Electronics > Beginners
How To Connect Two PSUs, To Get More Current - Solved
mvs:
--- Quote from: t1d on November 11, 2018, 05:14:42 am ---They are just basic models... Meaning they can set either voltage or current to constant mode, but not both, at the same time. Both are Linear/transformer and 30v/3a.
--- End quote ---
There is no power supply available, that can control the resistance of an arbitrary load. So its fine to have either CC or CV, but not both at the same time :)
Set desired voltage on both power supplies using some load and same DMM, then connect them in parallel. The resistance of connecting wires will do the load balancing.
PS It applies to lab power supplies that can only source current. If your PSUs have Push-Pull output, add diods or disable the current sink.
t1d:
Okay, I have been studying on this. I understand the use of resistors... Diodes? Not so much...
If using diodes, I do understand using Schottky's... They are faster... But, I am not sure I have any, on-hand.
I have also seen YT video tutorials that say "blocking" diodes can be used. I think this is just a term, in the solar power community, for bigger common rectifying diodes. Do I have that correctly? I do have 1N4004s and 1n4007s, in stock. And, maybe, 1N4002s...
I look forward to trying this, with diodes. I just have to get comfortable, with the idea... I don't want to hurt the new circuit that I have worked so hard on and am so proud of, my (precious; expense vs. personal financial budget) power supply, or me... lol
Thanks, for your continued help.
t1d:
I did try 1N4007 rectifying diodes. I could not get those to balance the two PSUs. So, I am looking at buying various parts/options; Schottky Diodes and Resistors.
With the supply only partially balanced, I could tell that the electronic load DUT can handle more current. So, woot, to that.
macboy:
You don't need to balance the two supplies if they have proper current limiting, which means that they will automatically go into constant current mode when overloaded. Essentially, you will allow one supply (whichever one tends to supply more current) to go into constant current mode and the other will then 'take over'. See below.
If you are using a basic dumb SMPS which has only over-current protection (like a computer power supply), this won't work. When the first supply overloads, it will shut down, then the other one will become overloaded and shut down. In that case, you need to very carefully balance the load, by matching output voltages and using balance resistors. The easiest type of high current low-ohm resistor is a sizable length of suitable copper wire. Actually, two lengths of wire: one between each supply and the load.
You can also use the copper wire balancing resistor trick to help balance the load between your two lab supplies, but you don't really need to. When you apply an increasing load (which means a decreasing load resistance) to two parallel supplies, the supply with a slightly higher voltage will supply all/most of the current until it reaches its current limit. Then its output voltage will begin to drop (which is exactly what must happen in constant current mode when load resistance continues to drop, because V=I*R). When that voltage reaches the slightly lower voltage of the other supply, that supply will begin to source current into the load, keeping the voltage steady at that point. If the load continues to increase (resistance decreases), then the second supply will also eventually reach its current limit, and the voltage will begin to drop again. You can see this automatic crossover at work very clearly if you set up one supply to a clearly higher voltage than the other, say 10 V and 12 V. Experiment with that setup to convince yourself that the two supplies can work in parallel without issue. Then set up both to the same(ish) voltage and let them take care of sharing the load. One will switch to constant current mode before the other, but that is OK.
t1d:
MacBoy, that is a brilliant explanation! Thank you.
I have a couple of Schotty's on order, at my local supplier. They will give me more options, in addition to the copper wire trick.
The PSUs are both linear. You said that one of the PSUs will follow the other. This coincides, with my original test observations.
Let's talk wattage. The PSUs are only capable of so many watts. As you explained, at their max watts, any change in current is offset by a change in voltage and visa versa. So, at the moment the PSUs have gone into constant current mode, this is the combined max wattage output. Therefore, this is the point where the e-load/DUT will see everything the power supply combo has to offer. There is no need to push beyond this point, because I would only be changing the v/c ratio. Correct? The e-load is CC, too.
One PSU has the constant current function. The other has both constant current and constant voltage. Is there any trick to combining these somehow, to an advantage, for this type testing?
Eventually, I think I will need more watts, than what the two PSUs can supply. The e-load design goal is 30v @ 3a. Buck converters are cheap and can supply a lot more. Is there any reason to not use one? They are switch mode, but it would be on the load side, of the circuit...
Thank you, so much, for your excellent help.
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