Problem – Solution …

Recently I purchased a programmable power supply.  I wanted something a bit on the lower price side but I wanted some features; adjustable voltage and adjustable current with current limiting.  A digital display for readout would be great as well.

a5710f0b-c8e1-45cc-bc62-48c83446dcceAfter some research, I decided on the RUI DENG DPS3005C available from many Chinese supplier websites.  I purchased mine from BANGGOOD.COM.  There is a metal case available for it as well, so I purchased that item along with the DPS3005C.  The DPS3005C will accept an input supply voltage between 6 and 40 volts and will provide an adjustable voltage between 0 and 32 volts with an adjustable output current between 0 and 5 amps.  There is also an adjustable over-current protection feature that immediately disconnects the load from the power supply should it be tripped.  Nice features for US$30 (the case is an additional US$19).

20171115_132423The case is meant to house different models of the DP and DPS series of programmable power supplies. One such model mounts to the bottom of the case and the display/control board mounts to the front panel.  For the DPS3005C, only mounting to the front panel is required and there is ample room inside the case for another (estimated) 3 x 4 inch board.

The DPS3005C comes with a USB2TTL bridge and a BlueTooth adapter that plugs into the DPS3005C to give the user the ability to communicate with the device using free software provided by RUI DENG. Johan Kanflo also wrote an open source firmware for the DPS5005 that likely works with the DPS3005C.

20171115_164032
Back panel with fan and DC/DC buck converter

In looking at the user feedback for the case on BANGGOOD.COM, several people were commenting that the heat exhaust fan was incredibly noisy.  When I first received my case and power supply, I tested the fan to see if it was noisy.  The fan runs full speed at 5 volts but other than the sound of air flowing, I heard no noticeable noise.  I assumed that RUE DENG had replaced the fan with a quieter one and proceeded to build the power supply.  After installing the device in the case, I powered up the fan for test and sure enough, the fan was noisy.  At the time, I decided to leave it alone and continue the build.

20171115_181213Once built and tested, I had a chance to use my new programmable power supply. Until my 24V/6A DC house-current power brick arrives, I used two 12V/7AH sealed lead-acid batteries connected in series, which worked fine but after only a few minutes of running the DPS3005C, the fan noise started to get to me.  I knew it wasn’t going to get any better, so I started looking back at the user feedback.  The most common “cure” was to simply run the fan at 4 volts by changing the 5V DC/DC converter’s feedback resistor.

With the case cover off, I performed some load testing on my unit by drawing the full 5 amps of current.  The current pass MOSFET’s did heat up the heatsink, so I figured that running the fan was probably advisable as opposed to not running the fan at all. Most of my project power needs are using micro-controllers that run at low voltage and current.  I didn’t see that the DPS3005C would be creating much heat but I could still see the need for the fan at higher temperatures.

attiny13_gI had some NTC thermistors from another project and pulled a few MTD20N03 nMOSFET’s from a defunct computer motherboard.  I have a few ATMEL (now MICROCHIP) ATtiny13’s left over from another project and they seem to be great candidates for simple controller projects like this one.

I made the decision to design a simple multi-speed fan controller using the parts I had on hand.

For the MOSFETS, I knew the fan drew 200 ma at 5 volts, so as a preliminary test, I tried using a 2N7008 nMOSFET but the RDSon (~7.5Ω) was too high and introduced a voltage drop nearing 1.5 volts. However, the MTD20N03 with an RDSon of 0.035Ω, could handle that load without a problem and so I stuck with that one.  The MTD20N03 is housed in a TO-252 package and is essentially a surface mount component.

Next post I will address the design criteria.

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