Fusion 360 adventures

[familyguy]

Just to give those interested in maybe taking up Fusion 360 is here's a short story of how it was used in one project.


Not that long ago I fitted a new moving coil movement into an existing meter housing as part of a vintage Avo valve tester restoration project, the original meter was fitted with a very sensitive and not very robust 30uA FSD movement - the coil of the movement had gone open circuit - on close examination I could see the meter coil had no former as is usual with moving coil meters, the coil of wire is very fine and simply held together in a rectangular shape with varnish - I could see one loop of the coil has come free and had rubbed against the magnet assembly eventually being cut through. Going through my drawer of salvaged meters I found a donor movement that fitted into the housing and had the right length pointer - it was less sensitive at 200uA FSD so I would also need to make a meter driver amplifier to increase the sensitivity of the movement, the upside was that it was much more robust.


Spent some time taking measurements and drew up 2 brackets in Fusion - one to hold the new movement in the original case and another to support the original meter scale - the original meter scale was opaque plastic and needed to be retained so the red low wattage lamp behind the meter (which lights up in the event of an overload) could be seen - the brackets made of brass and aluminium were milled on my CNC mill, and fitted perfectly first time (unusual for me). Next was the meter amplifier. One thing I learnt - do not work on meter movements in the workshop - as soon as the donor movement was removed from it's case all the metal filings nearby decided to leap onto the movement and cause the needle to jamb part way - they are a bugger to get out.


Fusion has an electronics module built in - spent a few hours coming to grips with it and came up with a meter amplifier design, this was then transferred to the PCB board section to layout the board tracks - Fusion will automatically do this as a double sided board but as I wanted only a single sided board I did it manually.


I designed the meter amplifier to fit onto the back of the meter housing piggy back style - I actually built 2 meter amplifiers - the first was sort hung out the side it worked fine but looked ungainly, even though it would not be seen once it was installed I made a second that looked much better, having the amp board piggy backed onto the meter housing meant it was easy to adjust the FSD and DC offset on the bench and then install the meter and amp in one. This went smoothly, using a 1.5V AA battery and variable resistor it was easy set up for 30uA FSD and once in the instrument it worked perfectly, and allowed me to tick off that part of the restoration project.

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[BaronJ]

Hi Ozzie,

I would have gone for a resistor to replace the original movement and a surface mount bipolar transistor current amplifier to drive the new movement.

[familyguy]

Yes, I get where you are coming from, a bipolar transistor would be simpler, however, the actual instrument circuit used to determine the 'health' of a valve is based on a bridge configuration, the meter is used as the null indicator for balancing the bridge so even though the meter scale is graduated from zero up in a positive direction it does need to swing negative to show that the bridge is not balanced, the instrument controls are adjusted to balance the bridge and bring the meter needle to zero, once balanced a third knob then applies a small voltage to the valve, the meter then indicates the 'health' of the valve on the scale, a differential input op amp is perfect for that.

[BaronJ]

Yes, I get where you are coming from, a bipolar transistor would be simpler, however, the actual instrument circuit used to determine the 'health' of a valve is based on a bridge configuration, the meter is used as the null indicator for balancing the bridge so even though the meter scale is graduated from zero up in a positive direction it does need to swing negative to show that the bridge is not balanced, the instrument controls are adjusted to balance the bridge and bring the meter needle to zero, once balanced a third knob then applies a small voltage to the valve, the meter then indicates the 'health' of the valve on the scale, a differential input op amp is perfect for that.

Hi Ozzie,

I didn't realise that it was wired like that, I agree an op amp is a good choice