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adc buffer with an opamp

Former Member

hi im in the process of designing myself a beefy bench psu as part of an arduino project.. i want to monitor the current drawn by the load and optionally take action to stop the smoke from escaping.... i was thinking of using a sense resistor an measuring the voltage drop to calculate the current flow through it, but depending on the draw( im a radio amateur, and some of my radios make the lights dim when i hit ptt)the voltage drop accross the sense resistor could go way out of  the allowable range of the arduino. to overcome that i need to use an opamp in a buffer configuration to clamp the voltage levels the adc will have to contend with.... does any one have any sample circuits  that i can usefor this type of configuration.

 

my psu specs are : 12-13.8v adustable by the arduino( LM317TLM317T or l2000) 30 A using 2n3055 bypass trannies.         7segment led displays+ 20x4 lcd

arduino will monitor  current draw and adjust vout in order to limit the current draw uC will also monitor heat on bypass trannies and adjust fan speed to compensate( using an 80mm  pc fan)

 

any comments?

  • Hello John,

     

    Need a bit more spec to help:

     

    Max current you want to measure

    Max current you might see

    Resolution needed

    Current measuring bandwidth

    High side or low side current sensing

     

    MK

  • in reply to michaelkellett

    hi, thanks for the reply here is what you needed

    max current - 30A resolution= whatever the arduino( uno) adc can supply

    i dont understand what you mean by band width. what i wanna do is set a max current drain  and then have the arduino cut the output using a mosfet of some shape when current goes above the threshold.   i know this can be done in hardware  with an opamp driving the  mosfet bu i wanna do it in my code soi that i can display the current drawn on the 7-segment  displays

     

    this will be highside sensing

  • in reply to Former Member

    Hello John,

     

    The bandwidth is a measure of how fast the circuit is - since you are going to process the signal with software it's all going to be quite slow so a sensor bandwidth of about 10kHz should be plenty. You may need to filter the signal more than that due to noise on the power supply.

    Just sticking  a resistor in series and measuring the drop is not  a good way to go at 30A. Even if your resistor is only 10mR you will dissipate 9W in it. The accuracy will be poor unless you buy/make a proper 4 terminal current shunt.

     

    A decent resistive current shunt will cost you £19.67 from Farnell (1339339) but it's only 1mR so the full scale voltage is rather low at 30mV.

    You can make your own but it's not that simple.

    You would also need a suitable amplifier - there are special chips for this - take a look at INA170 (Farnell 1212420).

     

     

     

    You could use a current sensor like 9135588 at  a more reasonable £9.54 - this is ideal since it can be powered from 5V and connected directly to the Aurduino adc input. You wont have any common mode problems and won't need amplifiers.

     

    So that's my suggestion - forget the series R and use a current sensor.

     

    MK

  • in reply to michaelkellett

    @John,

     

    if you are still working on this and you didn't like my current sensor suggestion you could take a look at Linear Technology's LTC2945 which is a complete power monitor on a chip with I2C interface.

     

    MK