Monday, 7 January 2019

Making Ruideng DPH5005 power supply case fan silent

I use Ruideng DPH5005 as a bench power supply and it is quite good for what I need, the only problem I have with it - 5V high speed fan of the original power supply case which runs always, regardless of load/temperature, making a lot of noise.

DPH5005 board has it's own small fan which kicks in only when power supply is overloaded, so probably I could disconnect the case fan completely or connect it to the heatsink fan, but I find no fun in doing so.

There are a couple of suggestions on the Internet how to deal with the noise of the fan - connect it in series of diodes to lover running voltage or to use Normally Open bi-metal thermal switch to turn on the fan at about +45 C.

I liked the latter, but decided to combine it with the first one to limit the driving voltage with a diode. And I had a nice and proven project for that: Madis Kaal tiny_pwm for ATtimy85 microcontroller.
Using pure PWM did not work with this fan - to run the fan stable at the lower speed I had to use quite low PWM frequency with clock divider 128, which was creating extra noise, diminishing the purpose of the project.

As power supply provides 5V, we can use bump-on-the-wire approach to control the fan. Modified schematics looks like this:
ATtine85 controls the fan speed using two FETs: one for "half" speed (~60%) and one for "full" speed. In the "half" speed mode 1N4148 and a FET reduce the voltage on the fan by around 1.4V. During the tests I found that 2N7000 FET adds a bit more voltage drop than BS170, so I used it for "half" speed mode.

On startup MC turns on the fan full speed for a second and then selects the running mode according to the internal temperature sensor. If temperature is above the low threshold, fan is running at "half" speed though 2N7000 + 1N4148 combination, if temperature is above high threshold, fan is running at "full" speed through BS170. If temperature is below low threshold, fan is off. Hysteresis in 2 C is used more smooth control operation. Code is avaibale at the GitHub.

To mount the board inside the power supply case I printed a simple box:



And used two-sided sticky tape:


Now DPH5005 power supply only makes a gentle "woof" on start up and then runs nice and quiet.

Additional read:
[1] Microcontroller for Variable Speed BLDC Fan Control System

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