Monitoring voltage and current ?

[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.

[mister_e]

High likely your signals will be pulses? If you want to make it simple and use ADC, you'll need to filter them to get only a DC voltage, not pulses. Simple capacitor usually works pretty well.

[Nicmus]

Hi iugmoh,

On your voltage sensing circuit you might want to change the lower side of R3 to ground to prevent your ADC input from getting full 150V when Q1 is OFF.
Also your R2/R3 ratio should be at list 30/1 in order to limit the voltage to ADC to maximum 5V. This might bring your output impedance low enough so you can use an ADC input directly. A filter capacitor and a Zenner diode parallel to R3 will help with the possible noise problem and make it safer.

HTH

Nick

[languer]

I like your idea. It is simple and to the point.

As Nicmus mentioned, R3 should go to ground, and a zener for limiting/clamping, and capacitor for filter is recommended.

Same comments from mister_e and Nicmus apply to current measurement side. An RC into an OPAMP is recommended since you will be seeing pulses (OPAMP gain of around 8 should get you there). You may also want another limiting/clamping zener so that the pulses do not exceed the OPAMP's common-mode range. As it is, R1 will see up to 1.8W on average. The key points here, "up-to" and "average". So you may want maybe 4 parallel 0.8ohms (0.5W resistors), or perhaps 5 parallel 1ohms.