Power supply (split current by 2 outputs) - Page 1
Forum section: electronics / projects

author: GoatZero
author_rank: Regular Contributor
timestamp: November 15, 2013, 04:43:57 am
content: So i just finished this basic Power suppy and its working as expected howeverI need to make a circuit so the currents dont swich outputs depending on the load, (they have to be de same in both outputs) currently dependindg of the load the channels will pull all the current needed even if that means leaving the other without any, i want to change thati have no idea where to start to make a circuit that can do thisthe upper circuit its what i have right now, fully working, rectified voltage is 20V DC, im losing a bit in the circuits of the 5v charger around (-3v) and the variable power supply Rectifier bridge and and LT317 (-3.2V) that explains my outputLower circuit its what i expect to do plugging the new circuit into my power supply and having 2 circuits with the same current no matter the loadIm usisng a 12v 500mA transformer, i can change it to a 19v and more curent later however thats not the objective right now, i just want to do a new circuit that splits the 500mA current by 2 for both channels and then i might consider a new transformerThe diagram explains itself (i hope) hopefully anyone here can give me a lead of how can i do this using transistors

author: AG6QR
author_rank: Frequent Contributor
timestamp: November 15, 2013, 05:09:17 am
content: It sounds to me like you're trying to violate Ohm's law. Good luck with that. Or maybe I've misunderstood.You've got a 5V section that is built of unknown components. Presumably it is some sort of voltage regulator, either linear or switching, it doesn't matter. It's trying to put out 5V.You also have a 13.7V regulator that's based on an LM317. I can see that you have configured it as a voltage regulator.You say you want to have the current in the 5V and 13.7V sections be the same, no matter what the load.But Ohms law says that V=IR. That equation has three terms. If you pick two of them, the laws of physics will dictate the third.You show 250mA at 5V, which tells you that you have a 20 ohm load connected to the 5V output. You show 250mA at 13.77V, which dictates that a 55.08 ohm load is on that line.But you also say that these currents and voltages must be maintainedregardless of the load. That part is impossible. Once you've specified the voltage and current, you've locked in your load resistance. If you change the load, you must change the voltage or current. If, for example, you put a 100 ohm load on the 13.77V line, Ohm's law insists that either the current will drop, or the voltage will go up. Since you've got a voltage regulator there, the voltage won't change significantly, but instead, the current will drop by about half.Just curious, why do you need the currents to match regardless of the load?

author: GoatZero
author_rank: Regular Contributor
timestamp: November 15, 2013, 05:49:37 am
content: Quote from: AG6QR on November 15, 2013, 05:09:17 amYes i didnt even think about that, you seem to be right in your explanation to the point that i cant refute itIm learning about transistors so was expectingto use a transistor in this "unknown circuit" in order to try to understand them a little bit more deeply,I was hoping to be able to do what i explained but somehow i found a way to forget about Ohms lawHowever im still left with the doubt if theres anyway i can use a transistor in my power supply circuit just for the sake of learning a bit more about how they work and practical experience with them anything would be nice since as i said im just learning about them