Interesting 70 mm AVC fans

Continuing the discussion about electronic loads. As everyone knows, a load is something that usually gets very hot, and when something gets hot, it sometimes needs cooling. The most common solution is a heatsink, and when that isn't enough, you add a fan. Fans aren't exactly cheap at local stores these days, especially high-performance ones, so I ordered some from China.

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I bought two fans, which worked out a little cheaper. One costs $2.50, with $2.39 shipping; ordering a pair came to $8.67.
Actually, a little later I ordered three more Sunon fans, the same kind I already have, and only afterward noticed they were from the same seller.

I placed the order on April 17 and picked it up from Ukrposhta yesterday, May 10. Not bad, really. No complaints about the packaging at all; it was packed securely.


They look new on the outside, or at least clean and tidy, with no unusual smells. Either they saw little use, were used in a clean room, or were thoroughly washed, though the last option doesn't seem very likely.


As the title says, they're 70 mm across and 20 mm thick. In the photo, one is shown next to a standard 80 mm fan. Of course, 70 mm is also a standard size; 80 mm fans have just become a bit more common.


According to the label, the fan's rated current is 0.7 A, which is quite a lot. Sure, there are much more powerful options, but rarely at this thickness, which was also very important to me.
I also needed a fan capable of generating high static pressure, since some fans are designed to move a large volume of air, while others are designed specifically for high pressure.
Unfortunately, this isn't really my area, and I don't know much about it. But I figured that a fan with an impeller like this should be good at pushing air through a tightly packed heatsink. If anyone knows about this, let me know if my reasoning is right.


I found a temperature sensor on the outlet side, and the fan has a four-wire connection. The sensor was originally bent closer to the impeller; the white marks on it are from my experiments.


Let's test it.
It starts at 5 V. Below that, the impeller twitches but the fan can't get going. At 5 V, it draws just 42 mA and spins confidently.
At 12 V, I measured a current of just 90 mA and power a little over 1 W. At 15 V, the current rose to 0.17 A, but was still nowhere near the rated 700 mA.
In the device it's intended for, my fans are powered from 15 V. Yes, that's overdriving them, but it noticeably boosts performance, which is very important to me.


The fan has a PWM control pin, which is pulled up to the internal supply. I tried applying 5–7 V to it, but that only caused a slight increase in current draw, probably due to the current in the control circuit itself. If you pull this input to ground, the fan switches to its minimum speed.
In that mode, it starts at just 7 V and draws only about 40–45 mA at 12–15 V. It's very quiet.


1, 2. For the sake of the experiment, I checked how it behaves with the air intake and outlet blocked. In both cases, the current draw increased. Someone might say that any fan like this will draw more current. Not so: depending on the impeller design, the current can either increase or decrease.
3. When I heated the temperature sensor, the fan sped up, and the increase was even somewhat gradual. At 12 V, the current draw rose to 355 mA and the power to 4.2 W. At 15 V, the current was about 450 mA, and the power rose to around 7 W.
4. This is roughly what it looks like in operation.

I don't know why I couldn't get the full 700 mA. The rated current seemed suspiciously high to begin with; fans like this usually draw up to around 400 mA, but this one is rated at 700 mA. I tried heating the temperature sensor more, and even shorting it, but the most I saw was about 360–370 mA.
But I can say that at 15 V, with the temperature sensor shorted, it really moves a lot of air. So maybe it's a good thing it doesn't draw 700 mA, because the noise would probably be unbearable.



And everything would be fine if it weren't so disappointing. One fan behaved beautifully, making the expected steady airflow noise, with no growling, rattling, or anything like that.
But as soon as I connected the second one, I immediately noticed increased vibration. Yes, it worked, drew the same current, and moved just as much air, but the vibration was still noticeable.
It turned out that the impeller was warped. A careful inspection revealed no signs of impact, cracks, or anything like that; everything looked practically perfect.


I wasn't happy with this and thought about opening a dispute, and I'm still considering it.
I removed the sticker, and it turned out that the impeller could be removed fairly easily—or at least I could take a look inside. I have to say that a design you can take apart is genuinely useful, since fans that are difficult to disassemble have become pretty common lately.


There are two ball bearings inside. I couldn't detect any noticeable play, and the shaft was fine too, although I can't imagine how it could have bent (that was exactly what it seemed like), since bending a steel shaft about 2.5 mm in diameter without breaking the plastic seems impossible to me.
My theory was that something had simply been resting on the fan impeller for a long time, gradually deforming it. So let's see if we can fix it.
I found the direction the shaft was leaning, inserted an improvised spacer to bend it the other way, and left it overnight.


This also shows that the fans clearly hadn't been washed—at most, they were blown out with air. Water would have made the stator metal start to rust.


The attempt to straighten the plastic didn't work, so to speed things up I decided to apply some heat.
Of course, you don't want to cook it; just warm it up. I used a couple of high-power resistors for this. I heated it to about 90 degrees, at which point my improvised spacer simply fell out—a sign that things were going as they should.
After that, I checked the result and found that the warp had almost completely disappeared. There was still a little left, but it was so slight that it was even hard to tell which way it went.


Unfortunately, the vibration was greatly reduced but didn't disappear completely. After a couple of tries, I found the side causing the imbalance, stuck a couple of pieces of electrical tape on the opposite side, and got an almost perfect balance—subjectively, even better than the first fan.


As I mentioned at the beginning, the fan has a four-wire connection. It turned out that one wire absolutely refuses to sit flush with the housing. It's not a dealbreaker, but it's annoying. I'll probably just cut off the tachometer wire, since I don't need it, while the control wire might come in handy.




And that's about it.
What can I say? Yes, I was disappointed that one of the fans was unbalanced, but it was fixed fairly quickly and easily (about 10–15 minutes of heating). The fans themselves look very good; I'd even say you wouldn't immediately realize they're used. I paid $8.3, or 310 UAH, for the two fans, while a comparable new one costs around 350–400 UAH, so I think it was a good buy.

I could say that the power doesn't match the stated rating—and it doesn't—but first, it's enough for me for now, and second, it may not actually be a defect. By the way, does anyone have more experience with fans, both with control and with choosing one to get high air pressure? I'd really appreciate any advice.

That's all from me. Thanks for reading, and I hope you found it useful :)