Electronics > Beginners
How to make an analog circuit to proportionally scale a sensor output voltage?
Dundarave:
--- Quote from: HwAoRrDk on July 04, 2019, 04:43:57 pm ---
--- Quote from: Dundarave on July 04, 2019, 05:34:53 am ---...which brings to mind the voltage drop of a diode... Perhaps by arranging the feedback gain resistance in the op-amp circuit so that below 0.7 v, a fixed resistance (calibrated for the initial slope) is used, with a parallel second resister + diode (which would thus conduct at 0.7 v) to handle the slope (i.e. gain) difference for the rest of the curve.
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I'm having trouble visualising what you're talking about here. Could you possibly illustrate it?
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The change in the graph data rendered the diode idea irrelevant, but it looks like @mikerj's solution has you well sorted! :-+
HwAoRrDk:
--- Quote from: mikerj on July 04, 2019, 09:36:15 pm ---Supply decoupling/filtering, overvoltage protection and input and output protection would really ideally be needed to make this reliable for automotive use.
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With regard to protection, if input signal over-voltage (e.g. in case of short to +12V) is not a problem and can be handled due to the large-value series resistance on the inputs to the op-amp, what about if the 5V reference going out to the sensor is shorted to either ground or +12V? Obviously nothing good will happen if the op-amp is suddenly hit with 12V on VCC! Especially if I am going to use something like an MCP6001, which is only rated for absolute max of 6V supply.
I had a thought: what about using a protective smart high-side switch IC on the outgoing 5V reference line? Maybe the kind commonly used on USB ports, as they're geared for 5V operation.
After a bit of browsing around, I came across the ON Semi NCP380. It has short-circuit, over-current, UVLO and reverse-voltage protection. It also allows to set a custom current limit using a resistor. So I thought maybe use one of these with the EN input tied to VIN (although not directly, as the datasheet warns against that - says to use an RC delay instead) and set a really low current limit with a resistor in the region of 100k-200k. As I understand it, GM MAP sensors consume about 20mA at most, so a current limit in the region of 50mA is probably okay.
How does this sound? Or are there better options?
mikerj:
That quite a nice part, but I don't think it provides any over-voltage protection and it's voltage operating range is lower than the MCP6001. I'd think I'd choose an op-amp with a wider supply range and then use a TVS and maybe a polyfuse for protection. You'll want to add some resistance on the op-amp output to limit current if that is connected to something bad, it can go before the feedback resistor so any voltage drop is compensated.
max_torque:
The question i'd ask is "how much money are you saving"?
Making long term automotive electronics robust is actually quite hard (i should know, i do it for a living...) and yes, you could probably knock together some circuit to modify the MAP signal to the ecu, but if that messing around takes hours of your time, and leads to a car with poor reliability, then you'll wish you just bought the "expensive" MAP sensor......
Zero999:
--- Quote from: mikerj on July 05, 2019, 08:57:15 am ---That quite a nice part, but I don't think it provides any over-voltage protection and it's voltage operating range is lower than the MCP6001. I'd think I'd choose an op-amp with a wider supply range and then use a TVS and maybe a polyfuse for protection. You'll want to add some resistance on the op-amp output to limit current if that is connected to something bad, it can go before the feedback resistor so any voltage drop is compensated.
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If 12V is available anyway, then why not use the cheap and cheerful LM358?
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