Dummy Load Characteristics

I am currently in the process of building a dummy load capable of 500 watts. With the amp project capable of potentially a couple hundred watts, being able to dissipate a lot of power for extended periods of time is kind of needed. Yeah i want to run full key down for like an hour into a dummy load to really stress test the whole amp design.

The dummy load will use 25 100 watt 50ohm resistors in a 5 in series 5 in parallel configuration, mounted on a huge heat sink with fans. Hopefully this will dissipate the amount of heat that will be generated. Anyway, this got me thinking about the dummy load that i built many years ago using 20 5w ebay resistors in parallel and how well it actually performed. Back then i did not have the test gear to actually characterise it. But i have had a VNA for a while now, well, not really a VNA more a glorified S11 meter that measures via vectors. 🙂

So i plugged it in and gave it a look and well, to my surprise it was not all that bad. 1.05 to 1 from 1Mhz to 60Mhz, which is the limit of measurement for my VNA. I am only interested in HF so more than good enough for the kinds of builds I make. I think it would be usable into the 2m band, but from there on parasitic’s would really take over and make it more or less unusable.

After doing this measurement, i kind of decided to order and better VNA and be able to do 2 port measurements and have extended frequency range. I have some other things i want to build like rf taps etc that would really benefit from having a full characterisation done. Oh and filters.

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Volt and Ammeter

Volt and Ammeter

So i discovered something last week or so ago while building power amplifiers. Its a total pain the the butt to be pulling everything apart to wire in the DMM to measure current when setting the bias for fet devices. Also that the meters in my Chinese power supplies are just about useless for anything less than an amp.

So with that in mind, i grabbed out a couple of analog panel meters that I have had sitting here for just about for forever, drew up some artwork in my favorite 2d design package and then went out into the garage to cut and bend some sheet aluminum into something that resembles a case.

Yeah the case is not my best work, but its perfectly serviceable and does the job that its meant to do. IE it houses and protects the meters. So now i can plug this into the power supply, plug the device under test into this and monitor both current and voltage while keeping my DMM free to be used for other jobs.

Yeah I know that you can use 2DMM and achieve the same result, but why spend a lot on an expensive DMM that is going to sit doing nothing most of the time, when for about $10 you can make a bit of test gear that will be good enough.

73, Rob.

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YiHUA Power Supplies

A couple of months back I bought a pair of cheap lab power supplies off ebay that turned out to be crap. 20mv quoted ripple was closer to 200mv. I managed to clean one up with some massive filtering, but it was impractical and still the ripple and switch noise was to high.
So while searching for something else i came across these on ebay. 28v, 10A and a linear supply. They weight about 8Kg each. So did some googeling to see if there are any issue and there is one, over shoot when you cycle the power on and off and there is an easy fix to that by adding a cap. But hey, who cycles power supplies on and off anyway.
So i grabbed one and then checked the ripple, about 10mv. Thats perfect, so grabbed another one, cause you just cannot have enough power supplies.

So it looks like I am becoming a HeeHaa fan boy. 2 power supplies and soldering and reword station. Oh the humanity LOL

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New Power Supplies

So i was having trouble the other day with a circuit I was testing and my oscilloscope going bonkers. I did not know what was going on, so I did some investigating and low and behold my homebrew power supply was acting up and had massive amounts of noise and ripple on all the power rails. So i grabbed a couple of these off ebay cheep, thinking well if they are crap i will send them back. Checked the ripple on the output and its not bad at about 100mv, not the greatest, but not bad enough to cause the scope to act up. Time will tell how they go. About the only thing I really need is a 3rd channel. I often need to run 2 12v rails and a 5v. So im going to have to hack up some dual leads. I went looking for double adaptors with 4mm banana and dual terminals, but they just don’t exist anywhere i can find anyway LOL

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Vacuum Desolderer

I am no fan of desoldering, so when i saw this tool on ebay i was skeptical that it would perform any good. So i gave it a whirl today to see if it was going back or staying. Well its staying, its actually pretty damn good. Its not cheep for Chinese crap but it does the job. Nozzle over a pin, wait a second or 2 for the solder to melt, hit the button and suck it out.

Proof is kind of in the pudding on this one, made short work of desoldering the headers on this Arduino Mega. Pins just fall out as you go and the plated though holes are nice and clean. Facebooktwittergoogle_plusredditpinterestlinkedin

Red Pitaya VNA

I got new toys. A VNA addon for the red pitaya. More on this as I work out how the hell it works LOL 😉

Well i managed to get it connected and working and I ran a test on my 40m dipole. Seems to be working now i need to learn how to use it for serious things. Not just checking swr on antenna 🙂Facebooktwittergoogle_plusredditpinterestlinkedin

Oh The Humanity

So i sat here this morning with a bag of 50 crystals playing a game of what is the resonant frequency. Thoroughly boring I can assure you and I am not sure how the ladies at uBitx Labs in India can sort through thousands to get sets of 8. But alas I have done enough for now to get 2 sets that are matched at a guess to within 50hz, being that my frequency counter is only good to 100hz.

One thing that I would not have predicted is the distribution in the resonant frequencies. Its like buck shot, all over the place and the other thing I would not have predicted is the number of dead crystals or ones with so low Q they might as well be dead being 8, that seems like a very high death rate to me. I still have 2 more bags to sort, but thats not happening today, i have 16 matched crystals I need for 2 filters and am moving on to the next thing.

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Crystal Frequency Tester

Need a tool, make a tool as they say. Well i need to sort these 100’s of xtals into groups of similar frequency for use in xtal filters for an upcoming homebrew receiver project I have started. I have like 200 12mhz to sort through, so i should be good for a few filters at least. Its just a matter now i sitting here for a few hours and sort them all. Not looking forward to that. 🙂

No rocket surgery here, just your basic oscillator and buffer circuit that can be found all over the internet. Its simple, it works and it is reliable.

Built it up Manhattan style on a scrap of copper clad pcb. Yeah i love MeSquares to all you MeSquare fan boys out there. LOL

The Joy Of Oscillation, it works. 3 xtals checked, 100’s left to go.

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Lab Power Supply Final Update

Just A Bunch Of PSU Modules?

All good projects need a name don’t they? Sadly I think just a bunch of modules has been taken by Pete Juliano for one of his many radio projects. Oh well, such is life. I was thinking Buck This or Well Switched or Ebay Wonder might also do. Anyway, i have now completed the project. It has its lid on and it works. Here are the final images, a couple of them are even hi-res using the SLR and not my mobile.

Here is one of those Tribal Knowledge moments that we often hear about. This is 6 core shielded computer cable, I used it to wire up the pots. It helps keep things nice and neat.

It worked out quite ok, next time I will spend some more time measuring then cutting just to the right size.

I do not like playing with mains voltage or anything that can kill you. It is why i went for a modular option, so that when I am done there is no exposed 240v anywhere inside the case that dummy hear can touch while doing da-finger-poken inside the case. Not that the lid will come off hardly every, i just am very cautious of anything that can kill me.

Here i have started to wire up the DC power side of things. I am not all that much of a neat freak like my friend Simon 6MSC, who can make toriod winding look like an art form, but I am trying to keep things somewhat tidy.

Now we are getting into the meat of it, things are all wired up and zippy tied all over the place.

All the power is done and dusted.    

An overhead shot of the whole guts of it.

Now i do not pretend to be a metal worker. I was really happy with the bottom half of the case. But, alas i turned the top half into an abortion. First off i folded the metal on the wrong side of the line and made it to narrow so that it would not fit properly. So I had to try and fold it flat and start again. Sure i could have scrapped it and started over, but hey, why waste such a large sheet of metal. So i folded it a 2nd time, this time it fit, but one side is 5mm short. I can live with that. I know that the next time i make a case, which will not be all that far away, will be much neater now that I know what I am doing. Thanks for reading, hope you enjoyed the project.Facebooktwittergoogle_plusredditpinterestlinkedin