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Friday, 15 December 2017

FeelTech - Really?


I bought myself a "FeelTech FY3200S" of eBay; this is a Chinese manufactured Arbitary Function Signal Generator/Counter that claims an output up to 24 MHz.

Now, the unit seems to work to spec and for the money it represents excellent value. The microprocessor and DDS board are good but there is a problem..... I can measure 97+V AC between the ground terminal of the output connectors and my mains earth - ouch!

This is because there is a very cheap SMPS inside with loads of leakage:

So, in the spirit of homebrew I have constructed a simple linear PSU:

I've gone to town a little on the heatsync, but you cant over do such a thing and the dissipation in the 5V regulator will be quite high. I am feeding the circuit above with a 1A rated 12-0-12 standard transformer:

As you can see there was loads of space in the case, I've mounted the three regulators on the heatsync and then attached that to the base of the case. The bolts securing the devices are countersunk on the underside of the lump of metal.

One thing I learned is that the 7912 -12V regulator seems to need a small load for it to regulate properly, hence I have added a 20mA drain on each rail in the form of an LED. More to add a small load than to provide an indication of function. I haven't located the LEDs on the front panel, they will stay buried inside the box.

Seems to work well:

I will probably now also add isolation between the USB port and the main board as currently when I connect my PC I am grounding the 0V lines of the generator; might be useful at some point to keep them floating and completely isolated from mains earth.

Good, egh?

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