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Registered Member #1525
Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 294
I've been banging my head on my workbench for several hours on this one.
I'm trying to charge the 200uF capacitor with a voltage doubler. The AC source is an inverter, car-plug type. The annoying thing about doing it this way is the output of the inverter isn't referenced to ground, so the capacitor negative terminal isn't at zero volts (the negative reads about -30 to -180V with respect to ground, weird).
The solid state relay turns the charging action on and off. It's supposed to be controlled by the comparator on the far right.
Here's my idea: Use an op-amp to subtract the capacitor - terminal voltage from the + terminal voltage, thus getting the actual voltage across the capacitor. First the 1M/22k voltage dividers divide down the voltages to IC friendly levels. Then the two voltages are subtracted by the op amp (with input and output buffers so the dividers don't screw up the subtracting action). Then the output of the subtractor (proportional to the voltage across the cap) is compared to a 2.5V reference by the comparator. The output voltage divider allows the user to adjust the cutoff voltage.
All of this works perfectly when I test it with voltages from my power supply. However, when I hook it up to the capacitor, the V1 voltage divider doesn't seem to work. When the - terminal is at -60 volts, the red node should be at -60*22k/1022k = -1.29V. But no matter what, it reads -0.58 volts even as V1 fluctuates.
What am I doing wrong? Is there a better way to do this?? Please help.
Registered Member #1875
Joined: Sun Dec 21 2008, 06:36PM
Location:
Posts: 635
Op amps have an input range that is governed by your V+ and V-. If your inputs are above V+ or below V- then the device cannot be expected to work reliably, as far as I understand. Also, depending on which 741 you are using, the input resistance can be from .3M to 6M, which will significantly influence your divider.
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
... and what's the voltage of the capacitor terminals with respect to the power supply ground?
let's put it another way, you have an IC powered by +12v, and you're trying to measure a voltage below the GND terminal.
A common way to solve this problem is to pot-down the two capacitor terminals to a voltage that's mid-rail for your opamp. String two equal resistors from +ve to GND, and ground both divider strings to this point. The impedance of this point to ground is not particularly important, as the voltage you're measuring is a differential one, just make sure it can control the measured voltages to always stay inside the amplifier common mode range.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
I recommend a string of zeners in series with the LED of an optoisolator. When the voltage exceeds the Zener voltage, the optoisolator will turn on.
If the charging voltage needs to be variable, replace the zeners with a variable high voltage reference. I've made these with a TL431 and a high voltage transistor such as the MPSA42, in a cascode arrangement.
The inverter may be a full bridge, where both hot and neutral have AC on them. A negative supply to your op-amps might help.
Registered Member #1525
Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 294
ScotchTapeLord wrote ...
Op amps have an input range that is governed by your V+ and V-. If your inputs are above V+ or below V- then the device cannot be expected to work reliably, as far as I understand.
I think that is exactly the problem. Thanks for bringing this to light.
ScotchTapeLord wrote ...
Also, depending on which 741 you are using, the input resistance can be from .3M to 6M, which will significantly influence your divider.
The dividers actually function accurately, to within about a 10th of a volt of what's calculated, so that's not a problem.
Dr. Slack wrote ...
... and what's the voltage of the capacitor terminals with respect to the power supply ground?
With respect to power supply GND, the negative terminal voltage is: When power supply and inverter are off: Just a few negative mV When power supply and inverter are on: -200V while charging, -37V while not charging When just power supply is on (no charging): -4.5V
Dr. Slack wrote ...
let's put it another way, you have an IC powered by +12v, and you're trying to measure a voltage below the GND terminal.
A common way to solve this problem is to pot-down the two capacitor terminals to a voltage that's mid-rail for your opamp. String two equal resistors from +ve to GND, and ground both divider strings to this point. The impedance of this point to ground is not particularly important, as the voltage you're measuring is a differential one, just make sure it can control the measured voltages to always stay inside the amplifier common mode range.
I don't fully understand how this works... The positive terminal already has resistors going to ground, via the V2 voltage divider. Is this what you mean (see attached pic)? Please let me know if that's what you meant, as I trust your advice and would like to try this.
Steve McConner wrote ...
I recommend a string of zeners in series with the LED of an optoisolator. When the voltage exceeds the Zener voltage, the optoisolator will turn on.
If the charging voltage needs to be variable, replace the zeners with a variable high voltage reference. I've made these with a TL431 and a high voltage transistor such as the MPSA42, in a cascode arrangement.
So I understand that the capacitor + terminal voltage causes the zener breakdown which activates the LED, but I don't understand how the TL431 and transistor can make it adjustable. Is there a circuit diagram? I'm very interested in trying this as well
Steve McConner wrote ...
The inverter may be a full bridge, where both hot and neutral have AC on them. A negative supply to your op-amps might help.
Yes, that's exactly how it works, and where all the problems come from.
Thank you so much to everyone for your replies- I could not complete my projects without the help of people on this forum.
Registered Member #816
Joined: Sun Jun 03 2007, 07:29PM
Location:
Posts: 156
Alternatively you can use some non-isolated voltage measurement, provided you take some precautions 1/ Treat the whole circuit as live and don’t touch any part of it, and not connect any grounded test equipment. 2/ use an isolated psu that can float the output voltage, 3/ not connect it to anything else without the use on an isolator of some sort, finally use a plastic shaft on the variable resistor.
You can then use V1 as the negative reference, without worrying about the common mode voltages, and do away with the differential measurement. You may want to use an op-amp designed for single rail supply’s for the set point to work down to low voltages.
Here’s what I was thinking about, there’s no grantee that it will work as I haven’t built it, and was just a quick (and badly drawn sketch) to give you a general idea. I guess that’s what you were aiming for in the first place, sorry if I have missed anything.
Edit, could'nt seem to paste the image, but what i was trying to describe is power your op-amps from an isolated source and use V1 as the common negative for voltage measurment.
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Measuring a isolated voltage??
