# Newbie power questions

Discussion in 'General Electronics Discussion' started by enigma65, Aug 22, 2013.

1. ### enigma65

3
0
Aug 22, 2013
Hi all, hope you can help me on some basic questions...

I have AC to DC power supply as follows:

Input from wallwart: AC to 18v DC @ 1000ma

Multiple Outputs:
9v DC @ 100ma
9v DC @ 500ma
12v DC @ 100ma
18v DC @ 100ma

I need to take a feed from one of those outputs and get 5.1V DC @ 2A. Is this even possible?

If not what about tapping directly into the 18V @ 1000ma input instead of one of the outputs.

Any info would be appreciated but keep in mind my knowledge of electronics is minimal.

2. ### Kam

19
0
Aug 21, 2013
if the maximum current the transformer can supply is 1000mA
then you cant draw 2000mA from it.

Is it unregulated ? if it is then when you hook it up you will get a
voltage drop anyway so 9v might drop down to around 5v

3. ### enigma65

3
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Aug 22, 2013
As far as I can tell from the specs the voltage at the outputs is regulated. I just thought that since I need only 5v of the 18v, somehow those 13 extra volts could be harnessed into more amperage.

4. ### Kam

19
0
Aug 21, 2013
It doesnt work that way unfortunately

you could easily drop the voltage to 5v using something like
an LM7805 which would give you 5v out from 9-18v input but that again is rated
at 1A

maybe make something up using a battery/s ?

5. ### enigma65

3
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Aug 22, 2013
Thanks Kam... I guess that's why nobody else invented this "great idea" I had...

You know those boards that guitar players use that are loaded with different effects pedals? Well a lot of us are now using a tablet (iPad or similar) mounted on our mic stand to display set lists, song lyrics, etc... The pedals on the board are generally powered by a DC power supply brick that typically takes a 18V 1000ma input and feeds it to a bank of outputs as mentioned above to power different pedals with pigtails (short power cords). Plugging in an iPad or other tablet requires access to an additional AC outlet. My bright idea was a box (just like an effects pedal) with USB ports on top that could feed off the power supply "brick" and somehow transform the low amperage of those outputs to 2 amps due to only needing 5V instead of say 12 or 18.

As they say... Oh well!

6. ### Kam

19
0
Aug 21, 2013
nothing stopping you making one from scratch,
sounds like a great idea though.

hope you get something sorted

7. ### duke37

5,364
771
Jan 9, 2011
You can get buck convertors which can take a high voltage and convert it to a lower voltage with higher current. The efficiency is about 80%. I have never used or made one.

8. ### Harald KappModeratorModerator

11,413
2,619
Nov 17, 2011
You're not going to be happy with this adapter and your output requirements.

9v DC @ 500ma makes for 4.5W. This is the maximum power you can draw from a single output. Even if you connect both 9V outputs in parallel (I do not recommend that because it can give you trouble due to load sharing problems) there would be 9V*600mA=5.4W.

You require 5.1V*2A=10.2W. No regulator on the world can give you this power from a 4.5W source (not even at 100% efficiency). You will have to get a more powerful adapter.

You could use the 18V/1000mA input source (18W) and a buck-converter (step-down converter) to get 5.1V/2A. A commercial DC/DC converter is 30€ and up. You may try to build one yourself.
However, buying a suitable wall wart is probably the least expensive thing to do.

9. ### BobK

7,682
1,688
Jan 5, 2010
The problem with a buck conveter is that it is supplying the full current during the ON period. The average current will be less than the output current, but the peak current will not. If it switches fast and has a large enough capacitor on the input, it could work though.

Bob

Last edited: Aug 23, 2013
10. ### Yaser43082

11
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Jul 20, 2013
So from what I understand, you have 18 volts DC rated @ 1A and you want to go to 5.1 Volts rated at @ 2A. This is really a two part question.

1) To go from 18 volts DC to 5.1 DC, you will need to design a Buck Convertor
At this stage of the game, you circuit sees 5.1 Volts @ 1A

2) Next you will need a Current Amplification system. You can do this with very basic transistors. Pay attention to the Beta value in the data sheet. Beta is dependent both on Temperature and Collector Current. This will get you to the 2A required

11. ### Harald KappModeratorModerator

11,413
2,619
Nov 17, 2011
Excuse me, but that is nonsense.
While a transistor can amplify current (i.e. collector current vs. base current), no transistor circuit can deliver more current than the source itself.

If you have [email protected], then this is equal to 18W. You can use a step-down converter to create [email protected] from that. The original post is ambiguous on this point: the table lists [email protected], the text says [email protected] What is correct? With [email protected] it is impossible to drive this scheme.

Last edited: Aug 27, 2013
12. ### Yaser43082

11
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Jul 20, 2013
Harald, thanks for the correction. Perhaps I did not express the solution well enough