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ADC and general peer review of idea

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Litch

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Building a power monitor for my car.

The following is a "probe" for the power monitor (which will talk to this probe via I2C). I'll build a few of these, one to monitor the feed into the audio amps (0 - 600A) another to measure the batteries (-50 to 600A) and one more for the alternator feed (0A - 400A). From this I'll be able to graph and analyse the power-performance of my (significant) car audio install, the charge rate of the batteries and other interesting stuff like RPM to Alternator output performance. The below (as a start/template for the rest) will be for the amp power feed (0 - 600A)

PowerMonitor-Probe v1.png


Theory:
  • Tolerance: Assume all resistors are 1%, all caps are 10% and everything else is of standard/popular values.
  • [POWERIN]/[POWEROUT] is the zero-gauge wire to the audio amps running through a current transducer (LEM HTFS series).
    • [OUT] is 50% of Vcc at 0A, 100% Vcc at +600A and 0% Vcc at -600A
    • [REF] is 50% of Vcc (reference for 0A)
    • [OUT] is fed into ACD's CH2, then grounded via a 5K resistor. (V=IR, I=V/R, I=5/5K, I=1mA drain, P=VI, P=5mW drain)
  • [+12v IN] is a direct feed from the car's +12v (which will be more like 12.6V when engine off, and 14.8v when engine is running)
    • This feeds a low noise DC-DC switching reg (that, in turn, powers the ADC and the current transducer via [+5V1A])
    • Power draw is likely < 50mA for the circuit, it's overkill but meh - room for expansion.
    • The [+12v IN] also goes to ACD CH1 via a 33% voltage divider (33% of 15V = 5V = ADC's max input voltage) and smoothed a little more via a 100nF cap.
  • The ADC is capable of 200kS/s
  • Ignore the +5v feed into the I2C connector - superfluous, won't be used.
Questions:
  • With 12bit accuracy - do I need better smoothing of the analogue inputs?
    • Is that 100nF on CH1 sufficient or should I be looking at an RLF filter?
    • But at what point of smoothing do I lose the degree of accuracy I'm looking for calculating power draw of the amps - it would be nice to sample all this at 10kS/s and graph it out.
  • Do you see any red flags? Comments? Advice?
I appreciate the input as I don't have much room to play with all this (or breadboard the circuit with such small SSOP packages) so I usually just order a few pcb prototypes which isn't cheap (isn't that expensive either) but the point is, I'd like to get it peer reviewed before I go and build it.

Thanks in advance.
-KB
 
I think you'll need to add transient suppression components to cope with massive load-dump spikes on the supply. All the components should be automotive-rated. The automotive environment is really harsh.
I hope your insurer is happy with the car mods? UK insurers probably wouldn't be.
 
Questions:
  • With 12bit accuracy - do I need better smoothing of the analogue inputs?
    • Is that 100nF on CH1 sufficient or should I be looking at an RLF filter?
    • But at what point of smoothing do I lose the degree of accuracy I'm looking for calculating power draw of the amps - it would be nice to sample all this at 10kS/s and graph it out.
  • Do you see any red flags? Comments? Advice?
alec_t's "massive load-dump spikes" suppression and component ratings suggestions are reasonable.
The 220μF cap is reversed. Other than that, it looks OK to me.
From this I'll be able to graph and analyse the power-performance ...
Just out of curiosity, how do you intend to determine "performance" (I envision a "number of doors blown off" graphic display...:woot:)?.
 
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