• My opinion on a Hi-Link AC-DC module

    06/13/2020 at 11:10 0 comments

    Some manufacturers make low power AC-DC modules for small appliances, IoT devices and smart meters. MeanWell may come to mind (IRM-05-3.3), but one of the most cost-competitive modules is the Hi-Link HLK-PM03.

    Here I will have a closer look at how the module is built. There are of course other teardowns out there, but not necessarily of the 3.3V variant, and you never know if you're going to find something others could not or had not seen.

    Unpotting

    So what's inside the HLK-PM03? With a silicone potting compound that readily unmoulds from the enclosure, the module was definitely inviting further investigation. 

    The assembly came out after gently pulling on the pins. Adhesion of the silicone compound to the electronic components is generally poor, and there is no primer / adhesion promoter on the surfaces.

    Quoting https://acc-silicones.com/products/primers:
    "Silicone adhesives and some 2-part rubbers have built in adhesion promoters - with these products the use of a primer will only be required for use with substrates that are exceptionally difficult to bond to. Most 2-part silicone rubbers will not have any adhesive qualities and the use of a primer will be essential if adhesion is required."

    I'm not seeing a lot of adhesion here, but the surface texture of the injection moulded plastic housing is reproduced flawlessly :) Slides right out of small gaps:

    The inner sides of the black housing have a satin texture added to better retain the silicone block. It seems like good practice, but only early models will tell the tale whether the mould texture was added later in production.
    There's a surprising amount of gas bubbles in the compound. In the picture they're mostly seen around the secondary side where they don't matter much, but they are also found around components on the primary side. While possibly acceptable for normal operating conditions, the porosity becomes a concern for high voltage transients. Silicone compounds break down at around 20 kV/mm, but then again there are no zones (except for the transformer) where the design would rely on high dielectric strength.


    Unfortunately I will not finish this article on account of the hackaday.io having deleted a good portion of the text, lost graphics and giving error 413 when I save the draft.

    hackaday.io is no hacker's "platform". If you can put up with a text editor that loses your work over and over again, that's your choice. I won't condemn your masochism, but I won't share it.






  • Hose Clamp - Style GT2 Belt Tensioners

    05/26/2020 at 21:26 0 comments

    A couple of years back I bought "MakeBlock" parts from the XY Plotter v1.0 kit for a low price that reflected the hot mess this construction was in its early conception.

    Here begins my journey which can be summarized as: I rebuilt an XY Plotter with most of the parts, machined my own bits and pieces and then time had other plans for me. And so this thing has been sitting around for another 3 years ... because the belt tensioners were missing.

    Concept

    The better isn't just the enemy of the good, and I've had many better ideas along the years, none of which ever materialized. So, it needed dealing with:


    You're looking at a hose clamp, a spade connector and adhesive-coated heat shrink tube.
    The version I settles on looks like this:
    Breakdown

    The hose clamp is what tied it together, really. I did digest some inspiration from [adamfilip] who built a tensioner from guitar tuners


    The tuner has a worm drive to create rotary motion and a toothed pulley to get back to linear belt movement. Consequently, one could attach the belt directly to the worm wheel, then unroll the wheel and replace it with a perforated metal band. You want the ones that have punched holes, not the embossed threads.High Torque Jubilee® Clips | PAR GroupThe proof-of-principle build I made was more about figuring out how to attach the belt to the clamp strips than about the tensioning or mounting. The hose clamp has a strip attached to the formed housing which becomes the fixed end.

    Build process (first shot, consider modifications mentioned later):

    • unroll the hose clamp - for me this was a 1/4-5/8" size one,
    • figure out how long the fixed end needs to be to attach the belt,
    • cut with metal snips,
    • Remove bridges. I used a Cr-V steel chisel held at 10-15° to score the two sides, then snapped the weakened bridges with a screw driver.
    • File the corners with a square needle file,
    • insert belt protection elements (see later),
    • attach belts.

    Seen every now and then in beginner solutions, the belt is pulled over a sharp edge. While cheap GT2 belts should come with aramide fibers, it's generally a bad idea to abuse them this way. Other belt types can be glass fiber-reinforced and really don't like minimum bend radius violations.

    https://blog.misumiusa.com/timing-belt-maintenance-and-belt-failure/


    If available, slotted spring pins can be inserted into the slot and rested against the sharp edge to roll the belt over with at 1.5-2mm radius. I ended up cutting spade connectors into pieces and filing them to the desired height:
    They can also be wedged open and crimped shut to lock them in place. Here they're inserted into the parallelogram-shaped slots and pushed to the sides where they are squeezed (without crushing) with pliers to stay put:
    Attaching Belt Ends


    I ended up using two layers of heat shrink - thin heat shrink tube to get the belt to where it needs to be and to do a test fit. It doesn't have adhesive and even in unshrunk condition it holds well enough to set the belt length reliably. The actual clamping would have been done with a steel crimp ferrule for belts, but I didn't order them in time. You might also get away with thin walled copper or brass tube which can be squished and annealed with a gas torch in preparation for crimping. Either way, I'm giving hot-melt coated heat shrink tubing a try, as it was available and can be trimmed with a scalpel.

    Below you can see how the heat shrink terminations worked out. The belt ends were pinched with needle nose pliers to allow the heat shrink to set in a fully closed shape.

    Mounting

    The final challenge is where to put the mounting bolt. At first I tried to be smart and put the hole in the fixed end in front of the screw, but that required the screw head to clear by ~1cm and made the whole tensioner extra long. Shown above is the revised version where the bolt head is right next to the belt attachment on the strip-only end.

    And that's the finished product in place:

    I'll see how this solution holds up over time. I've outfitted three belts with it,...
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  • PoE, dV/dt and ESD

    06/29/2019 at 17:29 0 comments

    Why I don't like USB in the lab

    When lab setups don't fall into disuse and get scrapped, they are usually passed down through generations of PhD students - and there doesn't always seem to be an incentive to spend time to improve a working setup.

    ... and then one ends up with such organically grown USB cancer:


    While USBTMC (USB Test and Measruement Class, [1]) allows ease of use of a subset of lab equipment, one is usually at the mercy of manufacturers to support newer platforms and control software implementations to properly handle USB devices.
    Practically such equipment communication ends up being vulnerable to faults by re-enumeration and broken handles to once-functional virtual COM ports.
    Let's change the requirements too and ask to allow multiple control PCs and setups sharing measurement equipment - and USB stops making so much sense altogether.

    To spice things up, I'm adding transients of 1 .. 20 kA/µs and 10-50 kV/µs into the mix. Under these conditions, and with setups not enclosed in metal test cells, with filters added to all cables crossing the shielding walls, USB struggles through common mode (CM) noise and yet-to-be-identified perturbations.

    On a side note: the only cables we've found to make a difference are "CU-HQ-20". They have honest copper cross-sections, proper shielding braid and ferrite filters on both ends. Expect to play 15€/pc.


    Ethernet and PoE in the lab?

    Ethernet enables multiple pieces of equipment, actuators, sensors, control interfaces and PCs to be connected, both locally and remote, and satisfies the requirements for maximum flexibility with cheap and ubiquitous components.

    Thus we should be looking at the 10BASE-T and 100BASE-TX physical layers in terms of noise immunity and voltage handling capabilites and see how PoE fits into the picture.

    Magnetics!

    Ethernet has become very popular in industrial applications [2]. As far as noise immunity is concerned, common-mode filters are often part of integrated-magnetics RJ45 connectors (commonly called "MagJack", which happens to be a name trademarked by BEL). Having the filter magnetics right behind the connector pins in a shielded can connected to the enclosure and bypass capacitors right next to the connectors is the a best effort approach to mitigating conducted and radiated emissions into a network enabled device. The PoE lines are commonly outfitted with ferrite beads to block common-mode transients coupling into the DCDC converter circuitry.

    Below two types of MagJack circuits are shown [3]. The right one is outfitted with separate PoE pins which hook up to full bridge rectifiers and an isolation converter that also handles the PoE communication. Power is transmitted courtesy of a common-mode potential difference between differential pairs.


    It is to be expected that the patch cable shield is connected to ground of the switch chassis. We have found our first issue: high frequency CM voltage components drop across the CM filter and to a lesser degree across the signal transformer. Additionally, low frequency and DC components are blocked by the signal transformer alone.

    Are they really being blocked?

    Let's look at the signal transformer:

    These parts tend to be produced as a multifilar toroidal winding, e.g. on a small NiZn ferrite core. The windings have maximized overlap to optimize coupling and to marginalize stray inductance, but then again this also enhances capacitive coupling. This inter-winding capacitance is a low-impedance path for high frequency common-mode noise.

    Faraday shields eliminate inter-winding capacitance. For transformers this requires the addition of one or mulitple shielding foils or wire windings in between primary and secondary sides, along with added insulation. I haven't seen them used in ethernet magnetics though.

    What would be the best effort? Lacking a teardown survey of PoE switches marketed for industrial applications (please contact me if you happen to have a box of interesting stuff...

    Read more »