Monitoring voltage and current ?

[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.

[iugmoh]

Thanks alot for all for your helping , but some notes about :
1- the capacitor is't electryletic or ceramic and what is its capacitance and rated voltage.
2- I know that the ratio of R2/R3 = 30/1 bur how I can choose the optimal value of R2 or R3.

[Nicmus]

Hi iugmoh,

A quick Google search on “current sensors” will give you a lot of choices on selecting a suitable current sensor for your application. This link shows one not so expensive choice, is flexible and you can also get the schematic which will give you an idea on how to build your own if you choose this avenue.
http://<br /> http://www.sparkfun.c..._id=8883<br />
On the voltage side I suggest to design your R2/R3 divider aiming about 1mA current. This will keep your power dissipated on R2/R3 to a minimum. For R3 using 1% series resistors you can get a 4.99K (for simplicity let’s call it 5K) value which in turn will give you a full 5V under 1mA. Your total resistance R2+R3 should be 150K. So now all you have to do is find a 145K resistor which can be “made” from a 140K+3.01K or any other combination of the standard values from 1% resistors list.
The values above should get you started with pretty good results. Depending on your PIC and its ADC input minor adjustments might be necessary due to the impedance added parallel to R3.
You didn’t mention what kind of precision you want on your readings but with a 8 bit ADC you will read (5000/256)*30=600mV steps on your main voltage and it goes down to (5000/1024)*30=150mV when using 10 bit ADC.

For the filter capacitor I suggest to start with a 0.47µF/25V ceramic cap X7R grade Digi-Key part # 445-1353-1-ND. If that is not enough you can increase the value.

Any Zenner diode (5.1V) will do and should work fine with your maximum 1mA current.

HTH

Nick

[iugmoh]

thanks to all and to Nicmus for his powerfull demonstration this subject , I'm now understand it 100% and working to apply it.

[iugmoh]

after I connect the circuit, I notice that I always measure the source voltage not the motor voltage because I connect the wire from 150VDC to voltage divider then to ground, so I think I need another voltage divider after motor windings and make a difference OPAMP to mease the voltage on motor windings ( before and after motor windings) , any one have comments ?