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| moving coil meter, biasing to bipolar |
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| timeout:
Hi All, I have a school demonstration project in which a slowly varying bipolar sine AC voltage (~0.1 Hz) is putting current through a coil. in order to demonstrate that a magnetic field is generated, two compasses are in close proximity and show the alternation of the polarity by constantly reversing. In order so illustratively show that the current flow is actually reversed I would like to use a moving coil meter with a custom "scale" showing N and S in red and green. For this the resting position of the needle has to made at half scale - i hoped for the screw adjust to be able to, but after checking three different meters, i came to the conclusion that this really only sets +-10% FS or so - which is reasonable i think. True bipolar meters (as used on some Kepco BOP models) are quite hard to find around.... The meter i would like to use is 60 mV to full scale. What would be a good strategy of offsetting the meter electronically ? using my function gen (sine, 100mHz using 30mV offset) to test it did exactly what i want in the application... Did anybody come across this need ? is it weird ? Would you use an AC coupled (10uF or so) op amp and bias it accordingly to half scale ? is this over-engineered ? :o Thanks in advance! |
| iMo:
For example you may bias the coil meter (0..60mV) with two resistors and 1.5V AA battery. With the generator running from -30mV to +30mV your coil meter will run from 0mV..60mV. The zero output of the generator will be in the middle of the coil meter (at 30mV). See the below schematics. You may use 22ohm and an 1k5 potentiometer (10turn) to adjust the middle perfectly. |
| BNElecEng:
Do you know much current is flowing at the peak of the sinewave? I found some cheap moving coil ammeters online which are centred at zero, so they will show the change in direction quite nicely. Here is an example, search for the term"DC 0-250uA Rectangle Analog Panel Ammeter Gauge Amperemeter Class 2.5" |
| capt bullshot:
These meters measure current, not voltage, though they're often specified for voltages - then just use Ohm's law. Take this as a starting point: put a current source that delivers half scale current in parallel with the meter. This current and your measured current adds up within the meter and brings you the desired result. No need for fancy buffering and AC coupling. How to implement this in practice, depends on your circuit. |
| Paul Price:
IMO, with your circuit, when there is -30mV Vin the meter will show 60mV, pegging FS, and with +30 mV Vin it will show zero, and only with 0-V Vin it will show 30 mV. or half full-scale. if you use a meter of 60mV FS. That's misrepresenting the generator voltage output polarity! It also feeds current into the generator. Also the meter open circuit will show 0 and only closed-circuit (connected) will it show midscale as zero. Ideally, what you want is 0-V Vin showing 30mV or half full-scale(60mV), and +30 mv Vin to show 60mV (full scale) and -30mV to show 0V on the meter, and showing 0-V, and do this whether or not the meter input connections to the generator are open or connected to the generator. The problem with your circuit is that you get a 30mV reading (1/2 FS) only if the input connections to your meter are shorted. However, it is easy enough to do this with a dual op-amp like a LM358 and a 9V battery and a slight modification of your circuit. It would need a high-resistance pull-down resistor from the + input of one of the op-amps to keep the very sensitive input from floating all over the place and allow a zero Vin reading correct. Hint: one of the op-amps forms a +/- 4.5V supply with a virtual zero-voltage ground as one side of the meter input. |
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