Jun 30

Seven Decade Programmable Resistor – A Low Cost Solution

I have spent a lot of time over the years prototyping electronic circuits and the amount of resistors that have ended up in the trash because they are so cheap you don’t bother to keep them tidy or organised once you take them out of their organised storage – you know the story. One potential solution to this is a programmable resistance box but the problem with these things are they are bulky and expensive and do not lend themselves well to breadboard prototyping. The cost of construction means they are typically the reserve of high-precision resistance boxes. I have a CROPICO RBB6E resistance box in my lab which I open up to have a look inside, its really well made, mostly by hand too, far too nice to abuse in prototyping….

I looked around at what is available but did not find a solution that met my own requirements so I decided to design something simple myself. I also wanted to make a simple project to get manufactured by machine which apart from other things requires reasonable volume, and I thought this project would be useful enough to others that I should get some made and make them available.

Various Pictures


Project Discussions

Project discussion thread on EEVBlog forums

Other Resources I mention in the Video

Cheap Resistance Substitution Box by @vtl on the EEVBlog
Cheap Resistance Substitution Box – Hack-a-day

If you do want one of these I have these for sale on “tindie.com”, the item listing is here:


I have also listed these on e-bay, you can search for “gerrysweeney.com” which will find the listing.

As ever, if you have any comments, suggestions or feedback, please use the form at bottom of this page

Thanks for watching

Jun 24

I Need 10MHz – how hard can it be!

It all started when I wanted to calibrate my HP 53131A universal counter, which as it turns out probably has one of the crappiest and most disappointing standard oscillators ever put into a frequency counter, HP you should bow your head in shame….oh of course I forgot, a half reasonable oscillator is an “optional extra” when you by HP/Agilent – of course it is….anyway, on with the job at hand

If you have or want to play with an FE-5680A Rubidium Frequency Standard or an OSCILLOQUARTZ OCXO 8663-XS or a HP 53131A Counter or a Racal Dana 1999 counter or similar then this video will most likely be of interest 🙂 what I am trying to get is a predicable and reliable frequency and standard for my home lab.

I guess I will let the video do the talking on this one….

Here are a whole bunch of useful links that relate to this video (there are many more too if you search around the web)

Thanks to all of the authors and content creators for the above information. Thanks for watching.

Jun 23

Omnitronic EDP 1000 Watt Class D Amplifier Teardown

Here I describe the basic operation and topology of a Class D amplifier and then tear down an Omnitronic 1000W amplifier to have a look and see what is inside. The big advantage of a Class D amplifier is the compactness and overall power efficiency. However, Class D amps are generally not used in High Fidelity application because of the limitations in dynamic range and distortion performance that can be achieved.

Here are the specifications scanned in from the user manual.

Hope the information is of some use. Thank you for watching.

Jun 16

Fully Programmable Modular Bench Power Supply – Part 14

I said last time I would look at the software and firmware for this post but I decided I needed to spend more time on the regulator to address the DC response issues I had observed, I felt this was more important to get right so this article focuses on that. The good news is, I have achieved good results and I believe the regulator design is complete – at least for the high current version of the module (0-8V 0-8A).

I had problems with the regulator over shooting by about 500mV when transitioning from a high power load to a low power load. There were two main problems, the first was down to the inductance of the wiring on the output, that creates a problem which in turn was amplified by the relatively slow response of the various amplifier stages. I have achieved significant improvement on my previous measurements by tuning response times through the various stages, this simply required lowing the impedance through the various stages to ensure the servo was able to respond more quickly to changes on the output. This was achieved by adding C24, C25, C39, C40, C41, C43, C45, C46, C47 and C48 to the input and driver stages. I also needed to throttle back the fast rise time of the control drive from the DAC to follow behind the response curve of the regulator circuit. These changes mean the regulator now delivers a clean and very acceptable dynamic response to fast-switching load conditions.

The Schematic

Here is the latest schematic which is at version 0.7 including all of the latest changes. The most significant changes are on sheet 3.

Various Scope Traces from Testing

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I now need to lay out the new PCB and get some ordered, I will do this over the next couple of weeks. I will definitely cover the development environment and the firmware in the next post as well as the serial protocol used to control and monitor the module.

Thanks for watching.

Jun 08

Compare Video Quality of a Canon HF S21 side-by-side a Sony HXR-NX5

This is a quick post to demonstrate the video quality of two different cameras. This is not meant to be a side-by-side review and there is no assumption on my part that these two cameras are in the same class, because they are not. What this shows is why you might want to consider an upgrade form a high end consumer class video camera to a low end professional class video camera. The two cameras I compare are the Canon HF S21 and the Sony HXR-NX5U.

I decided to upgrade myself for a number of reasons…

  • More dynamic colour range
  • Less noise in lower light conditions
  • Better optics, specifically optical zoom for close-up work
  • Pro audio connectivity and capability
  • I purchased the NX5 camera at a good price at just £1,400

The outcome is pretty self evident as you will see in the video. I have made no changes to the two clips, each is a native AVCHD import directly from the SD card into Final Cut Pro X, I made no changes to the video but FCPX did have to transcode because the two camera’s shoot at different frame rates.

The quality of the HD video I am able to produce now should be much better, it will be good to see how improved the detailing is when I am zoomed into those small surface mount components…

Thanks for watching, see you next time.