High voltage DC measurement  - Page 1
Forum section: electronics / projects

author: kiran_shrestha
author_rank: Regular Contributor
timestamp: June 12, 2014, 07:01:02 am
content: 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 ??

author: ruffy91
author_rank: Regular Contributor
timestamp: June 12, 2014, 11:51:01 am
content: 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.

author: robrenz
author_rank: Super Contributor
timestamp: June 12, 2014, 01:14:30 pm
content: Isn't 20 x 12V = 240V ? 20 x 13.75V = 275V that you are reading. Looks like it is reading reasonably accurately.

author: gardner
author_rank: Regular Contributor
timestamp: June 12, 2014, 01:50:47 pm
content: 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?

author: kiran_shrestha
author_rank: Regular Contributor
timestamp: June 12, 2014, 02:04:49 pm
content: Quote from: ruffy91 on June 12, 2014, 11:51:01 amI have used opamp as voltage follower.. still same problem . I have attached the schematic this time.

author: kiran_shrestha
author_rank: Regular Contributor
timestamp: June 12, 2014, 02:06:40 pm
content: Quote from: robrenz on June 12, 2014, 01:14:30 pmI have never reached 275 volts in my circuit. It reaaches 271 volts and also it fluctuates .

author: kiran_shrestha
author_rank: Regular Contributor
timestamp: June 12, 2014, 02:07:44 pm
content: Quote from: gardner on June 12, 2014, 01:50:47 pmi have averages 20 samples taken at rate of 200ms each. But still output is not exact of the actual voltage.

author: dannyf
author_rank: Super Contributor
timestamp: June 12, 2014, 02:29:18 pm
content: QuoteWithout 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.

author: gardner
author_rank: Regular Contributor
timestamp: June 12, 2014, 02:37:28 pm
content: Quote from: kiran_shrestha on June 12, 2014, 02:07:44 pmCan you explain how you are measuring the "actual voltage" of your battery bank?

author: kiran_shrestha
author_rank: Regular Contributor
timestamp: June 13, 2014, 09:48:52 am
content: 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.

author: kiran_shrestha
author_rank: Regular Contributor
timestamp: August 29, 2014, 03:35:16 pm
content: 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

author: Seekonk
author_rank: Super Contributor
timestamp: August 29, 2014, 04:00:28 pm
content: 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.

author: kiran_shrestha
author_rank: Regular Contributor
timestamp: August 29, 2014, 04:08:51 pm
content: 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.

author: Artraze
author_rank: Contributor
timestamp: August 29, 2014, 04:59:42 pm
content: 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.

author: David Hess
author_rank: Super Contributor
timestamp: August 29, 2014, 11:06:03 pm
content: 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.

author: kiran_shrestha
author_rank: Regular Contributor
timestamp: August 30, 2014, 09:18:39 am
content: 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.