in feet first and dive through Mouser, et. al. to see what I can find.

    Thanks very much!
     
  5. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Hi,

    I've never thought of isolation transformers as low loss items. The usual practise is to grab a step down transformer and use it. That way you get your isolation and a safe voltage to play with. Put a pot across the output and adjust until the voltage off the wiper is a precise proportion of the line and digitize away.
     
  6. mrmeval

    Distinguished Member

    Jun 30, 2006
    833
    2
    Another way to do that is to use an optoisolator. By varying the input to the LED you vary the conductivity of the output phototransistor and you can use that representation to drive the A/D circuit.

    Here is just such a circuit
    http://www.edn.com/article/CA189472.html
     
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