US$1 5VDC Power Supply for the Kiwi

I just got some samples of a very inexpensive SMPS that seems to work very well with the KiwiSDR. Rob, AI6VN, alerted me to an offer on Amazon that was ridiculously cheap but seemed to have good potential so I ordered it. What attracted me was not just the price but that it switches at near 1 MHz where edges are easier to filter.

Although I know some on here have expressed the opinion that SMPSes and KiwiSDRs don't belong together, for many months now I've been very successfully using SMPSes that switch at 160 kHz on all my KiwiSDRs, along with a Guanella type balun on the PS and another on all the lines going into the Kiwi. This has suppressed the SMPS family of signals below -80 dBm , such that they haven't been very objectionable. One of the KiwiSDRs I run is at a solar powered remote site so I have been loathe to spend power in the wasted heat of an analog supply.

So seeing this new possibility I thought it merited a look. To switch efficiently at 1 MHz it uses an integrated controller and FET, the entire board is just about the size of an RJ45 connector.

I powered it up from 13 VDC into a 10 ohm load and adjusted it for 5V. Everything seemed fine and the board barely got warm, perhaps ~40C. Scope says that there is a ~960 kHz family of lines coming out of it but the SA showed even looking right at the output they were -50 dBm or smaller. Since the KiwiSDR has proven itself to have very good PSRR I was encouraged and hooked it up to a Kiwi/Wifiadaptor/guanella balun that was sitting on the bench.

At first look, I couldn't see anything other than normal -157 dBm/1-Hz noise floor. This was with no antenna on the KiwiSDR. Adding any reasonable antenna brings the noise up much higher than this. After identifying the precise frequency of the fundamental with the spectrum analyzer, I tuned to it in the Kiwi. Here it is, the worst and only line I could find making it into the Kiwi's detection:

This thing has been operating flawlessly and cool with no heat sink at all for several hours. Perhaps this might be a good and inexpensive solution to the PS problem for others on this forum.

I offer this only as an encouraging first glance that might be useful to others on this group but I probably will convert all of my units to this SMPS.

Glenn n6gn


  • Addendum: After trying a few different arrangements, I think I do note one caution: There are pretty big and fast edges present near the board. It seems it may be important to either shield the PS or keep any open high-Z antenna circuits away. If I bring it close to an 'open' antenna line, I can see .96 MHz lines in the spectrum. This isn't unreasonable for a buck converter, after all we are operating a 1W MW transmitter close to a receiver that can detect below -155 dBm.

    I also see in the reviews that some people have had problems, though many haven't. I think I will replace the voltage adjustment with fixed resistors just to be sure the setting doesn't change.

    If any of this bothers you, don't use one!
  • A few months back I saw mention of these on here, already had some for powering other devices but none in use with a Kiwi. I had assumed from their simple PCB and cost they would be bad.
    I normally run with one of these (hot glue on the adjustment) but recently went back to a linear while trying to check my own house for noise changes.
    I tried them also against a Traco Power step down (5.1V/4A) that retails for about £60 , the Traco is back in the box (visible noise very low down).

    I also made a post visually comparing the Kit switcher, a relatively expensive Chinese linear and one of these. (something like spot the 34p PSU)
    Remarkable items that I probably should order some more as they are so useful for wide input voltages. I broke the Micro USB socket on a tiny NVR, rather than fix it, it was easier to put a 5.5mm socket on the side of the NVR, one of these inside and run the NVR off the same voltage as the cameras.
  • Recently bought a Kiwisdr + Beaglebone. So far tested it with a switched 5v power supply. Works poorly - lots of noise (as one would expect).
    Will build old fashion power supply. I have a transformer salvaged long ago from an old BC-receiver (from the gloriuos era of vacuum tubes) - one of its output is 6.3 V AC.
    My question: would it be enough to rectify the 6.3V AC and smoothe the crude DC with capacitors to ground combined with an inductor coil.
    I.e. can the Kiwi handle 6.3V without risk of the voltage harming it? If the answer is that six volts could burn the Kiwi I will have to try to do surgery on the transformer to tap 5 V from it - an operation I would if possible avoid.
  • Please don't do this. But there is a much better way using that 6.3 VAC transformer. In fact, it's perfect.

    Use a modern low-dropout linear regulator chip (LDO) to get exactly 5V. E.g. a modern LTC / Analog Devices LT1085-5 in a friendly 3-pin TO-220 package can handle 3A output. It's $2-3 USD. It will need to attach to a small heatsink. There are complete schematics in the data sheet and on the web.

    Just because you have a 6.3VAC transformer doesn't mean you'd get 6.3 VDC. If you use a full-bridge rectifier you'd get 6.3*sqrt(2) = 6.3*1.4 = 8.9 VDC. But this is fine for the LDO input which always needs to be higher than the output anyway. Yes, the 8.9 to 5V difference will be burned away as heat by the regulation (and you'll need a good heatsink for the LDO to handle that). You don't need an output inductor with an LDO. Just the recommended input and output capacitors to the chip (see data sheet).

    You would essentially be building one of those ~$60 Ebay power supplies we recommend on our webpage. They use the same design.
  • Many tnx! Yes I remember that the peak voltage from a four diode bridge is the AC voltage * 1.4 - but after the smoothing of that crude DC it will be lower again (RMS of 8.9 - will it not?). Anyhow, I will instead try your suggestion LT1085-5.
  • @jks: Now I have built a linear 5 V power supply with a salvaged old transformer and the LT1085. Works excellent. Thanks again for good advise.

    Off topic: Next project will be a suitable antenna for the urban and interference rich surroundings. I'll try a magnetic loop + a broadband amplifier ? la LZ1AQ -
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