Calibrating sensors to your Traqmate

#21

[quote=“jlevie” post=64444]

Since the DAC input is limited to 5v, I would drive the measurements with a 5v regulated source. That would eliminate any risk to the DAC should the resistor get shorted. That is how the analog inputs to my IQ3 are set up.[/quote]

I respectfully disagree with the 5V source. You’re on the right track Chris, a 5V source would give you some tiny measurement input range of 1.5V or so. Something that is designed to work with each other and the sensor has it’s own ground can work ok with a 5V reference source but to be reliably accurate and keep the calibration ya gotta start with a higher reference V.

IMO the power resister shorting out isn’t the most likely way to hit the DAC with too much V. If the input gets hit by too much V some day it will probably be a wire to the car’s battery brushed it or it got killed by welding.

#22

[quote=“Ranger” post=64434]

Get a chip that is rated at 500mA@15V so it will be nice and stable. Then fasten it to a piece of AL also.[/quote]

When you say chip do you mean the DC-DC converter, the resistor, or something else?

#23

So what are you using anyway. I would guess some kind of usb powered system that has built in 5v outs. I would go the 5v route. I don’t think the supply voltage matters as much as the consistency of it, as long as you don’t burn it out.

#24

The 5V out doesn’t give me that great of resolution and based on Ranger’s experience it sounds like it doesn’t exactly provide stable measurement. Ranger’s solution allows me to keep the sensor power in the engine bay and then just run the signal wires back to the DAQ while likely achieving a better signal quality from them. I’d prefer to not waste time reinventing the wheel and Ranger has spent considerable time working this out already.

#25

[quote=“MrDomino” post=64464][quote=“Ranger” post=64434]

Get a chip that is rated at 500mA@15V so it will be nice and stable. Then fasten it to a piece of AL also.[/quote]

When you say chip do you mean the DC-DC converter, the resistor, or something else?[/quote]
Well, I suppose you could call it a DC-DC converter, but what it’s really called is a regulated power supply. Something like 15V out and 12-24V in. That is to say that it will tolerate a wide range if input V but can be depended upon to maintain output V at 15V.

#26

I think it depends on the ohm range of the sensor your using. I can’t be bothered to do any math right now. you want 0 to 5v in the signal wire. If the minimum ohms of the sensor gives you 5v or less at 15v input then go for it. I don’t think that in any situation increasing the supply voltage will improve the resolution of your sensor.

Ranger had a very specific reason and application for the 15v input which I can’t remember now.

#27

The datasheet of the part number you listed earlier calls it a DC-DC converter which I agree is misleading since it gives the impression that it’ll take a fluctuating DC voltage input and turn it into a fluctuating DC output just scaled.

#28

[quote=“turbo329is” post=64469]I think it depends on the ohm range of the sensor your using. I can’t be bothered to do any math right now. you want 0 to 5v in the signal wire. If the minimum ohms of the sensor gives you 5v or less at 15v input then go for it. I don’t think that in any situation increasing the supply voltage will improve the resolution of your sensor.

Ranger had a very specific reason and application for the 15v input which I can’t remember now.[/quote]

The “very specific reason” was “did the math”.

#29

Yep. The only way to get good resolution is to input 12 or 15 VDC.

#30

Is there any reason in particular that you said to not buy 10-180 ohm sensors? I want to buy ones with floating grounds since I won’t be able to properly ground the water temperature sensor since I think I’m going to use something like this I think http://www.frsport.com/Greddy-16401636-Radiator-Hose-Water-Temp-Sensor-Adapter-36mm_p_15545.html

#31

There’s 2 common sensor resistance ranges, 10-180 Ohm and 240-33 Ohm, or near enough. The first is often called the “VDO range” and the other the “US Range”. I might have those reversed, but I think it’s right.

Note that the two are reversed so full sweep VDO range is high resistance whereas US range it’s low resistance. The US range is easier because the resistance doesn’t drop so low, and also generally the sensor won’t go full scale so you’ll likely never drop below 50 Ohms. In contrast the VDO range sensor is going to be at 10 Ohms every time you start the car. Consider what this means in the context of current limiting. Every time you start the car the VDO sensor is going to pull a lot of current so your power supply and power resister are going to get hot and stressed.

The floating ground business is a non-issue. Just buy a sensor with a ground prong and wire it to a ground. That’s a separate issue from VDO or US range. Alternately imagineer a way to connect a ground wire to the coolant hose adapter.

#32

The problem is that there aren’t that many 240-33 ohm temp sensors with a floating ground. I figure running a dedicated ground from the DAQ to the sensors would help cut down on the amount of noise and eliminate any offset issues with the senors. I’ll have to calibrate each sensor individually based on the exact resistance of the current limiting resistor and all should be good.

#33

So after looking at the math some more and trying to decide on resistances I think that you’re incorrect about 240-33 ohm sensors being better.

So let’s assume I’m using the power supply thing that gives me a stable 15 VDC signal and a 500 ohm current limiting resistor.

When the sensor is at 240 ohm I’ll see a voltage of 4.86V and when the sensor is at 33 ohm I’ll see a voltage of 0.93V which gives me a measurement range of 3.93V (not too shabby).

Moving on to currents (I = VDC/Rtotal). The max current draw will be when the sensor is at 33 ohms (28 mA) and the min current draw will be when the sensor is 240 ohms (20 mA). This is very reasonable.

Finally, moving on to power absorption (P = I^2*Rsensor). The most power is absorbed by the sensor its resistance is 240 ohms (0.10 W) and the least when the resistance is 33 ohms (0.03 W).

Unless I’ve messed something up somewhere along the line, it seems like a 10-180 ohm sensor would be better in terms of power absorption.

#34

Don’t worry so much about the sensor being able to dissipate heat unless it’s explicitly not designed for 12V. Automotive sensors are designed for 12V and yours won’t see half that. What you want to worry about is the power supply being able to maintain an accurate V if the current demand goes up, and also the power resister being able to dump enough heat.

You won’t get to 33 Ohm in a 240-33 Ohm sensor because you’ll never run the gauge at full deflection. What you will see will probably be something more like 240-100 Ohm. That high sensor resistance means better current limiting then the VDO range 0-180 Ohm.

#35

So I bought an arduino uno r3 today. The goal is to use my iq3 external warning output to tell the arduino to start my camera recording the same time that the iq3 begins logging with an infrared led. Unfortunately I don’t think I’m going to find the output codes that panasonic uses online so I’ll have to record them myself.