Monitoring voltage and current ?

[iugmoh]

Hi all,
I need your help how to monitor voltage about 150VDC and current up to 3 Amp using PIC , with protection circuits to protect pic from damage when any problem occurs such that high voltage or short circuits?

[languer]

For the voltage, a simple voltage divider should work. Scale the voltage down to 5volts or less, depending on your supply to the PIC, and use an ADC input (PIC should have built-in ADC for this to work) to monitor the voltage. Since the resistors should be somewhat large, you may need to use a unity-gain buffer amplifier between the voltage divider output and the PIC-ADC input so that the ADC is not upset by the voltage divider's resistors.

For the current, I would use something similar to the LT1787 (high-side current sense amplifier) and feed (also) into a differen ADC input for the PIC. There are several current-sense amplifiers from the likes of LTC, MAXIM, TI, etc. Or you could even use a high-precision OPAMP to do this (this may be a bit more tricky though). The key here is the sense resistor (it must be low enough so that there is very little drop, yet there is enough of a drop that can be sensed - wattage is also a consideration).

[iugmoh]

Thanks languer for your help, but for the unity gain amplifier is't posible to built it using voltage follower of OPAMP 741 ?

[mister_e]

I would consider a better op-amp for this purpose. LM741 is noisy enough, and probably not good enough for really low voltage range. CMRR spec must be good, LM741 CMRR specs are poor. Check Microchip AN894, it has some nice theory in. Not sure if I agree them<all but, good enough to give you some ideas.

http://ww1.microchip.com/downloads/e...tes/00894a.pdf

[Byte_Butcher]

MisterE is right, a 741 is a poor choice for low voltage instrumentation work.

I've become rather fond of the TS921.
It's a nice low noise, low voltage (down to 2.7v) op-amp with rail to rail input and output...

http://www.st.com/stonline/products/...re/ds/5560.pdf

[iugmoh]

Thanks for all for your help and powerfull information.
I built this circuit for current and voltage sensing and I want to take your notes. then after that I can build the operational amplifier stage.