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MOSFET Circuit Help

Gage Pelon

Mar 6, 2018
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Hi all,

New the forum here. I'm working with a data acquisition system which has an internal fan. The fan cycles on for 10 seconds, and then off for 5 seconds. When the fan cycles on, the internal barometer reading drops by about 0.6 inHg. Once the fan cycles off, the pressure returns to the original value. Using an old circuit (shown below), we tried to reduce the current draw of the fan load by using a MOSFET (BS107P). The circuit still cycles the fan on and off, but the pressure drop remains unchanged. Please help! This pressure drop causes a big issue for our data. Thanks in advance.
 

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Kabelsalat

Jul 5, 2011
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I don't understand your problem. So there is a fan that make lower pressure inside a box, and that is the problem? Then why is the fan there in first place.

You haven't mention a single word of what the system is supposed to work and what purpose the fan is ment to have, so reading your post assumes that system just being a very poor design.
 

Gage Pelon

Mar 6, 2018
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I don't understand your problem. So there is a fan that make lower pressure inside a box, and that is the problem? Then why is the fan there in first place.

You haven't mention a single word of what the system is supposed to work and what purpose the fan is ment to have, so reading your post assumes that system just being a very poor design.

The fan is for internal air circulation because components get warm. The barometer reads ambient air pressure, and SHOULD NOT be affected by this fan drawing power, but it is. That is what the problem is. I'm trying to develop some kind of MOSFET amplifier to avoid this (maybe a common source amplifier).
 

WHONOES

May 20, 2017
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Look up Bernoulli's theorem. It basically states (simplifying it) that air or any fluid for that matter is at a lower pressure than the surrounding air. The fan moves air hence pressure drop. You may be stuck with what you see to a degree unless you find a solution that doesn't involve a fan.

Edit. I should have said "any air or fluid in motion"
 
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BobK

Jan 5, 2010
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What do you think your MOSFET circuit is doing that would alleviate the problem?


Bob
 

Gage Pelon

Mar 6, 2018
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What do you think your MOSFET circuit is doing that would alleviate the problem?


Bob
Bob, I'm trying to solve the problem using a MOSFET by creating a common source amplifier. I would like the input to be a small signal, then get amplified by the MOSFET to avoid drawing too much power from the main board.
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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This problem sounds akin to someone complaining that the steering wheel on their car causes the car to change direction when driving, and then asking which steering wheel cover will prevent this from happening.
 

Gage Pelon

Mar 6, 2018
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We have HUNDREDS of these boxes out the door and VERY FEW of them have this issue. The enclosure is not completely closed, nor is it air tight. This same issue even occurs when the lid is OFF and only connected to the enclosure VIA THE WIRES FOR THE FAN. I know, for a fact, that this issue IS AN ELECTRICAL PROBLEM.

Instead of trying to come up with a solution for something I didn't ask for, focus at the task at hand. I have already done all of the preliminary diagnosing to find out where the problem is coming from. This is an ELECTRONICS forum, and I have yet to see any of you respond with an electrical engineering solution.
 

(*steve*)

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If the fan is removed from the box and placed some distance from it, but still electrically connected and operating, does the problem persist?
 

Chemelec

Jul 12, 2016
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Have you checked your Supply Voltages to see if they Change when the Fan goes from Off to ON.
It Could be a REGULATION PROBLEM.
 

kellys_eye

Jun 25, 2010
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We have HUNDREDS of these boxes out the door and VERY FEW of them have this issue. The enclosure is not completely closed, nor is it air tight. This same issue even occurs when the lid is OFF and only connected to the enclosure VIA THE WIRES FOR THE FAN. I know, for a fact, that this issue IS AN ELECTRICAL PROBLEM.

Instead of trying to come up with a solution for something I didn't ask for, focus at the task at hand. I have already done all of the preliminary diagnosing to find out where the problem is coming from. This is an ELECTRONICS forum, and I have yet to see any of you respond with an electrical engineering solution.
This information would have been useful at the start of the thread then. We can only act on information received and if you want to keep salient facts to yourself what can you expect as an answer?

Power the barometer independently. Does the fault still occur?
Power the fan independently - does the fault still occur?

If the fault is caused by the fan 'loading' the supply then you won't fix it by 'buffering it but having it powered from the SAME supply'.

Can you change the fan to a 12V version (assuming there is a 12V supply handy?)

Either way, as above, I reckon you have supply regulation issues.
 

WHONOES

May 20, 2017
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Have you tried running the fan in the box in its normal position but power by an external supply?
 

hevans1944

Hop - AC8NS
Jun 21, 2012
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Other than a few hen scratches of an image you submitted of "MOSFET CIRCUIT" I see NO evidence of electronics involved in either the "problem" or your attempted "solution". How was the fan turned on and off before you added a MOSFET to perform this switching function? A relay contact perhaps, with the relay coil driven from a digital output of something?

If this is a production product and you
have HUNDREDS of these boxes out the door and VERY FEW of them have this issue.
then this is a defective design. There should be ZERO boxes out the door with this issue.

So, why don't you get down off your high horse and tell us what is really going on? You said the problem is the barometer indicates a falling pressure when the fan is on, but recovers to its normal reading when the fan is off. This factoid lead you to diagnose this as an electrical problem associated with how the fan is powered on and off, rather than an electrical problem associated with actual power delivery to the fan. Further, you said the indicated pressure drop (which apparently is erroneous) occurs when the fan is remotely located from the box, thus eliminating mechanical effects associated with air flow.

The electronics expert conclusion here at EP is there is an interaction between the power supply for the fan and the power supply for the barometer whenever the fan is on. This has nothing to do with how the fan is turned on or off, and everything to do with what happens to the power supply rails when the fan is turned on or off.

Since your MOSFET switching circuit works just fine to turn the fan off and on, and draws zip power from the "2 pin MOLEX for fan" then the problem has to be related to the current consumption of the "910 Ω" fan, supplied with power from what you indicate is a "5 V" cell. Your drawing is BS is that respect. Where does the "5 V" for the fan actually originate? Is this "5 V" supply also used by whatever circuitry implements the barometer? Is it perhaps the sole "5 V" supply for the entire box? Just for shits and giggles, try using a real battery to power the fan while using your MOSFET circuit to switch the fan off and on. Does the problem magically disappear?

Thanks to @kellys_eye and @Chemelec and @(*steve*) and @BobK and @Bluejets and @Kabelsalat and @WHONOES for some penetrating and relevant questions and comments.

What more detail would you like? I can try to provide anything you ask for.
Why should we have to try to pry the relevant information from you?

Power the barometer independently. Does the fault still occur?
Power the fan independently - does the fault still occur?
This is from the Troubleshooting 101 Course. You should have performed these two tests from Day 1 and then perhaps would not have needed to visit here.
 

Gage Pelon

Mar 6, 2018
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Thanks to none of you guys, I fixed the problem myself. And incase you're wondering why I've been such a DICK on this post, it's because since the first reply, I've gotten nothing but attitude on this thread. I told you guys I was new here and I tried to give only relevant info in the original post. I mentioned electrical jargon there thinking you guys could infer that I knew that was the issue - but I guess I needed to spell everything out for you.

For those wondering, it was a voltage regulation issue and was fixed using a NCP1117DT50G voltage regulator and a couple of caps.
 
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