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DC Voltage Measurement to RS232

Discussion in 'General Electronics Discussion' started by Steve_O, Aug 19, 2015.

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  1. Steve_O


    Aug 19, 2015
    Hi, Can anyone advise on a product that can measure a DC voltage between -30V DC and +30V DC and convert to an RS232 data string for output to be read on a computer? No visual representation of the Voltage is needed on the device it only needs to be read by computer.
  2. Minder


    Apr 24, 2015
    Do you want to convert RS232 to some other voltage level? 0-5v?
    A MAX232 IC does this.
  3. Steve_O


    Aug 19, 2015
    M, Thanks for this info. Its for a piece of test equipment and the sensor measures a voltage which will range between -30V & +30V DC which I want to read in Hyperterminal on the computer. I'm wanting to measure in increments of milli volts. Ideally am looking for a product that can do this alternatively if the MAX232 IC will do what I want then I can look at building a PCB.
  4. BobK


    Jan 5, 2010
  5. hevans1944

    hevans1944 Hop - AC8NS

    Jun 21, 2012
    @Steve_O One millivolt resolution in a 60 volt span means you need at least a 16-bit analog-to-digital converter. Adding ancient RS-232 communications protocol increases the cost somewhat compared to a USB solution. Try this web page for a 16-bit A/D with RS-232 communications protocol.

    If you choose the product cited on the web page, you will need to attenuate and then offset your ±30 VDC signal to accommodate the 0 to 2.5 V input range of their A/D. In other words, a 1:24 attenuation followed by a precision +1.25 V offset will do the job. Thus a -30 V input becomes -1.25 V, which then becomes 0 V after offsetting by +1.25 V. The +30 V input becomes +1.25 V, which then becomes 2.5 V after offsetting by +1.25 V. The A/D output then varies from 0x0000 to 0xFFFF as the input varies from -30 V to +30 V.

    You should build a small circuit board containing buffer operational amplifiers and an IC-based precision voltage source to perform the analog signal attenuation and offset signal conditioning described above. Be sure to mount the circuit board inside a shielded metal enclosure to avoid noise pick-up. If at all possible, use precision resistors to perform the attenuation and to scale the offset voltage. Avoid using trim potentiometers if possible because they will introduce temperature-induced drift as well as noise from their wiper contacts. If a trim pot is unavoidable, its value should be in the range of one to ten percent of the value of the resistance you are trimming. That is, a 100 ohm trimpot is appropriate for trimming a 10 kΩ resistor, but a 1 kΩ trimpot is pushing your luck.

    If you are comfortable rolling your own hardware and software, there are many A/D solutions that can be integrated with a microcontroller that implements an RS-232 interface. This would possibly be a "fun" project, but an off-the-shelf solution is probably a better idea.
    Last edited: Aug 20, 2015
    davenn likes this.
  6. hevans1944

    hevans1944 Hop - AC8NS

    Jun 21, 2012
    Besides wanting to measure -30 V DC to +30 V DC with a resolution of one millivolt, how fast do you want to make these measurements? A few thousand samples per second is probably feasible with a fast RS-232 baud rate, but limiting it to a few hundred samples per second (or less) will prevent a lot of headaches and the A/D will cost less. For example, a delta-sigma converter (MCP3424) from Microchip will do 16-bit conversions at about fifteen samples per second and costs about US$4.00. However, it requires a lot of support circuitry and software to make it do anything useful.

    So, how fast do you need to take samples, and what baud rate do you want the RS-232 link to run?
  7. HellasTechn


    Apr 14, 2013
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