No, that’s not a euphemism.
Recently, I purchased a new multimeter, the now widely used UNI-T UT61E, which as far as cheap chinese multimeters go is pretty darn good. After watching various teardowns and tests courtesy of Mr. Martin Lorton, I was actually reasonably impressed with the functionality and accuracy of the meter and really wanted one to replace my Mastech “shitty piece of shit” meter which I’ve grown to loathe over the years. Most annoyingly, it had stopped displaying decimal points almost entirely. An internal inspection revealed it to be the switch contacts not quite making contact on the volts ranges (which is… the most important range!). This led to some almost-accidents as I had to guess where that decimal point would be. Is it 2V? 20V? 200V? Bugger this, time to upgrade and get a decent meter. Not that I won’t use the Mastech still, always good to have two meters, and the Mastech measures temperature which the UT61E doesn’t.
It’s been a week of tweaking and assembly, mostly. This is both the part I love the most, and dread the most. On the one hand, it’s awesome to see the project come together and not just be a dangerous collection of bits on my bench. On the other hand – drilling into aluminium is such a chore!
Anyway, here it is thus far:
Front panel view, not perfect, needs labelling, but good enough!
As I said in my last post. I am mostly happy at how the Loaded Dummy performs, but with the caveat of some inaccurate voltmeter measurements.
After some more tuning and testing, I’ve identified two issues that leave the project just shy of my expectations. Neither are deal breakers, but are enough to make me want to investigate further and see if I could improve if not solve them.
Bit of a rats maze, but she works! All the bits are built.
For the past week and change, ever since getting my boards from the fab in the post, I’ve been assembling and testing and fiddling with my dummy load project. I outlined a few beginners mistakes in my last mention, and since, I’ve disovered a few more! Don’t worry, it still works fine enough and I’m still generally pleased with it. I’m even laughing at the absurdity of the bugs I’ve found.
Anyway, here’s the state of it now:
All the bits waving in the breeze, but she works!
Hurray! I got my PCBs via DHL this morning. So happy to receive them and hold in my hand my first professionally made PCB.
Digging up an older idea to fill a newer void
Buzzing along my electronics wave, hot off the success of my nixie tube clock, and dummy load projects, I looked at what else I had going on and found that several of them were in need of panel meters, something I’ve had a really hard time finding at a decent price. You’d figure in this day and age they’d be easier to acquire, not harder.
Anyway, so I dug up my original idea of using an ICL7107 based 7-segment panel meter. I always loved this idea, mainly because I love meters but in particular I love LED 7-seg displays over LCD just for readability and sheer cool factor.
My earlier attempt at a panel meter
Well after humming and hawing, I finally just pushed the button on the dummy load project and ordered the PCBs from JLCPCB (via EasyEDA). I figured I can’t go too wrong, it seems most electronics youtubers seem to like it fine. I think the only thing that could go wrong is I made some mistake on the PCB layout, though I did check it over three times. You never know, Murphy might show up. Still, either way it will be a learning experience and if this works out (as it likely will) I will have more boards made up. I find this all very exciting!
Here is the layout I sent them:
Completely re-done from yesterday, includes the 12V supply
Institutionalized Dumpster Dive
Over the weekend, I had occasion (in other words I made an occasion) to visit A1 parts and surplus in Etobicoke. I have mentioned it before as a candyland for the junk enthusiast and this is essentially correct. Despite its remote location, it is well worth the visit, just be prepared to dig and expect no help from the rather cantankerous guy at the counter. Total comic book guy for the electronics store which I find endearing rather than offensive. Anyway, if you ever end up there, be prepared to wade through tons of unsorted junk, find a few gems, and score a few surplus deals.
Just a quick test and a quick solution
Had a few minutes to just check up on why the load would change when the voltage does. A simple test of trying a higher voltage power supply seemed to do the trick. Stayed within 10mA of where it should, and probably did better than that, but my multimeter can’t measure that. The new power supply is a 200mA 12V wall wort which is very unregulated. I measure 16V off the bugger with no load. Makes me wonder if what is printed on the box is merely a suggestion. Still, nice to have the headroom. Now the opamps have enough swing to really kick that MOSFET into regulating the current flow reliably.
Some quick tests work out quite well
Had a tiny pocket of time with which to test a few things with my aforementioned dummy load I just build. In looking through my collection of junked wall wort power supplies I found a low power one, a 9V 210mA. I wanted to check and see if the voltage headroom for the opamp would be worse. As it turns out, the stated 9V is actually 11.3ishV but whatever. I knew the supply would only need to deliver a few milliamps so I ventured to see how much exactly.