Very Low-Cost Pulse-Width Modulation (PWM), and Isolated Analog Measurements

RE: Very Low-Cost Pulse-Width Modulation (PWM), and Isolated Analog Measurements

shabaz — Tue, 12 Dec 2023 22:27:25 GMT

The high voltage measurement boards arrived, and I have soldered one up, to see if there are any glaring errors! Turns out there are. I'll write a separate blog so that it can be tracked outside of comments here!

RE: Very Low-Cost Pulse-Width Modulation (PWM), and Isolated Analog Measurements

shabaz — Sun, 26 Nov 2023 02:44:16 GMT

I was thinking of using a linear optocoupler, there is this figure 14B in the HCNR200 datasheet.

Translated into this circuit, for up to 1kV measurement with about 1uA loading, and I hope with OK bandwidth:

It's large enough that it may be better to just build this on a PCB rather than other prototyping methods. It would be awesome if people could review it, and I'll lay it out on a PCB in the next few days once it has been scrutinized. Also, anyone interested to build it? It would be nice to debug together!

Although the £1/$1 price is left in the rear-view : ( it is maybe £20-£25 of parts in single qty.

Obviously not laid out like this, but just to see the approx soldering effort:

RE: Very Low-Cost Pulse-Width Modulation (PWM), and Isolated Analog Measurements

DAB — Sat, 25 Nov 2023 20:56:05 GMT

Very nice find.

Now I have to go look in my stash to see if I have any of the chips.

RE: Very Low-Cost Pulse-Width Modulation (PWM), and Isolated Analog Measurements

BigG — Sat, 25 Nov 2023 12:09:39 GMT

Awesome. Thanks for sharing. I'm still digesting the datasheet. As a starter I'm curious to know why the recommended minimum input voltage is 7V. EDIT. Ah, I think you've answered it in your blog - namely for the 5V regulator.

Another question. I really like the wide PWM frequency range and there is even a fancy "soft start" feature. But I cannot work out how you adjust the duty cycle. It looks like R1 and C1 just set the frequency. Is this what does the feedback input (pin 3) does. I also could not quite make sense of these voltage outputs shown in the datasheet.

RE: Very Low-Cost Pulse-Width Modulation (PWM), and Isolated Analog Measurements

michaelkellett — Sat, 25 Nov 2023 11:05:02 GMT

Interesting - last time I wanted to this I designed a 4 channel PSU with a different kind of switching power supply feedback coupler (FOD2741A). It was horribly non linear and I was left in a bit of hole with boards that didn't work.

Luckily the FOD2741A came in an SM package with 8 pin DIP dimensions so I designed a tiny board with an ST 8 pin ARM with a 12 bit ADC. The board was small enough to solder in place of the FOD2741A and used the same pin out.

The only problem was that the main PSU expected to get analogue signals from the FOD2741A - so I reconfigured its analogue input pins for slow bit banged NRZ serial input and the little add on processor sent continuous analogue readings at 300 baud or so.

FOD2741A

Replacement using micro with 12 bit ADC

Board layout - careful trimming of the board needed to expose "mousebite" contacts. There was enough space to overlap the FOD footprint a bit.

Another time (I'm hoping there won't be !) I'll consider the TL494.

RE: Very Low-Cost Pulse-Width Modulation (PWM), and Isolated Analog Measurements

dougw — Sat, 25 Nov 2023 06:06:06 GMT

Nice application. There are a bunch of interesting old chips like the LM10 and the LM2907 that found strange uses.