A quick video about an unusual circuit board I was sent a picture of the circuit board by Vic who was maintaining a printed circuit board processing machine and the Machine uses rollers with Scotch bright material on them to clean the surface of copper clad circuit boards and I actually use these these standard pads to clean my own circuit boards and I'm processing them because uh, it's a very good way of taking off the off the oxide layer and revealing shiny copper. For the photo, Sensi Coatings and in this case, the machine has adjustable rulers and if you adjust them too tightly, it causes excessive wear. Also causes increased abrasion in the circuit board, so there's a a fine balance. and this circuit board is designed to help the operator tune that balance by using a meter which is connected between these two holes here to give an indication of numerically it's not actually it's not giving amps, it's not giving uh, kilog or Pounds or whatever, but it gives indication the force that you can uh, set it to the same force and watch the meter to get this, an indication of the uh, the force that's being applied.

And it does that by measuring the current through one phase the motor via these resistors and then coupling it via this. Transformer I'll show you the schematic because it's really odd. H I'll in fact, I'll cut straight to the schematic. Things to note: There is a little potentiometer here for calibrating this unit, but there's an external potential.

Two, let's go straight to the schematic and it's an odd one, as shall Zoom down. This is very retro. as you saw from the style of circuit board there, it looks like it's homemade. Let's just double check.

This is super focused. So here's the motor, the motor, the meter, and it's a 1 milliamp FSD meter. FSD stands for full scale deflection. It's one of those traditional analog meters with the needle and, uh, that's just as well, Because it is.

There's no smoothing on the output of these uh Transformers here, and it relies in the sort of ballistic behavior of the meter that it's going to just average the value. So live comes in here, although in the drawing, it's kind of shown as being connected to the motor connection I Don't think it really matters too much and neutral is here. The live neutral goes straight to the primary winding of the Transformer that powers one side of the circuit cuz this is a strange balance circuit and the Transformer actually has two windings. If we take a look at it, we've got the two 6vt windings that are separate and two 120 volt windings are separate.

The way they're used here is that for the 240 volt Supply, they were just connected in series of those windings and likewise the output. In this side, the two 6vol windings are connected in series and that gives a 12vt supply. It's rectified, and then there's a potentiometer and it kind of wipes between the rectified roughly 12vt Rail and the effectively kind of Zer volt rail. It's notable that the positive is common between these two sides, so that's presumably then the Zer volt reference.

In a way which is odd, the sensing side is very odd. It's got the resistors two in parallel that add up to about 34 ohms total, and as the motor current increases, there'll be a voltage differential across that, but that is actually coupled to the circuitry by the Transformer on that side being used in Reverse. This time, they've just used a single one of the 6volt windings, and then they've paralleled up the 220 volt windings, so that will basically provide a voltage an amplified voltage back to the secondary side here. that is then compared to this voltage.

It's very strange. The voltage drop across this resistor, though, will be very, very low, so the output of that rectifier has a slight resistive shunt across it. I'm not sure if this also effectively provides uh reference to the other side. It's quite an odd circuit.

and then there's a calibration pot. This is the one that's actually in the panel, though. And uh, from the look of it, the manufacturer recommends that the machine is run unloaded and then it's calibrated to a position of two on the display. This is all going to be mumbo jumbo if you don't really know what's happening in the machine as I don't 100% know what's happening in the machine.

And for me, uh, To calibrate this I'd put this calibration potentiometer on the panel into a mid position. that and then I'd use the printed circuit board one the 1K There's a question mark, so I was wondering about the value of that, but you'd then fine-tune that to the desired two reading and then after that uh, that can allow a fine tuning of that, but the manufacturer in the instructions says you know specific pressures might be about 3 3.5 but again, this is just using this as an indicator I Have to say if I was it looks like an old system, it actually looks like it's from the 70s or ' 80s and uh, if I was doing it I'd probably just use a digital meter and I'd get the correct setting by trial and error and then I'd write the number of the actual current that was being read through that Transformer But it's interesting. It was well worth looking at. Um, quite an unusual design, particularly the fact it does look handmade.

These are just sort of hand etched Pcbs, presumably made in small batches for this particular task. But there we have it: an interesting and very retro Uh design for measuring motor current and therefore roller pressure in a circuit board making machine. Quite interesting.