Today's Message Index:
----------------------
1. 01:46 PM - Re: Re: DAS system (Steve Williams)
2. 05:00 PM - Re: Re: DAS system (Charlie England)
3. 06:16 PM - Re: Battery capacity test load (blues750)
Message 1
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Yesterday, discussing my project to create a data logger to measure
charge and discharge paths in my VW camper, I wrote that I'm planning to
use one of Adafruit's Feather microntrollers. Bob suggested the LTS
series hall effect current sensors. I wrote:
> (The Feathers have) six ADC pins, which should be plenty. The ADCs can measure
> 0-3.6mV.> > ...> > The LTS6NP requires 5V and outputs 0-5V, whereas
the Feathers can > measure only 0-3.6V. The LTS 6-NP outputs 3.6V at
2.64A. So I would > adjust the parallel wires to carry only about 2.5A
through the aperture, > if that makes sense.> > But here's the next
gotcha: I want to measure separate currents FROM the > charging sources
and FROM the battery, so I don't need any of the > negative current
range of the LTS devices, which output 2.5V at 0A. That > wastes most
the resolution! The Feathers use 12-bit ADCs. Measuring > only over
2.5-3.6V leaves only about 1,250 bits of resolution. At 60A > (from the
alternator) that's about 50mA per bit. Is that enough > resolution?> >
Ideas for using more of the range?
Of course, I already know the answer: Use a simple op amp circuit to
shift and scale the 2.5-5V output range of the LTS to the 0-3.6V input
range of the Feather ADC.
I think I've found the resources I need to learn how to design that
circuit. More on that below.
But adding an op amp to adapt the output of the very elegant LTS series
devices makes me feel like I'm not using the ideal hall effect device.
So let me ask: Is there a device that can be applied more directly? The
ideal device would:
- Work with a supply voltage of 3.3V, like the Feathers, and output
0-3.3V. (The Feather ADCs can use the 3.3V supply voltage as the
reference or an internal 0.6V reference and 1/6 divider to measure 3.6V.)
- Or require a 5V supply voltage, but also a configurable full scale
voltage.
- And configurable to produce 0V output at 0A current.
But assuming I use the LTS series, here's what I've found so far to help
me shift and scale its output:
I re-read Wikipedia on op amps:
https://en.wikipedia.org/wiki/Operational_amplifier
Then I asked DuckDuckGo to find me an "op amp designer":
https://ddg.gg/?q=op+amp+designer
That led me to TI's Design Resources page:
https://urlzr.mp/gqb (ti.com)
I selected "Amplifier circuits":
https://urlzr.mp/hqb (ti.com)
Under "Signal Conditioning," I selected "Non-inverting op amp with
non-inverting positive reference voltage circuit" to download the PDF:
http://www.ti.com/lit/sboa263 (PDF)
That's very informative, and it has a link to another PDF, "Designing
Gain and Offset in Thirty Seconds":
http://www.ti.com/lit/pdf/sloa097 (PDF)
I haven't yet applied that, but it looks like a great resource for a
DIYer like me! I'll try to create a proposed schematic and run it by
Bob for refinement.
In particular, I note in the DAS schematic Bob sent me that his signal
conditioning op amps have capacitors in various configurations around
the inputs. The TI tool produces just a basic circuit with resistors
for the offset and scale, so I'll be curious to find out what Bob's
capacitors add to the signal conditioning, and whether my use case needs
that.
I'm tempted to adapt TI's "30 Seconds" tool to Javascript, so
experimenters can use it through a web browser!
Message 2
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[snipped]
> In particular, I note in the DAS schematic Bob sent me that his signal
> conditioning op amps have capacitors in various configurations around
> the inputs. The TI tool produces just a basic circuit with resistors
> for the offset and scale, so I'll be curious to find out what Bob's
> capacitors add to the signal conditioning, and whether my use case
> needs that.
>
> I'm tempted to adapt TI's "30 Seconds" tool to Javascript, so
> experimenters can use it through a web browser!
I haven't dug out my old linear device books in a couple of decades, but
the old 'cookbooks' and application notes for opamps and similar devices
typically included a section describing 'best practices' for bypassing
values/locations. If you do a little careful reading, I'll bet it's in
there somewhere, unless the new-tech stuff includes internal filtering.
If that's the case, it should be documented, as well.
Charlie
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Message 3
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| Subject: | Re: Battery capacity test load |
Finally received my 4 ohm 100W resistors to make up a resistive load for doing
my battery capacity checks. I get 12.8A through the load and plenty of heating
of the heat sink (so I added a fan, and a couple of cooling fins) to the heat
sink which is a 4"x6" slab of 3/4" aluminum. Will be interesting to find out
what the EarthX battery will do for time before disconnecting.
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=496579#496579
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