Workshop portable vacuum project

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16 Apr 2020
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For years I have been relying on old domestic vacuum cleaners to suck up the sawdust generated by my power tools, not the best solution. So recently I decided to build a dedicated dust extraction system that I can move around and put in the back of my truck when doing onsite jobs.
Its based on a Vacmaster 20lt 1250 watt vacuum cleaner combined with a vortex dust separator and plastic drum to hold the sawdust. All located inside a custom built trolley with lockable castors. I liked the Vacmaster as it had a PTO facility. However, there is a drawback with the PTO system. You can only power tools through it with a maximum wattage of 1740w (I have 2 tools that exceed this wattage, my table saw and my planer/jointer). This would be the case with any vacuum/PTO system that runs through a single 13 amp UK standard plug as the maximum wattage you can draw is 3 kw and this power has to be shared between the tool and the vacuum.
So, I designed a custom power sensing device that takes power from its own 13a plug/socket to supply the tool you are running (now max 3kw) and when it senses a current flow it applies a small load on the PTO which turns the vacuum on.
The current sensing is done by a nifty little current sensing switch and the load is a 1k Ohm 100w finned, chassis mounted resistor. This consumes 57.6 watts at 240v which is enough to trigger the PTO. The system also has a 40mm cooling fan to aid heat dissipation. The resistor is also mounted onto a 1mm thick 35 sq. cm plate with thermal transfer compound to act as a heat sink. If anybody is interested in the component details then let me know and I'll supply them.
Here's some pictures, probably makes more sense than my words