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Swichmode voltage regulators in series

I have two of these adjustable voltage regulators ( http://www.dimensionengineering.com/DE-SWADJ3.htm ) and two 12 volt battery packs (from electric drills, their maximum voltage is actually around 14 volts).

I want to power a computer that needs 19 volts. I am not sure how much power the computer uses but I think it is less than 50 watts.

My plan is to regulate each 12 volt battery down to 9.5 volts and then connect the outputs of the regulators in series for a total of 19 volts.

Do you forsee any problems with this setup, could the switching of one regulator interfere with the other? I am asking for your opinion first because these thinghies are somewhat expensive and they blow up very very quickly if mishandled (one of them released its blue smoke instantly when I plugged it in backwards).

 

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Tricky. I'm not sure. Here is one thread I found on the subject. No definitive answers, but some things to consider are discussed.

http://cr4.globalspec.com/thread/37432/SMPS-in-series-connection

My only concern is that each of those babies can only handle 3A and they are each rated for only 25W. In series they can still only handle 25W because they will both be handling the same current. You said your PC is rated 50W, this may be an issue. Please correct me if I am wrong.

They even mention that older models were connected in parallel to achieve higher power ratings, however I can't say if it is still safe for a series connection.

Why not connect your two batteries in series so that they output 24V and then use your two voltage regulators in parallel to step that down 19? That way you won't go over the power rating, and you will be doing something they have tried before.

power = voltage x current so either I do

voltage/2 x current ( series connection )

or

current/2 x voltage ( parallel connection )

I get the same thing :)

The thing is those regulators can only output a maximum of 12-13 volts - that's why I had to use them in series.

I should mention that I tried it and it worked :D, however this doesn't always work with mains powered sources (for example I couldn't test the setup with 12 volt wall warts because the output ground is the mains ground on both wall warts so connecting the plus from regulator 1 to the ground of regulator 2 would actually short regulator 1 and the blue smoke would escape)

True that  P = IR

But this is how I think of it, correct me if I'm wrong. Say you have a constant load that requires a current C at 19Volts.

If the regulators are in parallel outputting 19V then each of them will provide roughly C/2

If they are in series then the first one will give a voltage from 0 - 9.5V with a currect C and the second will give a voltage from 9.5 - 19V at a current C. 

If you have a constant load in both cases that draws a current of 4 amperes. In the first case each regulator would only provide around 2 amps, while in the second each would have to provide 4amps. It is true that the power provided by each is the same but in a series connection the regulators would see a current (4 amps) that is outside their operating range of 3amps.

Parallel: 19 x 2 + 19 x 2= 38 + 38 = 76 W
Series: (9.5-0) x 4 + (19-9.5) x 4 = 38 + 38 = 76W

Either way you are right that the regulators can't fo 19V (didn't notice that). Glad you got it to work =)

Cheers.