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  • Author Author: michaelkellett
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A Really Simple HF True RMS detector

michaelkellett

In a recent thread Shabaz was wondering how easy it might be to make a wideband true RMS detector  using a surface mount thermistor and resistor.

This is a first very crude and simple experiment to find out.

I soldered tiny bare wires to a 10K 0603 thermistor and a 100R 0805 resistor and glued the pair together. Its supported by its wires on a piece of strip board.

image

It needed a little bit of wind shielding since its a warm day and I had a fan on.

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I drove the resistor with a range of pulses and settled on 0.66V at 0.01Hz 50% duty for plotting. This is 4.35mW. An 0805 resistor can take 1t least 100mW so we are operating at 13.6dB below full scale.

image

The response time is a bit slow but I think 0603 or 0402 parts would sort that !

The signal was clearly detectable but rather swamped by ambient thermal noise at 70mv (-43dB ref 100mW).

A differential sensor with an ambient sensing thermistor and better thermal design might well fix that.

Time for tea, more later.

MK

  • in reply to shabaz

    It is ENIG. Because its so small (20mm square) I had to order 40 before the cost of the boards and stencil was about the same as the shipping so I shall have plenty to try.

    I'm hoping the mill will do a decent job with a proper fibreglass cutter. If I make it too thin there may be problems with it warping when being soldered.

    MK

  • in reply to michaelkellett

    Hi Michael,

    The board design looks fantastic. Looking forward to seeing how this goes! It's a brilliant idea to physically remove off the material. 

    Especially if it's ENIG, hopefully it should be super-flat. I tried removing material a while back for trying to make a PCB slightly flexible (for a Halloween project, not for any technical reason), and only got to about 0.5mm, but mine wasn't a very flat board to start with, it had bits of solder on it etc. I was using FR2, so it was super brittle, and cracked occasionally (whenever the milling bit got close to any hole, it snagged on it I guess). 

    image

  • I've complete the initial pcb design.

    After  a lot of reflection and thinking about how other devices are put together I decided that it's important that there should be a clear thermal path from the resistor to the "cold sink" at controlled and measured temperature and that heat loss via this path should be the dominant source of cooling for the resistor. I could have done a lot of sums but decided to just make it instead.

    The design uses one of   ideas for layout with the resistors soldered to the pcb and the thermistor glues crosswise on top. The twist is that I shall machine most of the board away from the back rather than just have  a hole under the resistor. The machining will be 4mm diameter and I'll attempt to get to within 0.2mm of the top surface of the board.

    It'll be  a couple of weeks before i see any boards.

    image

    The additional 4 thermistors are to measure the temperature of the board. It has 4 layers and will be 0.8mm thick so a lot of it is copper.

    MK

  • in reply to shabaz

    I quite like this one - I may get time to do a pcb today.

    I've been thinking about thermal isolation from the rest of the board.

    MK

  • in reply to michaelkellett

    Another idea for the PCB layout:

    In this layout, the hole is 0.8mm. Since a typical resistor is a bit thinner than a typical thermistor, the resistor is soldered to the PCB first, it is the orange part in the diagram. It is likely about 0.3mm tall.

    Them, the blue thermistor is glued on top, and then soldered (the vertical gap is merely 0.3mm since that's the thickness of the resistor underneath, so it's easy for solder wire or paste to accommodate it.

    image