I had not planned to continue with the Ar2uino Dummy Load any more for the moment until Step 2 – but the day before yesterday when I replaced the normally(?) noisy fan of my Rigol 1054z oscilloscope with a more silent one (Gelid Silent 5) I suddenly had a fan of the right size left over! Not such a bad fan either but to noisy to listen to over extended periods with the oscilloscope! But that did not matter in this case – any fan would do for the dummy load! Just one critical thing wrong with it – it was a 12V version! The 5V fan that I sent for was $10 with shipping and even if I have gotten that money back now – I still have no fan! Then I realized that I could use this “left over” fan just by adding a DC-DC Voltage Booster to bring those 5V up to the stipulated 12V – for less than $1/€1/10SEK! No need to send for another 5V fan and pay $10 again! 🙂
That is really a good idea that I will have to remember in the future! Power something by 5V (or even 3.3V) and need a fan? Then use a 12V fan and a DC-DC Voltage Booster and get variable fan speed thrown in! So now all my 12V fans can be used at much lower voltages! That is really cool! Eh!? 😉
(Albeit at the expense of higher current requirement.)
At least for the moment what I have is a manual speed control that I can use to lower the fan noise a bit when the load – and the cooling capacity needed – is not so high! 👍 I have had plans to make the fan temperature controlled in Step 2 or controlled by the ESP8266 depending on the load – but this might turn out good enough … I will see!
It is a scary moment when you have cut the wires at exactly the right length and then have to remove the insulation! You know that if you fail and cut through the wire – it will NOT fit very well and you will have to extend the cut off wire in some not so very elegant way! The critical de-insulation – went well after all! 🙂 Just to calm my nerves I added a back-EMF diode! That is to protect the booster from the high voltage generated by the fan motor working as a generator when the fan spins down. I just soldered a 1N4007 across the output terminals! (See the red arrow!) No particular reason for picking the 1N4007 – just because that was what I had at hand. Well it has to handle the desired max forward voltage naturally! The 1N4007 does that hands down! 😉 It was an SMD type with solder pads made from bent flat “wires” so I “unbent” the solder pads somewhat to be able to solder them a little easier.
Then is the question of if the fan is most efficient when “blowing” or “sucking” air? I will need to test this just to find out how it is – at least in this case! Not that I think the difference will be big enough to matter that much! It is just fun finding out! 🙂 Judging from the manufacturers sticker (assuming they want to be seen) it should probably be “sucking” the air out of the heatsink – but to my … ehrm … extra sensory perception – tells me it would be better with “blowing” air into the heatsink! 😉 A small test will give the answer … and the answer is …
To quote a nice song: “The answer my friend, is blowing in the wind…” Well, as expected the difference is not very big degree wise but it is definitely clear and big enough to motivate choosing it! Blowing into the heatsink is the most effective! See diagram above! As the fan was blocking the view of the heatsink innards I could not use an infrared thermometer so I decided to use the temperature add-on to the multimeter to find the temperature. This time I measured it at the transistor side of the heatsink, at the transistor metal tab. Measured with the fan set to be powered with 12V – that is “full blast”. Room temperature was 21°C and it was open space all around the heatsink.
The loads I measured was 20V and 300mA and 500mA – translating to 6W and 10W. The temperatures (above) that those loads gave translates to temperature increases of 0.5°C/W when blowing and 0.75°C/W when sucking air. That is around 50% worse when sucking air – not such a small difference after all! The temperature measurement was only possible in whole degrees Celsius so it is a bit coarse – and the measurement points were not very far apart – but the difference is clear! I removed the fan and let the temperature climb a bit and then replaced the fan again – and every time the temperature settled on the above levels! To go by these values a 50W load should raise the temperature around 25°C from room temperature and that puts it in the vicinity of 45°C – if the fan blows into the heatsink!
(I broke off the measurement of 500mA without fan as the temperature broke 48°C and was still climbing!)
I used heat glue to stick the DC-DC booster to the side of the fan – out of the way. The thinking was right – but NOT the glue! I was only left with a small “glue-coin” that did not stick to either surface! So I switched to superglue and that seems to have done the trick with a small distance put in between to cater for the soldering bumps! One thing that initially had me flabbergasted (just for a little while, mind you!) was that the booster would just “pass through” any voltage that I fed to it and no matter how much I turned the little trimmer screw (clockwise) nothing happened – not even when I turned it counter-clockwise did anything happen! 😦 Then I realized a small lead bridge over two pins at the small IC on the booster so I tried to fix that with a solder wick – but no improvement! I decided to try my other sample of the same DC-DC booster – but the same result! Both broken? Murphy hit me twice? Unlikely … so then I decided to turn it all the way counter-clockwise! Lo and behold – after about 5 to 10 turns the output voltage started to go up! I set it for 12V and connected the fan – and it was as noisy as before! But now I can adjust it! Lovely! 👍😁
Those small DC-DC Voltage Boosters are certainly both handy and cheap. But they are switching which means they are generating high frequency electrical noise. Will this disturb the dummy load maybe? We are dealing with some very minute voltages there! A look at the old oscilloscope would not be amiss!
See you sooner!