I needed to house a general-purpose lab filter and its supply in a case.
The case had to be metallic, and steel, to minimize the influence of external magnetic and electrostatic perturbations.
The device had to be class II, to avoid ground loop issues, and it had to have a minimal primary/ secondary capacitance, because the tens of µA it creates can induce pV or nV drops along the shields of unbalanced devices.
In short, it had to be as quiet as possible.
A 50 Hz transformer inside was out of the question, because of the huge shield it would have required.
This left me with two possible solutions:
-A traditional wall-wart, connected with an umbilical
-An ultra low noise mains supply, small enough to fit inside the case
I didn't want the inconvenience of a wall-wart: I wanted something completely self-contained.
For the supply, I used this concept:
https://www.diyaudio.com/community/threads/symetrical-colpitts-oscillators.329968/post-5601529
The frequency of operation is ~100kHz, sinusoidal, and the I/O capacitance is <10pF, of which most is "dead" capacitance, from GND to GND.
This means that no dreaded "Y" capacitor is required, unlike a regular switcher.
The power required is +/-18V @ 30mA, + the LED, which the supply can easily provide
The case had just enough room to accomodate all the circuits, + the shielding and the class II insulation, but it would have meant an awkward ergonomy for the front panel, making the instrument unpleasant to use.
The solution I finally opted for was to attach the supply outside of the case, but it needed to be protected, mechanically and electrically.
I found this ad-hoc cover, fulfilling all the requirements:
Do you guess what it is?
Yes, a sardine can. It was the right size, and had all the desirable properties.
I over-painted it, maybe should I have left it natural?
The case had to be metallic, and steel, to minimize the influence of external magnetic and electrostatic perturbations.
The device had to be class II, to avoid ground loop issues, and it had to have a minimal primary/ secondary capacitance, because the tens of µA it creates can induce pV or nV drops along the shields of unbalanced devices.
In short, it had to be as quiet as possible.
A 50 Hz transformer inside was out of the question, because of the huge shield it would have required.
This left me with two possible solutions:
-A traditional wall-wart, connected with an umbilical
-An ultra low noise mains supply, small enough to fit inside the case
I didn't want the inconvenience of a wall-wart: I wanted something completely self-contained.
For the supply, I used this concept:
https://www.diyaudio.com/community/threads/symetrical-colpitts-oscillators.329968/post-5601529
The frequency of operation is ~100kHz, sinusoidal, and the I/O capacitance is <10pF, of which most is "dead" capacitance, from GND to GND.
This means that no dreaded "Y" capacitor is required, unlike a regular switcher.
The power required is +/-18V @ 30mA, + the LED, which the supply can easily provide
The case had just enough room to accomodate all the circuits, + the shielding and the class II insulation, but it would have meant an awkward ergonomy for the front panel, making the instrument unpleasant to use.
The solution I finally opted for was to attach the supply outside of the case, but it needed to be protected, mechanically and electrically.
I found this ad-hoc cover, fulfilling all the requirements:
Do you guess what it is?
Yes, a sardine can. It was the right size, and had all the desirable properties.
I over-painted it, maybe should I have left it natural?
To be truly creative you should have kept the sardines in the can as well. They would have added some more shielding 🙂
A new project I am busy with is a metal baking form to bake cakes in. It will be an amplifier. We probably have to reuse/recycle as everything metal seems to be rare all of a sudden.
I love a creative re-purposing solution to a non-obvious problem!
I would have painted it matt black to match the rest of the casework, but that's a personal preference.
I would have painted it matt black to match the rest of the casework, but that's a personal preference.
Nice!
An old cookie tin makes a "sweet" cover for a toroidal transformer. Keeping the original paint adds to the DIY "flavour" / character.
An old cookie tin makes a "sweet" cover for a toroidal transformer. Keeping the original paint adds to the DIY "flavour" / character.
Cookie tin the preferred method of shielding at Linear Tech before they were purchased by Analog Devices. They have other benefits as well.
Tried to buy the shielded laboratory from my University, but they has torn it down before I had a chance.
I have one "ultra-ultra quiet supply" which uses a crystal oscillator, self wound trafo and LM4780 chipamp to supply a bipolar voltage for measurement purposes. Regrettably LM4780 is no longer in production.