Electronics > Beginners

DC dummy load circuit calibration

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Ian.M:
As the circuit relies on millivolt level signals, any noise capacitively coupled from nearby metalwork is likely to upset its operation.   That's the price you pay for a simple circuit with low cost, low-value current sense resistors and a low minimum load voltage.  Bonding the case to the circuit 0V rail prevents any disturbance to the circuit when it is touched.

Anodising forms a fairly tough insulating layer, but its usually broken in tapped or countersunk holes unless they were formed before the anodising process was performed.

You can get washers with solder tags that are big enough to slip over the pot mounting bush to make attaching the ground wire easy.  If you cant get one easily, you can always cut one out of thin brass or copper - shim stock or heavy foil or even out of thin tin plated steel, though that's a lot more work to cut.   If you sand off the anodizing on the inside of the front panel round the pot hole before fitting the pot, it should ground the front panel well enough.

However, personally, I'd drill a hole in the case bottom, countersink on the outside, and sand off the anodising round the hole on the inside then use a countersunk head machine screw, with a serrated washer, a ring terminal for the ground wire and a plain washer and nut to firmly bolt the ground wire to the case bottom, then apply some lacquer to the bolted connection to exclude air and moisture so the serrated washer's connection to the aluminum doesn't deteriorate.

VEGETA:
I remembered that I have a hand drill! I can use it to make a hole in bottom layer then solder ground wire in it very easily! I just don't make he hole reach the other side but enough to expose the internal aluminum and remove anodizing. As for POT, I would just solder a breadboard jumper wire to it then to ground pin. No need for more complications.


I don't get why you need to go outside the case.

Ian.M:
Good luck with that.  Soldering aluminium is *DIFFICULT*.   You have to exclude  all oxygen from the surface, scrape off the oxide layer (which reforms almost instantly at elevated temperatures in the presense of oxygen) and solder while maintaining an oxygen free environment.    Its possible to do that in an inert atmosphere, or under oil (but the oil must have a smoke point significantly higher than your soldering bit temperature), and you may have some success with aggressive flux, a large pool of molten solder + scraping through the solder pool, but it will be very difficult due to the massive heatsinking effect of the large thick case walls.

Soldering to the pot bush is *NOT* recommended for plastic body pots as the heat is likely to damage them.  Its acceptable for metal body pots - just solder the ground wire to the back near the terminals.

VEGETA:
But as you see there is no way to screw something on the main bottom case so I need to get dirty.

As for POTs, I didn't quite understand. You don't like soldering its metal part to ground? OK, but the only option left is to put the ground wire under the associated nut\washer while fixing it. I thought solder would be a better connection. What ground and black wire are speaking about?

Ian.M:

--- Quote from: VEGETA on May 20, 2018, 04:01:06 pm ---But as you see there is no way to screw something on the main bottom case so I need to get dirty.

--- End quote ---
Looking at the case again, it looks like there's enough wall thickness in the sides to drill and tap them so you've got threaded holes for grounding screws.  Due to poor access  you'll have to drill right through and tap from the outside, but it will be easy enough to cut the screws to length and clean up the ends so they are flush with the outside when tightened on a ring terminal + toothed washer from the inside.   With a drop of black lacquer paint on the ends, they'll be barely noticeable.    I'd add grounding screws to both case halves due to the risk of poor contact beween them.     If you've never tapped aluminum before, use WD40 as lubricant and practice on something extruded that doesn't matter.   Personally I'd go with M3, but M4 would have a lower risk of breaking the tap off in the hole.  The tap *MUST* be held absolutely straight as if it starts on an angle it will cut a 'drunken' thread that will be much weaker as the hole forces the tap into line and part of the initial drunken thread is cut away.   It can be helpful to drill a slightly tight guide hole through a block of wood, tap the guide hole and clamp it to the part you are tapping so the tap is held straight and encouraged to advance as it first starts to cut.   See https://en.wikipedia.org/wiki/Tap_and_die for more details.

Another method that requires less skill with precision hand tools would be to use self-adhesive copper tape with conductive adhesive.   Sand off the anodising round the end plate screw holes on the inside of the end plates and the ends of the two case halves.  Stick a strip of tape to the end of the case half, extending a short distance down the inside of the case side, covering the hole the screw will go in,  pierce it for the screw and clamp it firmly between the case half and the end plate using the screw.  Dismantle again, trim the outside edge with an Xacto knife, and solder your ground wire to the copper tap on the inside of the side.   Similarly, stick a strip of tape to the inside of the face plate over the pot hole, cut out the hole with the Xacto knife and solder the ground wire to the tape clear of the mounting bush footprint.  If you decide to improvise with copper foil and ordinary adhesive tape, only put the tape to stick down the end of the foil you are going to solder to as its an insulator and mustn't be between the foil and the area round each hole from which you've removed the anodizing.


--- Quote from: VEGETA on May 20, 2018, 04:01:06 pm ---As for POTs, I didn't quite understand. You don't like soldering its metal part to ground? OK, but the only option left is to put the ground wire under the associated nut\washer while fixing it. I thought solder would be a better connection. What ground and black wire are speaking about?

--- End quote ---
I said 'back' not 'black'

A metal bodied pot usually has a tin plated pressed steel back cover.  Because its fairly thin, it doesn't take a lot of heat to solder to - just scrape off any lacquer to expose a small spot of bare metal, tin it then solder the wire to it.  OTOH soldering to the bush of your pot would take a *LOT* of heat, which would cook the lubricant and may melt plastic parts of the mechanism or loosen the bush in the body moulding.

If you clamp something between the bush and the front panel it needs to be thin and *FLAT*.   Trying to directly clamp the wire is likely to make the pot sit at an angle, and due to the small contact area there is a high risk of the wire squishing and the pot loosening.  Its much easier with a tag washer even if you have to make it from brass shim yourself.

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