EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: kiran_shrestha on June 12, 2014, 07:01:02 am
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Hi, I am using a 1 MEG resisitor and 10 K resistor voltage divider to measure the voltage range of 500 volt of DC . But currently I am measuring only 275 volt max of my 20 12v battery bank supply . I have used PIC16f1518 controller as adc and used the isolated 485 signal is given to datalogger after measurement . But the output is oscillation the vltage changes by about 10 volts and when comparing with actual voltage its way more than actual value. What can i do to solve it ??
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The maximum recommended source impedance is 10k. You are near this value. You could use lower resistor values or an opamp as a voltage follower. You could also try to lower your sample rate to get better results.
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Isn't 20 x 12V = 240V ? 20 x 13.75V = 275V that you are reading. Looks like it is reading reasonably accurately.
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Assuming lead-acid chemistry, the 13.75V would be the float voltage of a 12V battery under maintenance charge. OCV would be in the 12.7V range. Therefore I would infer the presence of charge current/voltage from maintenance circuitry. Is it possible that voltage variations you see are due to the operation of the charge circuit?
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The maximum recommended source impedance is 10k. You are near this value. You could use lower resistor values or an opamp as a voltage follower. You could also try to lower your sample rate to get better results.
I have used opamp as voltage follower.. still same problem . I have attached the schematic this time.
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Isn't 20 x 12V = 240V ? 20 x 13.75V = 275V that you are reading. Looks like it is reading reasonably accurately.
I have never reached 275 volts in my circuit. It reaaches 271 volts and also it fluctuates .
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Assuming lead-acid chemistry, the 13.75V would be the float voltage of a 12V battery under maintenance charge. OCV would be in the 12.7V range. Therefore I would infer the presence of charge current/voltage from maintenance circuitry. Is it possible that voltage variations you see are due to the operation of the charge circuit?
i have averages 20 samples taken at rate of 200ms each. But still output is not exact of the actual voltage.
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What can i do to solve it ??
Without knowing both the hardware / software, it is hard to tell what went wrong.
Generally, you will need to consider divider's input resistance vs. the source's output resistance, the divider's output resistance vs. the pic's input resistance...
I would do a couple things to start:
1) put a small capacitor from the 1M / 10K resistors to ground. This minimizes the effect of charge transfers during adc.
2) +/-10v out of 500v is a little bit extreme but you should really expect ~2 - 3 bits of fluctuations so a 10-bit adc provides usually 8-bit noise free results. Design your software around that.
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But still output is not exact of the actual voltage.
Can you explain how you are measuring the "actual voltage" of your battery bank?
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I have a chinese battery panel monitor with 20 channel which sums up all the individual voltages and gives it to datalogger. Also i have multimeter.
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This is the circuit I have used. Does the adc of PIC is not so good for the application .http://shortedwire.blogspot.com/2014/08/isolated-voltage-monitoring-system-1-5.html (http://shortedwire.blogspot.com/2014/08/isolated-voltage-monitoring-system-1-5.html)
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Not trying to be rude, but are you sure you are reading the right input? It is amazing the ghosting you can get from an adjacent channel. It will look very close to what you expect.
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I have used P1 for the input voltage with goes to the divider and op amp as buffer and to the PIC. Then the 485 signals are isolated using optocouplers for communication with data logger. I cant understand what ur trying to say about ghosting.
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Have you verified the accuracy of your voltage divider? If you have an error of 10V when measuring 275V that works out to be 3.6%, which is well within standard resistor tolerances.
Also, 275V into 1Meg is only 275uA, which isn't much. Some op amps can have bias currents in the microamp range, which could give a couple percent error too.
Finally, is this breadboarded? If so, the relatively long wires and poor connections can pick up a lot of noise and would easily explain your fluctuating measurements.
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I worry that your 1 megohm resistor is not rated for that high a voltage. Often several resistors in series are used for safe voltage derating.
The OPA237 is a pretty good operational amplifier for this sort of thing however if you operate it on 3.3 volts as shown in your schematic, it's output will be limited to about 2.55 volts typical which is suspiciously close to the 2.72 volts that your voltage divider would produce from 275 volts.
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Thanks guys . I didn't know about that 2.55 v thing and now i am building a new circuit and with some good dividers and noise resistance. I will be posting after i complete the circuit.