Hi there - I have an application with two separate -48V (nominal) power supplies, each of which includes battery backup, so the supply voltage can vary from almost -60V (when the batteries are being float-charged) to quite a lot lower (when the battery voltage is insufficient to power the load, and the load shuts down).
I'd like to build an Arduino-based monitor for this application, and have it (a) powered by the dual power supplies, and (b) monitoring the voltage from each supply.
For powering the Arduino (and Ethernet shield, I need to be able to monitor this remotely) I'm planning to connect both supplies to isolated DC-DC converters, which will give a stable +12V output from each of the -48V inputs, and then I'll combine the +12V outputs through Schottky barrier diodes. This will give a continuous supply to the Arduino as long as either of the power supplies is functioning.
I'm a bit stumped as to how to go about measuring the supply voltages. If it was a positive supply, I'd use a voltage divider to reduce it by a factor of 12, bringing the 0-60V range down to 0-5V. As the supplies are negative, this won't work. I thought about using an op-amp to add +5V to the output of the voltage divider, translating a range of (-5)-0V up to 0-5V, but I have to confess that it's getting beyond my fairly basic grasp of electronics.
Anyone have any thoughts about how they'd go about this one?
The circuit configuration, however, can be tweaked to measure any negative voltage range by changing out some of the resistors. If it looks promising for your application I can try to work out the resistor numbers.
Are you interested in measuring the voltage all the way down to 0? For a 0V -- 60V range it might make sense to have some voltage shifting first to get the voltage to the range you actually expect (-60V to -24V maybe?) and get some more precision out of it.
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The Rugged Motor Driver: two H-bridges, more power than an L298, fully protected
That circuit does look promising, RC. The idea of a smaller range for higher precision is an attractive one also.
I would ideally be looking for an op-amp in a DIP format (SMT packages really won't work for this project), and I guess (since I need two) the best option is a dual op-amp on a single DIP IC. I'd be much obliged if you could recommend a suitable component for the job - or at least the parameters I should be looking for in a suitable component - as well as the resistor values for -24 to -60V.
OK....try R1=14.7k, R2=180k, R3=191k, R4=3.6k. That will map the -60V to 0V input range to 5V and 0.1V, but it won't handle the compressed range of -60V to -24V. That's a bit more complicated and am not sure it's doable using only 5V single-supply op-amps.
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The Gadget Shield: accelerometer, RGB LED, IR transmit/receive, speaker, microphone, light sensor, potentiometer, pushbuttons
I was looking at a National Semiconductor LMC6082AIN dual op-amp; does that look suitable? I'm not sure how the choice of op-amp influences the circuit design.
Another question for y'all. I've expanded the circuit to monitor current as well as voltage, using these current transducers.
The data sheet contains this application note:
I'm guessing that this is more relevant to AC loads than the DC I'll be measuring, but for the sake of a resistor and capacitor, should I add the filter anyway? What effect would it have on the output of the transducer?