Current capacity of 5V sensor circuits & grounding strategy

[Desmo Power]

Hello,
I'm working with a FSAE team who runs an M130 ECU. We're developing an in-cylinder pressure measurement system, and we'd like to capture both the crank position sensor (12-1 pattern, with AC waveform) and the cylinder transducer output (0.5-5V output, 9-18 VDC/85 mA excitation) on a single USB data acquisition box with 8 single-ended channels. Eventually we'd also like to input coil and injector pulses to the same box.

To avoid grounding problems I was planning to convert the AC crank waveform to a positive square wave, and then pass through an opto-isolator to the same ground as the pressure sensor. A similar strategy could also be used for the coil and injector signals.

Another alternative is to use a small DC step-up converter on either sensor A or B 5V sources/ground (matching the current crank input) to get to the 9-18 VDC to power the transducer. I could record at least those 2 voltages on the same ground then. Do those circuits have adequate current capacity for an extra 200 mA load (fairly conservative estimate)? How about the 6.3V +V supply? I don't see a dedicated ground for that one, could that work referenced to A or B 0V?

Third alternative is to purchase a different data acquisition device that supports differential measurements and sidestep some of the electronics trickery. The current hardware has nice sampling rates for the price however, and I'd like to explore nerdy options before throwing money at the problem.

Suggestions?

Thanks in advance,
Mark

[jcolley]

Also curious to the answer to this. I can't find anything in the documentation for which sensor ground to use for the 6V3 output.

[David Ferguson]

Have you considered just mounting an additional crank position sensor for your cylinder data acquisition system?

Since your pressure sensor has flexible voltage input, why not just connect it to your 12V Battery/Alternator supply?

[Desmo Power]

jcolley: I never found any info on that, but didn't look around much after posting this. Currently we're using a cheap Amazon DC-DC step up board to boost voltage from a 5V supply up to the window for the pressure sensor. We found one model that ties the grounds of both sides together, and another which isolated grounds, so we have options for the future. 5V supply seemed to handle it just fine, believe 5V supply dropped by a couple hundredths when sensor was plugged in.

David: That would be an option, one that I'm trying to avoid however. The commercial ICPT system I used in school required an add on crank signal wheel, which made setting it up a long process. It also discourages use in smaller applications, like FSAE cars or motorcycles. So far the stepped up voltage has worked well, so until I try a data acq box with true differential inputs this might be the best strategy.

[adrian]

All of the 5V rails and the 6.3V rail reference the same sensor ground internally. It doesn't matter which 0V pin you use. The only consideration when wiring them up is spreading the current load across them.

[Desmo Power]

All of the 5V rails and the 6.3V rail reference the same sensor ground internally. It doesn't matter which 0V pin you use. The only consideration when wiring them up is spreading the current load across them.

That's good info adrian, thanks. Got any idea on current capacity for each, mainly for someone who finds this thread through a search?

[adrian]

The 5V/6.3V rails are rated at 200mA continuous with a peak of 400mA. The output voltage isn't guaranteed at anything above 200mA.

[Desmo Power]

The 5V/6.3V rails are rated at 200mA continuous with a peak of 400mA. The output voltage isn't guaranteed at anything above 200mA.

Aha. I'd better check the actual power requirements of my pressure sensor, and also the efficiency of the step up board. Probably prudent to put it on the 6.3V source also, which I believe is currently unused.

[SprinterTRD]

Although it does not matter where you put the 0V, For visibility sensors with a 5V-A supply going to it should also be wired to a 0V-A pin, 5V-B sensors connect to 0V-B and so on.