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power supplies in series question Answered

Just asking before I blow up 2 of my power supplies that I just made/salvaged/fixed/modified. I need 19 volts at 2.95 amps (practically 3) for a laptop that I found (yay dumpster diving, don't worry I'm not a guy who steals identity, it was actually my dad's lab and they're throwing away a bunch of stuff, it's a goldmine!) and it was marked dead. I pull out the battery and it has 0 charge. It had no charging adapter, so can I just make one? I have to power supplies, one from a computer that I modified and an adjustable one the goes up to about 15 volts at 6 amps (and they were throwing it away! it only need a new fuse). After all of that background info, here's my question: can i put the 5 volts from the comp supply in series with the 14 volts from my adjustable supply to get 19 volts at enough amps to power on this laptop? I don't want to buy a 60 dollar charger just to find out the comps broken cause of something else.



sorry for posting on the dead thread,but...
youd need to float the psu, that is, disconnect it to ground...hold on

It worked unfortunatley my laptop didn't

keep the laptop. im working on an instructabel on what to do with failed computer parts.

ok, I will. I cracked open the battery to find 8x 18650 lithiums. I think it's 4 in series in parallel with the other 4 in series. The combined voltage of everything is 6 volts, and it should be 14.4. I'm going to take the batteries out, hook up my power supply (do you think 12 volts will work, it'd be much more convienient) to where the batteries should be and see what happens. If all goes well I'll go buy some 18650's. If this is a bad idea tell me soon, lol.

The only real "bad idea" here is anything that tries to charge the batteries without going through the laptop's charge control circuitry. If you know what you're doing, fine, but just make sure you know what you're doing before trying anything like that.

alright, I'll test it, what's the worst that could happen? I think it should work because of the ground. For example: Lets say you have a 7805. +12 volts in, +5 volts out. If you change the ground (relative to another ground, as in the 1st power supply in series) to something like +2 volts, then the output should be 7 volts, right? I'll try it once I buy some connectors for my comp supply. I feel like I've made sooo many power supplies, I have: a mini one (about the size of the old fat ipods) that supplies 5 volts at 2 amps, and 12 volts at 2 amps. I just finished it. a variable one, I couldn't fit capacitors in the case, so they're external. It has 2 outs, 8 volts at 6 amps, and 3-15 volts at 6 amps. It even has an ampmeter built in! I didn't make this, I just fixed it and changed the ac output to dc. a computer one, I put on my load resistor today and everything works, all I need now is connectors and I'm done.

 So did it work? I'm trying to do the same thing with 2 ATX power supplies salvaged from old computers to get different voltages

to be truly honest, this is something i want to know too. seeing as its dc, its possible it works. you could possibly dry run it and use a multimeter to find out if it gives 19 volts. however, somehow, i dont think it will work.

If they are regulate supplies, the regulator will clip the voltage to the prescribed voltage.

so im not the only one having a bad spelling day

for me, it is a combination of bad spelling and horrible typing skills. The backspace button is always the first one to have the letters on it worn off LOL.


10 years ago

AFAIK, there's nothing wrong with doing this: it's essentially a bipolar power supply, but with two separate transformer / regulator circuits. Unless the two power supplies are mismatched, in terms of current capabilities (or one is poorly filtered, etc.)

Regulated bipolar supplies from a single transformer need a positive and a negative regulator.

I agree with NachoM. Two separate unregulated transformers together in series is (for practical purposes) one center-tapped transformer. I also would be more nervous about supplies in parallel....

Well, series batteries add up to the voltage increasing, but it probably will not work with power supplies, as they have a voltage regulator in them, and the voltage of the last power supply will or should be regulated at its rating. In parallel, the amps add up, but not the voltage.

. hmmmmmm A battery, for the most part, is pretty well regulated. . If you put them (batteries or PSUs) in parallel, the the voltage will be limited to that of the "smallest" unit (until it burns up) and the amps will be additive. In series, the amps will be limited and the volts will add up.

Yes, with batteries. But wouldn't the voltage regulator of the last power supply in series limit the voltage "to" what it is designed to ? If variable, what it is set to?

. None of the PSs/batteries sees the total voltage - it's all relative. . I wouldn't recommend hooking up PSs that way (and especially not in parallel), but, in theory, it ought to work just like batteries. . Be an interesting experiment if one had some expendable PSs laying around. I'll guess that something in at least one of the PSs will burn out due to something like an imbalance in the output circuit. Might be one of those deals where the cheaper the PS, the better it would work.