Monitoring voltage and current ?

[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 ?

[Nicmus]

Hi iugmoh,

You are right that you are reading the input voltage not the voltage on the motor (maybe I missed this requirement) but here is the possible solution:
Your IRF840 has a 0.85 Ohms ON resistence and your current sensor 0.2 Ohms (total 1.05 Ohms). Under maximum current allowed, 3 Amps, your maximum voltage drop will be 3.15V so the difference from the input voltage will be on your motor. You can compensate for that in your PIC program as one fixed voltage drop (read it under normal running conditions if such thing exists) or a variable one by using your current reading and apply it to the known fixed resistance involved (1.05 Ohms) if you want to get fancy.
This will change with the load, input voltage or how worn the motor is.

I’m pretty sure that we can better help you make decisions and select the best approach if you define your requirements in more detail.
Here are just a few questions for you:

• What will be an over voltage or over current situation? Define it.
• How are you going to address the inrush currents at start up?
• What is your target resolution and accuracy on voltage and current readings?
• Do you want to continuously have and monitor/display these readings or just be able to set trigger points at max/min conditions?
• Is your motor running under constant or variable load?
• What is your power source and its specs?
• Are there other requirements that you have in mind and didn’t mention yet?
• Is your project cost sensitive?
• Is this a one time deal or it will become a production item?
• Is this a hobby style product or used in a sensitive application (medical field or so)?

Please keep in mind that the answers you get from the forum are as good as your supplied information on what you trying to accomplish.

Regards,

Nick

[timmers]

Hi,

All the comments are valid, but in my experience the voltage accross the motor, even with smoothing caps in the potential divider, will allways be near HT. The only way to measure the voltage ACCROSS the motor is to have two dividers, one on each terminal of the motor then motor voltage is the difference. But bear in mind the voltage accross a PWM driven motor is pretty much the supply voltage plus the back emf effects.

I suspect you are wanting to monitor the motor drive, and to take meaningful readings I would dispense with the voltage reading and use the current monitoring. To isolate the circuit from the PIC, use an opto coupler.

Tim.