Felt I needed a shopping trip, just because I could. If there is one thing about the electronics hobby you should know, is that you never have enough parts. This time I visited Supremetronic on college street again, if it is still called that since it merged with home hardware.
Though I realize I could probably order the parts online and not have to go anywhere, I brick and mortar shop for two reasons. First, I want to support the few remaining shops still in business. They are having a hard time competing with online retailers. Second, I like the experience. Sometimes it’s damn hard to tell what I’m buying online and it helps to logically find stuff in bins of parts. It’s important also to keep through-hole components and by extension, hobby electronics alive. Radio Shack has become a pale shadow of what it once was, nothing more than a gadget retailer now. Especially in Canada where it has become “The Source” which is just laughable and has nothing I want for too much money. I do not shop there.
Anyway, I was able to get many parts I was missing given the revisions going on in my power supply design as well as my nixie tube clock project which has been on the back burner.
I picked up a 2x6V 12VA transformer, which means I can get 1x12V out of it at 1A. Just what I needed. Smaller and lighter than the 24V ones I was messing with since they are no longer necessary. This will power my relays and output board switching as well as my meters, keeping it nice and independent from my main power board.
As I mentioned before a couple of posts ago, I’m moving the voltage and current reference settings to the main board so that they actually have a common reference. To that end, I was able to find some zener diodes that work perfectly. Picked up 8 or so 30V zeners (didn’t know they existed), a bunch of 12V ones and some 5.1 (i think) volt zeners. This should give me rough voltages to start from and regulate for the references.
As a bonus, I figured out I can use another darlington transistor biased with a zener at it’s base to drop the voltage from the main rectifier (currently at 42.6V) to just shy of 30V so I solved the problem of the voltage being too high as well. This will save me a lot of heat and keep me well away from the voltage differential maximums for my regulators. This now means that I have to use three pairs of darlington transistors: first, as a pre-regulator, second as the current regulator, and third as a pass transistor for the voltage regulator. This seems complicated to me (as I am sure it does to you) but I really want to keep these functions separate if possible. I’m sure I could combine the first two just fine, but I want both to function 100% independently just in case. This is a learning process, so things may change when I breadboard it up and see if it catches on fire.
So I’m homing in on my final design. It’s a bit sad that my control voltage board basically has nothing on it anymore apart from power to the relays and the regulators for the meter supply. I may merge this with the output control board just to save boards. It also occurs to me that I may not have enough headroom on the new transformer for a 7812 regulator to give my output board a stable 12V. I do believe after rectification and smoothing with a 1000µF cap, I should have plenty to overcome the dropout voltage of the 7812.
In addition, I also picked up a few parts I was running low on. Notably, a bunch of dual and quad opamps like the TL084, LM3900 (which I heard was discontinued yet still available), and the NE5532 (one of which I fried by accidentally using it in place of a 555 timer for the nixie supply). Also picked up a bunch of 555 timers and 556 dual timers for various applications. I was thinking of improving my soft start circuit with a more accurate (and more customizable) timer using 555s though this remains to be seen. I may revert to the earlier design anyway. I still haven’t figured out to create a latching cutoff condition during current overload without putting the sense after the relay. A minor, but important detail.
the nixie clock now will also have MPSA42 300V switching transistors for it’s digits. I’ll write on this in a separate post.