Saturday, September 23, 2017

Electronic "variac" / Variable AC power supply

To be fair I was looking for a real autotransformer, since I could not find one at a decent price or in stock on the local electric material shop, decided to build one electronic version that would do the job.
The need is just a way of testing old mains powered electronics devices with increasing input voltage. Another way of doing the same would be with a serial light bulb, in any case that's not "elegant" enough.

Anyway, here's the outcome, basically a shunt regulator between two AC transformers. Circuit is not mine (original from here) I only gave a little adaptation based on experimenting and what I had available.

 Some output values:

Here's the box:
I ended up not placing the voltmeter in the pictures since it would be a very close fit to the box so placed just a small battery indicator, 300uA with rectifier diode and 180K series resistor. The dashes mark 50, 100 and 150V AC.

The transformers were from salvaged electronics. Still visible leftovers from experimenting with other circuit.

Some more testing:

Based on the design and the components used, on a 15W load I cannot output more than 170V, that is enough for now, if I need the real "deal" then would for sure buy an autotransformer with more capacity.

The original schematic from I2VIU didn't gave full regulation from 0 to 220 so I made some small changes (also due to the fact of using different transformer values:
My schematic: (changes are with white background)

On the original page there is one schematic with regulation by feedback and with a comparator, I tried that design but was not having full voltage swing and used an extra transformer.
Here's the one that was tried but didn't suit.

One fact, there is not many AC variable power supplies schematics available on the Internet.

Hope this is useful to someone.

 Have a great day!

Tuesday, September 19, 2017

XR2206 frequency generator

...Looks like I have to build all the projects that I didn't managed during my youth.... back then my preferences were power supplies, audio circuits and transmitters.
In the audio department something useful was an audio frequency generator , unfortunately an interesting project back then used a XR2206 IC which cost'd the equivalent to 6 Eur by today's money, that's not much, it was 30 years ago.
So back then the best I could manage was the finger test and check for audio output...

Recently come across one of those chips and decided to give it a go.

Here's the final outcome

It would be cheaper to buy one ready made... half the fun.
Also added an Arduino frequency counter with bubble display.

Inside plenty of space:

...but not everything fitted on the front panel:

And some outputs from it:

..I didn't used the full potential of the chip, it can do also some modulation but left that as a possibility for the future, it also only outputs AC component on the triangle and sinusoidal output.

Schematic was similar this one with small adaptations:

 In the initial testing of the counter code...

In all fairness, do I need it? No. Did I used it already, No... but less one project in my "youth" list.... If I remember still have a multimeter with the ICL7106 and an STK audio amp to do :)

Have a nice week!

Saturday, September 16, 2017

MAX7219 8 Digit Display Module Interference

While building the 50 Mhz receiver I notice after boxing it that there was a lot of noise, whistle type, in the audio overriding any possible reception.

Interference most of the time comes from supply line and I was pretty sure that was the source since the Arduino and the display module were previous connected to separate usb power without it being noticed. I knew the problem was in the display because the original one got "toasted" and interference was over when removed.

This week a new display module arrived and the interference started again after assembly.

...pesky little things, next time will go back to the usual LCD type.

First action was to kind of "shielding" cable from Arduino nodule to display, that made no difference, the cabling carries control signals plus power and I roled thin foil over it.

Second action was to add a small filter in-line with power supply. That placed the interference at a much lower level (not noticeable in operation). I also added a filtering capacitor of 220uF at the Bitx board vcc input. In terms of power I have the main line in the box to the Bitx board then Arduino, display and VFO are regulated downstream, one regulator for the Arduino and display module (module power is supplied by the Arduino) and the other regulator for the VFO.

Filter for the MAX7219 module that cured the interference is this one:

..worked fine first time, components values were eye balled, I have used similar ones to audio amp's (specially the LM386).
after installing and probing, looked like this:

The initial interference was like this at the speaker terminals:
and like this at the MAX7219 display module vcc input:
...this is AC coupled at the oscilloscope, on a perfect system should be a flat line, that is; no AC component on the DC line.
From the Arduino side at the supply line for the module,

looked like this:

...the VFO signal is all over the "shop".

Now with the filter and even more audio output the signal looks more of an proper audio one:

I like when it works first time! Now I can continue with testing and validating the receiver design (it's already fully boxed), when done I will post more details.

Bellow the current state in terms for from panel an inside:

Have a great weekend!

Update 2018-12-29:

Lex, PH2LB, had similar interference from the MAX7219, he sorted with inductors in low pass filtering and some shielding on the power lines, here a nice build from him along with description:


Thursday, September 14, 2017

Metal sponge for soldering iron tip cleanup

I think i'm melting solder for over 30 years... the first Iron used was one built by my father with a large tip, even so, I managed to work some stuff with it.
In my teen years I bough the cheapest I could find (no other choice), basically crap and still manage to so some work.

Only in 2005 or so I bough a decent one, still no soldering station and all that good stuff, the support for the iron and the cleaning sponge was still in the low end of the quality scale...

That good soldering iron "died" in 2013...

...back to crap equipment and to miracle making (on the left). That was in Portugal, in Ireland I have the cheapest one I found along with a "control" station that is always in the max temperature.... it works!

The iron is not the original because I broke it when replacing the tip. The tip shakes a bit but still OK.

Today, 2017, I have something completely different and new, as you can see I got the latest and greatest of the cleaning devices.... a metal sponge!!! (anyhow was the cheapest)

Need get used to clean the tip with this new this method but now I feel a professional.... still no proper soldering station, the things have "pornographic" prices!

Previously, the methods I used to clean soldering iron tips are, in no particular order:

1 - Slam the iron against the bench
2 - Pass rapidly the tip of the finger on the tip of the iron, it will not burn if your fast enough (took me some time and burn skin to master this one)
3 - Use the wet sponge

Have a nice week!