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| MarkF:
--- Quote from: Pirateguy on August 17, 2018, 05:07:39 pm ---anyhoo, wat do you guys mean when u talk about 'double bananna plugs'? and does the 3/4" measurement have to be exact? or is it a minimum? --- End quote --- There are a lot of varieties. The spacing is 3/4". Just Google them. Here are a few: |
| MarkF:
--- Quote from: Pirateguy on August 17, 2018, 05:07:39 pm ---speaking of dave's videos, i found the perfect one for me and this project: Video removed i haven't watched the whole thing yet, but it looks like this will teach me the math i need for the adjustable one. --- End quote --- Yes. Dave's video would be a perfect learning experience. Follow it with two LM338's and get it working on a breadboard. Then layout a PCB and get it made. You can expand on the layout I showed above by adding the current limit stage. The 4 diodes form a full-wave bridge for an AC input. Or, you can bypass them and feed in DC via a wall-wart plug. You can get five 100mm x 100mm PCBs made for $5.00 US at Elecrow. |
| Pirateguy:
lol but postage would be $8 :P for now i think ill just use some perfboard. so i had an idea: using this calculator: http://diyaudioprojects.com/Technical/Voltage-Regulator/ i determined that i need 5.9k to get exactly 32v. so i was thinking what if i just put a 10k multi turn pot parallel with a small 20k pot on the pcb, and use the 20k to tune the max resistance till it gives me 30V? |
| MarkF:
If you do two LM338 in series (1st doing current limit, 2nd setting the voltage), you are probably only going to get 28V for the output. That's assuming 2V drop-out volts for each LM338. Designing for 25V would be more realistic. Using that calculator, you have R1=240 and R2=5K. I would only use a 5K ohm 10-turn pot mentioned earlier for the voltage control. I built an Electronic Load with a 10K 10-turn pot and it gave me plenty of sensitivity to set the current. One 10-turn pot for each LM338 should be enough for the current and voltage adjustments. You would probably only use two pots if they were the 3/4 turn type. If you want two pots, they would be in series with one big (say 5K) and one small (say 500). Parallel pots would be a NO NO. You will need to determine the value for the current limit pot. Dave's video mentioned that the LM338 needs a minimum load and to adjust R1 and R2 to provide that load. I haven't worked through the math to determine the best values to use. I know the postage would cost more than PCBs from Elecrow if you only get one design made. I try to order more than one PCB design to make the postage worth while. But, I suggested it so you have the experience of going through the whole process and learn how to do a PCB layout and get it built. Maybe even a second board with all your polyfuses and whatever meter switching you want to do. |
| Pirateguy:
yeah i'm not quite at the point where i want to order commercial boards like that yet (though i am excited about these options we have now). i want to be better at designing circuits first so i can have some confidence of not having to order the same board 5 times due to tiny mistakes. and also i want to transition from THT to SMD first, at least to the point of having confidence in being able to solder a board full of 0805. y no resisters in parallel? with what i had in mind the 2nd one is just for fine tuning the range and then maybe fixing it in place. the idea was that i would not be bound by standard values for resistors... anyway i totally spaced on the 2nd 338 for the current :P back to the drawing board... still, you sound like no pots parallel is a bad idea in general? if so, how so? also the minimum load, isn't that what the 240ohm resistor is for? my circuit would have been the diode protected version that is in the datasheet, only adding that sencond pot... |
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