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Thread: Trying to replace Q1. Got wrong part. Is 2N7000 interchangable with BS170

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    Trying to replace Q1. Got wrong part. Is 2N7000 interchangable with BS170

    So I'm having issues with my fan. I'm trying to replace the part Q1 on my HM4.3 board since i read that that is the main culprit of my issue. I went to the parts store today and got a new MOSFET to install. However when i got home, i noticed that the part looks the same, but it is not labeled as "BS170" and is instead labeled as "2N7000". I'm a bit afraid to install this part and ruin my board. Anyone know if they are interchangable and if i can use this 2N7000 part in place of the BS170?

    Wikipedia has this to say about the two: "Packaged in a TO-92 enclosure, both the 2N7000 and BS170 are 60 V devices. The 2N7000 can switch 200 mA. The BS170 can switch 500 mA, with a maximum on-resistance of 5 Ω at 10 V Vgs."

    Thanks

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    TVWBB Guru Steve_M's Avatar
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    It should be fine. Q1 is just used to control Q3. All the blower current is running through Q3

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    Thank you for the reply. I honestly ended up ordering a BS170 online just to be safe. I did replace the Q1 and that fixed the fan issue though! Seems to be the explanation for a lot of people with fan issues

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    TVWBB Honor Circle Bryan Mayland's Avatar
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    Argh sorry I am so slow on these replies, I was out of town last week through Sunday. The 2N7000 has identical specs to the BS170, however its pins are typically reversed (unless it is some weird repinned version). To use a 2N7000 in place of a BS170 you just need to rotate it 180 degrees and install it backward. Like if the flat face is supposed to be facing the front, you'd rotate it so the flat part faces away.

    I wonder why Q1 is always an issue. I've physically broken one or two when they used to be a lot closer to the front of the case. I wonder if it is still to close and gets bumped or something.
    I'm that HeaterMeter guy what ruins everybody's free time.

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    Brian, I have another theory about why Q1 could be an issue, it might not be the explanation but it was just a theory. I followed your assembly video on YouTube. In the video, the first of the BS170s you install is Q1. If most people are like me, we haven't don't a lot of soldering and Q1 is the first time we've soldered that type of triple-lead. Q1 is the first one we attempt and it's sort of practice. The other two come next and we are a bit better by this point after practicing on Q1. I know my soldering looked the worst on Q1 compared to the others before I replaced it and I think I had the bends in the wire much to close to the board, possibly causing an arch? Maybe most people with an issue followed your video and had the same experience where they just didn't do a good job on their first BS170 and then figured it out by the next few. Maybe you could make a new assembly video and install them in reverse order and see if the issue starts cropping up on the other mosfet haha jk

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    another question for Dusty is, did you test the HM prior to installing in case. If it worked prior to case install, you likely bent the leads of Q1 when you installed into case. That is how I shorted my Q1 when I did my build. What I did when I installed Q1 again, was to shorten the leads and soldered quickly and made sure it was mounted closer to center of PC so when I slipped boards into case, the case did not contact Q1. Just be aware this side of the PC is very tight when inserting into case

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    I did test before it went in the case and the issue was definitely present so I don't think my issue was related to Q1 getting damaged when inserting into the case. I think it was a soldering issue

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    TVWBB Honor Circle Bryan Mayland's Avatar
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    That's a good point Dusty, it being the first one that is installed so nobody's had practice yet. I also find it very strange that the "TO92" package parts come like that, or with the leads bent outward ("kinked lead") but not one designed to fit in the regular 3-pin-center-pin-set-back arrangement that is most commonly used. I think the bending adds stress to the part, and can easily snap the lead if it is bent to the side a bit so the whole setup isn't ideal.

    Oh see here is all we need is just send the reel out and have the leads formed to specification "TO-92 Component Formed To TO-18 Pattern (.10” Pin Circle) Center Lead Bent Away From Flat Side". I bet that is cheap.
    I'm that HeaterMeter guy what ruins everybody's free time.

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