Friday, March 4, 2016

The Death of Electronics as a Hobby?



"The reports of my death are greatly exaggerated"

I tried to reorder some chips recently, that I have had no issue getting over the years. I'm told they are no longer manufactured. I contacted Maxim about the part, and received the following response:


We have very few through hole devices, and are actually moving away from through hole designs. So unfortunately, we do not have a through hole device that matches your requirements.

Industry is following the commercial customer, the money. Manufacturers use robotics to build boards, they order millions of a chip. Hobbyists use their hands, they order thousands of a chip. Does it pay to make a through hole version when most of your sales are smd?
This will continue the trend for companies like Sparkfun, Adafruit and ICBreakout to produce breakout boards so that those of us who like to experiment and build stuff aren't reduced to burning our fingers and destroying our eyesight trying to manipulate parts designed to be deployed by machines, but at a much higher cost than the chips alone, and reduced availability.

Monday, February 29, 2016

AC Current Monitoring, Current Transformers and Burden Resistors

In past articles we have covered monitoring DC current for solar and wind based applications.

Monitoring AC (grid or grid tie solar) is a bit trickier as voltage flows bidirectionally, and it's harder to insert a shunt inline. We use what is called a Current Transformer, or CT. It clips over your existing wires into your breaker box, so no disrupting the existing lines is necessary. In order to monitor AC current with a CT, you need to put a resistor across its output. This is called a burden resistor. To determine the correct resistance of this resistor, we need a few pieces of information.

As you can see in the picture, the maximum current rating, and the current output at that rating is displayed on the CT. Its rated for 120 amps, and outputs 40ma at that draw. If we divide the 120a by the .040 amp (40ma) output, we get 3000, which is the number of turns on the secondary.

Now, we need to know the Primary Peak Current (PPC), so we Multiply the RMS Current (120A) by the Square Root of 2 (1.414). This gives us a PPC of 169.68 Amps. We can calculate the Secondary Peak Current (SPC) by dividing the PPC by the number of turns (3000), so the SPC = .05656 amps.

Now, to determine the Ideal Burden Resistance. We need to know the AREF voltage of our microcontroller, which in the case of the Arduino UNO, is 5v. We divide the AREF by 2, and that by the SPC, so our calculation looks like this:

(AREF/2)/SPC
(5/2) / .05656 = 44.2 Ohms

Therefore, 44.2 Ohms (a standard 1% value) is the ideal resistor to put across the output of the CT to measure current. A complete measuring circuit and Arduino code to follow shortly.


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