SOAR confusion

We're thinking that nichrome wire would be pretty tough, namely a wirewould resistor. I'm testing some now and they seem fine.

Next we'll need a tiny fan.

The microstructure of thickfilms seems to make them vulnerable to progressive damage from short high-power pulses. They seem to grow surface cracks over time, especially near laser trims that cause current crowding. I saw that during my RF attenuator experiments too,

John Larkin Highland Tech Glen Canyon Design Center Lunatic Fringe Electronics

For 2us pulses?

What is your long term accuracy requirements?

I would have thought carbon MELFs would have been fine. And cheaper.

But still a whole lot better than air.>

You'd need very regular bits of diamonds to make a difference

The two statements aren't necessarily related. Badly constructed experiments can mislead potential customers. You obviously produced adequate gradient drivers. This makes them adequately designed. Optimisation requires a fairly deep understanding of what you are doing, and it's rarely worth expending that much effort for a relatively small market. The last time I was in hospital - for just two days - I was scanned a couple of times, so the market may be growing. If you aren't still selling gradient drivers, maybe someone else put in the extra effort.

But inductive. There are non-inductive winding schemes, but they tend to be more less-inductive than non-inductive.

Carbon resistors have a negative temperature coefficient of resistance, and if you drive them sufficiently hard, you concentrate the current into hot channels.

You can't use them in intrinsically safe designs for that reason. Early on one Colin Hunter, who had sat on the intrinsic safety committee, showed us a demonstration set-up up that put 1A through a 10k 600mW film resistor for a few minutes. It still measure 10k afterwards, but there was a narrow dark line on the colour bands where the hot channel had formed.

You can form the hot channels very quickly - Colin Hunter's demonstration depended on that. The core of a hot channel is a very narrow path indeed and doesn't have much thermal mass.

There's a upper limit on the current you can put through an NTC thermistor for exactly the same reason, though the problem doesn't show up as a hot channel but rather as an instability of resistance as the various possible current paths change resistance as the different regions within the thermistor warm up differently.

Would it be better to anneal the copper after machining, so as to relieve the stresses and make it more malleable? It would then flow and conform to the surface of the devices even better than just a machined surface.

For the ultimate thermal contact, machine the surface flat and then interpose a thin layer of pure gold between the copper heat spreader and the copper base of the transistor. Heating this under pressure will create a diffusion weld. The pressure could come from the mounting clips. The gold could be electroplated so as to avoid having to handle delicate gold leaf. Avoid using the type of gold used for pcb edge contacts as this contains a small amount of nickel or cobalt to harden it. It might be necessary to strip off the tin plating from the transistor heat sink tab for this to work properly. Back in the real world, simply soldering the transistors onto the copper heat spreader might be almost as effective. John

The 1-ohm 5-watt surface mount part that I'm testing measures 220 nH, and I'll use two, essentially in parallel. That's OK for my laser driver.

In another product I put two inductors nearby, which doubles the net inductance. If I flip the direction of one, it will reduce inductance.

Right. I expect that any material with an amorphous film element will be damaged by big power pulses.

John Larkin Highland Tech Glen Canyon Design Center Lunatic Fringe Electronics

It might also effectively wreck the transistors.

Transistors are made by forming semiconducting junction by doping the silicon surface. Make the device hot enough for long enough and the dopants move around.

Soldering surface mount components onto a printed circuit requires you to set up a thermal profile that guarantees that the active components aren't too hot for too long.

This is cool,

John Larkin Highland Tech Glen Canyon Design Center Lunatic Fringe Electronics

True, but it is usually the plastic encapsulation that fails first (except for extreme overloads).

Our gradient drivers were for analytical chemistry NMR, not for medical MRI. But some were actually used for micro-imaging.

Agilent bought Varian and killed off the NMR division. Superconductive magnets are expensive and there are new techniques. Bruker still makes NMR systems.

NMR was fun for a decade or so. But nowadays, things keep changing faster, so no company should bet the farm on one technology or one product line.

John Larkin Highland Tech Glen Canyon Design Center Lunatic Fringe Electronics

It is wirewound, combined with local capacitance it may well resonate in the GHz range.

You missed the point. If the amorphous film element has negative temperature coefficient of resistance, it won't be damaged - it will just form a hot channel and let lots of current through without sustaining any permanent damage, though anything in series with it may not be as lucky. Once the hot channel cools down it will work just as it did before.

Being 1 ohm and over an inch long, leads included, of course it will resonate somewhere in the GHz range. A paper clip will too.

If I bend and kink the leads to space it a bit above my board, the loop will be ballpark 50 nH, dominating the circuit.

John Larkin Highland Tech Glen Canyon Design Center Lunatic Fringe Electronics

Multiple experiments consistently disagree.

Try it.

John Larkin Highland Tech Glen Canyon Design Center Lunatic Fringe Electronics

The experiment in about 1975 that I witnessed did not destroy the carbon film resistor. It was carefully designed to show how hot channels worked. A less expert experimenter might not have got the same result, and a less receptive audience might not have understood what was being demonstrated.

Sounds like you're not insulating the heat source.

If so, then a soft copper foil introduced between the device and an aluminum spreader works pretty well as a gap filler and a conductive interface.

RL

Curious to know if these boxes are rated by a safety agency? (TUV, CSA, UL, etc.)

I see the CSA logo on the AC power entry module, but don't know if the whole box itself needs approval. One company I worked for had a safety engineering group that I could never pin down to give me an answer what the rules were.

Look at the AC wiring! There's no way it complies with UL or CE standards.

And there are hints of a single-point grounding scheme, even worse.

We prefer, when we can, to run our products from 24 VDC, from a wall wart or have the customer supply DC power. That helps a lot.

John Larkin Highland Tech Glen Canyon Design Center Lunatic Fringe Electronics