06/13/2020 at 11:10 •
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.
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.
05/26/2020 at 21:26 •
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.
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:
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.The 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.Read more »
06/29/2019 at 17:29 •
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:
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.
Ethernet has become very popular in industrial applications . 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 . 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 »