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### Topic: help with DC offset for mass air flow sensor (Read 2480 times)previous topic - next topic

#### mitch_79

##### Mar 23, 2011, 11:19 pm
I'm hoping some of the EE's or educated hobbyists here can help me with this challenge.

I have a Mass Air Flow sensor for which I want to increase the output signal for the lower part of the output range.  Here is a link to the closest thing to a datasheet that's available (pages 1-3)...
http://www.autoshop101.com/forms/h34.pdf

My particular sensor has a DC offset of ~0.7V at STP, and a max. output of ~4.8V.  The lookin resistance (measured with a voltmeter) across VG and E2G = 28k.

What I want to do is add about +200mV DC offset to the lower range of the output where VG < ~2V.
Specifically, where VG = 1.5V currently, I want the modified output VG' = 1.7V.

I'd be happy with either of the following solutions that have VG' = 1.7 when VG = 1.5...
- an offset of 200mV across the full range [VG' = VG + 0.2]
- offset > 200mV at VG = 0 , and nearly-zero offset for VG=4.8V [i.e. VG' = VG + (4.8-VG)*0.06]

Whatever the solution I strongly prefer passive components.
I've played around with LTspice and can almost get what I want by simulating the sensor output as a voltage source, and connecting +2V to the +ve side of the source through a diode.  But not quite, and I don't *really* know what I'm doing.  (I tried a reverse-biased Zener diode too, which seemed even closer, but was sensitive to series resistance of the source and again, found I didn't know what I was doing.)

I've looked at op-amps but I'd prefer not to build a voltage summer circuit, mostly because I think it's possible to achieve what I want through careful selection of some sort of diode and a few resistors.

Help, either in the form of search keywords or comments and discussion is appreciated.
Thanks to anybody interested enough to offer some friendly help

-Mitch

#### RuggedCircuits

#1
##### Mar 23, 2011, 11:37 pm
How about just connecting the output to +5V through a large resistor, say 100k or so? I don't think any diodes are required. If that doesn't give enough offset, then lower the resistance. Be warned that the lower your resistance gets the more you're actually going to affect the mass air flow if I correctly understand the datasheet you posted.

--
The Rugged Motor Driver: two H-bridges, more power than an L298, fully protected

#### mitch_79

#2
##### Mar 24, 2011, 12:04 am
Hmmm... pretty straightforward idea.
I guess I'd assumed the MAF signal output would be able to supply enough current that it would bulldoze any sort of pull-up resistor.
But running a quick experiment it seems this might work.  It seems pretty sensitive to the R value.  20k doesn't have enough effect, 10k has too much (pulling the output to a pretty stable 2.5V).

#### RuggedCircuits

#3
##### Mar 24, 2011, 12:07 am
Me, I'd go for the op-amps, for the reasons you've discovered in experimenting. There's only so much you can do with passive circuits.

--
The Quick Shield: breakout all 28 pins to quick-connect terminals

#### dcb

#4
##### Mar 24, 2011, 08:59 pm
let me understand, you want to "tweak" the mass flow sensor reading?  i.e. for a car?

so if you add a touch of voltage to the existing signal, it will look to the ecu like there is more mass airflow than there actually is, and it will add fuel and  make the mixture overly rich (then thrash on the o2 sensor).  Are you sure that is what you want?

If you want to lean the mixture then a diode in series with the signal would probably do that, .7 volts for a small silicone or .3 for a germanium.

#### mitch_79

#5
##### Mar 24, 2011, 09:11 pm
Yep, those are some of the implications of what I want to do.  I do *not* want to induce a lean bias on the signal.

What's actually going on is I've put the MAF sensor in a larger diameter intake tube, and have scaled the fuel injectors by a similar amount.  But it appears the MAF sensor response isn't linear, probably exponential.  So the lower airflow at idle leads to a reading that the closed-loop OBD-II control system can't compensate for - signal has dropped too much.

This is a small piece of a project with much larger scope: 2jzduino (visit my profile web-site or try google).

#### dcb

#6
##### Mar 24, 2011, 10:31 pm
Sorry, can't help you.  I only know how to get 'em to use less gas anymore

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