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Voltage and current monitoring — Parallax Forums

Voltage and current monitoring

Damon HenryDamon Henry Posts: 2
edited 2004-09-21 16:46 in BASIC Stamp
I am planning a programable battery charger with a motorized variac.· The variac is rated 0-180v and 20 amps, so I need to be able to monitor both the voltage and current at these levels and make decisions in the software to change the position of the variac to fit the battery profile.· Any good suggestions for parts and methods for collecting the voltage and current data?· Once I have that figured out I don't think the code part will be too hard.

thanks
damon

Comments

  • K de JongK de Jong Posts: 154
    edited 2004-09-04 10:31
    You will use a transformer after the Variac, I presume.

    It will be far more easy, and safe for you and your Stamp, to monitor voltage and current at a lower level. AND isolated from the mains !!!

    If you are on this lower level of say 0-20V and 0-1.5 Amp there are more ways to regulate voltage or current. In one of my applications I use a LM317T to do this job to charge NiMh batteries.

    Regards,

    Klaus
  • Damon HenryDamon Henry Posts: 2
    edited 2004-09-04 19:36
    Nope, no isolation transformer and no lower currents.· This is for charging different size strings of large capacity (30 - 100 ahrs) batteries.·If no one has a better idea, I will simply use a shunt for measuring the current and a voltage divider on the voltage.· By picking the right values I should be able to get the shunt values and the voltage divider values in the·same ballpark and in both cases end·up measuring millivolts.· I was hoping there was a more turn key solution however.· Also does anyone have an opinion on using an RC Time value as opposed to an ADC for measuring voltages?



    damon
  • K de JongK de Jong Posts: 154
    edited 2004-09-04 22:31
    Ok, that's clear.

    But consider to use a current coil to pick up the Amps signal and some optocoupler device for the voltage.

    Remember that a variac is directly connected to the mains. It is realy more safe to insulate your Stamp and the PC (and yourself) from the mains power supply.

    Success,

    Klaus
  • adaleadale Posts: 6
    edited 2004-09-07 12:44
    I agree that you need to insulate your measuring devices away from the mains.· I work with many high voltage (like 100 Vdc) at 50 Amps and they all use sometype of non contact measurement for current.· Most use simple current transformers, which are readily available.· Be careful with them also since they can still pack a helluva shock potental.· As for measure voltage, a ladder will work but there are many good ways to isolate the high voltage away from you and your project.
  • PaulPaul Posts: 263
    edited 2004-09-08 17:45
    Tracy Allens pages would be the first place to look. <www.emesystems.com/BS2rct.htm>
    Current transformers will work on the AC side of the charger..but not on the DC side. You could uses a precision 'shunt' resistor of say .1 ohm to determine the DC current by measuring the voltage drop acrossed it. I have been able to measure up to 90VDC using an RC type circuit. (the 90 volts never gets to the stamp, btw) The measurements were a bit erratic tho. I would suggest using a 40:1 voltage divider to step the DC voltage down to a reasonable 0-5v range then apply that to the (much smaller) capacitor and the stamp.

    Paul
  • DominicDominic Posts: 1
    edited 2004-09-21 16:46
    I also am trying to find a way of measuring voltage. I think the RCtime will be too insensative and any help would really be appreciated. My voltage is only 0-5 and I am trying to find the maximum point as a sensor is rolled along. I don't want to destract from Damon's question but since it is related I hope this is OK.

    Dominic
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