TENMA 72-10495 2CH 0-30V 5A Power Supply

This is my new power supply,  I was tried of using my old one which was a complete pain to use as the voltage and current adjustment was far too course;  this TENMA one has a digital pot which allows for course and fine adjustment.  Initially when it arrived I let it warm up to ambient room temperature before I switched it on;  then I tested both channels with both light and moderate loads and it all seems to work really nice;  the fan speed is variable based on the load,  and when you turn the pot through the entire voltage range you can hear it using relays to switch through the various transformer taps.

A few weeks later...

Well now I've had chance to play around with it a bit I must say it's really quite nice.  my only niggle with the unit is in the way that when you set either a voltage or current directly it always defaults to voltage and always at the volt unit,  this is a pain if you need to periodically tweak the millivolt, say if you were simulating a 0 - 5 volt sensor.  the other niggle is you can only change the voltage and current while in edit mode,  and after a few seconds it auto exits edit mode;  and entering edit mode always takes you to voltage and the volt unit and not the last unit (millivolt) setting.

I haven't tested the power-on voltage curve under max current conditions as I don't have a dummy load, but I'm guessing there wouldn't be much overshoot to worry about.

GM Tubes

Some GM Tubes for a project I plan to build some time next year.

A Posse of Transformers

Nice little Ebay snip;  30 transformers for £12 - perfect for my kiln controller boards  ;-)

DC Voltage Calibrator

This is something I bought from Ebay for £30.  It can output a voltage between 0.01uV to 9.9999v @30mA,   It contains a precision zener diode which is used as a reference input to a FET chopper amplifier operating in feedback stabilized mode.  The gain is determined by a set of precision metal film resistors which are selected by the 5 decade thumbwheel.

Looks like it was circa 1987.

The battery pack consists of 11 x 1.2 Volt AA Ni-Cads @ 700mA.  They were assembled in 1998 and they starting to leak.

Because I didn't want to go to the trouble and expense to replace the batteries, so I just removed them from the unit instead.

Experiment - Simulating VR Signals

Over the last couple nights i've been looking at creating some simulated variable reluctance sensor signals.

The scope picture above shows the finished crank and phase signal waveforms being output from the bread-board circuit into channel 1 & 2 of the oscilloscope.

Although I was quite pleased with the results it would be nice to develop the project a little more perhaps when I have more time to do so.

The circuit consisted of a small PIC uC that outputs the desired shaped waveform into a 6 bit resistor ladder DAC;   then the DC signal was fed into a single operational amplifier which negatively offsets the DC signal to create a nice AC signal;  after that the signal is simply multiplexed to create both the crank and phase.





Once I had increased the total number of sample points in my waveform the shape started to look much better.

As you can see I used quite a lot of trimmer POTs;  I couldn't find any 2R values in my parts box, so instead of using two-of  the same value Rs,  I just used some cheap POTs I had.

Overall the circuit worked well.

First SMD Board

The other week I ordered some boards to be made up from a project i'm working on, this was my first SMD board so I wasn't sure how it would all turn out.

My original plan of attack was to turn a low cost toaster into a reflow oven;  but after I wrote the PIC code and then chewed it over for a day or two,  I realised that the 10 sec steps I was using would be too much and I might potentially damage some parts if it ramped up too quickly.

So I opted to use a cheap DIY hot air gun;  so I stuffed a DMM thermocouple into one of the board mounting holes and then using a timer, manually ramped the temperature keeping a close eye on the elapsed seconds and temperature on the DMM display.

Although this seemed to work fine, I think I'm going to rework the PIC code to use 3 sec steps instead, that way we shouldn't have any big ramp jumps. I much prefer the idea of putting a board into an oven and simply pressing a button, instead of waving a hot air gun over the board.

The results of the hot air gun can be seen below; although It worked ok I was a bit shy using the solder  paste so I should have used more;  in the end I had to apply more solder using the soldering iron to a couple resistors and capacitors;  but the ICs reflowed really nicely.

GenRad 1657 RLC Digibridge

For a while now I've been after a LCR meter but I wasn't sure what to get - now I'm always a sucker for older test gear; so, in the end I saw this on Ebay and decided to buy it.

It's quite an early piece of test equipment and apparently it was the first ever digital bridge meter.

As you can see from the PCB build style and the white ceramic 6503 processor with a date code of 1977 it's pretty vintage stuff.





I had to hook up some temporary 'croc' clips but I will order a proper set of kelvin clips when I get time.

It was sold with a display fault which turned out to be a problem with one of the 7 SEGMENT LED drivers; So I ordered some replacements and fitted them,  that seemed to fix the 7 SEGMENT fault but it was still glitchy for some reason;  I then noticed one of the range LEDs was dull,  so I replaced that too;  after that the display seemed to work fine!

I checked out the power supply; the 5V rail was slightly under but looking at the datasheet for the 5V regulator it was still within the manufacturers specifications.

I measured some CAPs, and it seemed great;  I then measured a few Inductors and it gave sensible results.

Below are a few pictures of the internals;  notice the very large smoothing CAPs on the power supply.

The above pic shows how the display board fits to the main board.

The above pic shows the white ceramic 6503 processor;   the other white package to the right of the CPU is a precision 25 Mhz crystal oscillator.

The two chips to the left of the CPU are ROMs;  one of the ROMs provide the code to run the CPU;  and the other provides the sinusoidal waveform data which is fed into a DAC.

The display board and the row of 9 driver chips;  one for each of 7 SEGMENT displays.

Whilst de-soldering and removing one of the LEDs, the LED literally fall apart.