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Filtering from car alternator

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Hi,

the circuit attached works fine when connected to 12V battery. The question is what would happen to the performance when connected to the car battery with the alternator charging.

Would I require any additional filtering and would the output of the comparitor alter with the voltage fluctuations?

Cheers
Andrew
 

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mbarazeen

Member
no problem in connecting so, since the battery would be online with the alternator, it acts as a very large capacitor to filter out all variations.
 

MikeMl

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Most Helpful Member
...

the circuit attached works fine when connected to 12V battery. The question is what would happen to the performance when connected to the car battery with the alternator charging.

Would I require any additional filtering and would the output of the comparitor alter with the voltage fluctuations?

When running at a fast idle or faster, modern cars regulate the battery/bus voltage to a narrow range between 13.8 and 14.4V. There are times when the voltages are different: with key-on but engine stopped, the voltage will be ~12V. During cranking, the voltage will drop as low as 8V.

With engine running, the alternator ripple is "filtered" by the battery. The battery is equivalent to a capacitor of hundreds of Farads, and an effective series resistance of a few mΩ. The peak-to-peak ripple voltage at the battery is usually only a few tens of mV.

Most of the problems that people have with alternator ripple and other noise in automotive systems (particularly with audio systems) are due to resistance and common-mode voltages developed along the vehicle/ grounding system, which is usually the car's frame and body.

There are times when large short-duration inductive spikes (both positive and negative) can damage electronics. The spikes are usually triggered by high current inductive loads, like the starter motor, the starter solenoid, the A/C clutch, electric fuel pumps, etc. Most ICs developed for automotive applications are specially designed to tolerate +-50V short duration transients on their power inputs...
 

MikeMl

Well-Known Member
Most Helpful Member
I went and reread your post. Are you asking if the normal voltage fluctuations in the car would effect the temperature at which your comparator changes state? If so, the answer is: yes, but not much. Since your basic sensing circuit is a balanced bridge, it is quite insensitive to the supply voltage. What makes it slightly sensitive is the positive feedback through the 1meg pot...
 
Thanks Mike, it was a bit of a worry. So in effect it will be the hysteresis that will be impacted not so much the preset temperature.
 

MikeMl

Well-Known Member
Most Helpful Member
Thanks Mike, it was a bit of a worry. So in effect it will be the hysteresis that will be impacted not so much the preset temperature.

No, the temperature set point will be effected slightly. When the comparator is high, the ΔV in the supply will change the current through the 1 megΩ positive feedback resistor, which will in turn create a small ΔV at the non-inverting input...
 
Hi Mike

Cool thanks for that, considering I never had cooling whilst on the road this will still be a great improvement.

Cheers
Andrew
 
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