# Question on dropping voltage.

Discussion in 'The Projects Forum' started by Bob Obo, Nov 16, 2014.

1. ### Bob Obo Thread Starter New Member

Nov 16, 2014
7
0
Hi, i am fairly new to this site and electronics in general so please excuse me if my question seems redundant.

So me and my friend had a cool idea for a project and i wanted to see it through. The main goal was to have a set of two 12vdc 10ah batteries and one 6vdc 5ah power my 15vdc 5ah Wii U, so that i could play the Wii U tablet on the go. I figured i would just hook them up in series but then i would have 30vdc 10ah? So i think that using OHM's law, V/I=Ω, i can solve it. This is what i worked out:

30vdc to the 15vdc would need to drop 15vdc so 15vdc/10ah=1.5Ω

So does that mean all i have to do is put a 1.5 ohm resistor in between the negative battery terminal and the Wii U (straight into the system bypassing the ac to dc transformer)? Would i have to add something else to make the current or voltage stable? I would like to make this as safe as i can (safe enough to put in a small brief case or backpack) while still making it low cost, i also don't want to use a regulator because i've heard it wastes alot of watts and gets very hot. Also if you have a solution please explain a bit in lamence terms so that i can learn a little. Thanks for any help!

2. ### ISB123 Well-Known Member

May 21, 2014
1,239
524
Cheap switching regulator costs like 1\$.And you are limiting the current not dropping the voltage.You would have to use voltage divider to drop the voltage but its going to be useless since it won't be good enough for Wii.First tell us how much current does the Wii draw?

3. ### MrChips Moderator

Oct 2, 2009
12,234
3,282
Do what you are suggesting and your Wii U is toast!!!

12VDC 10Ah is not the power consumption. It is a rating of how much energy the battery can supply over time.
10Ah means 10 amps for one hour. It means that at 12VDC output the battery can supply 120W for one hour.

The Wii U tablet does not draw 10A. In order to power the Wii U tablet you need a 15VDC regulator rated for the maximum current draw of the Wii U.

You had better find out how much current the Wii U tablet really needs.

Ohm's Law for Noobies

4. ### Bob Obo Thread Starter New Member

Nov 16, 2014
7
0
the wii u uses 5ah for current and 15vdc

5. ### Bob Obo Thread Starter New Member

Nov 16, 2014
7
0
I only mean to power the Wii U console from the battery not the tablet(as of right now). I think i get what you are saying but is there a way that i can not use a regulator since it wastes a lot of watts and makes alot of heat (which i don't think would be ideal for use in a suitcase).

Last edited: Nov 16, 2014
6. ### MrChips Moderator

Oct 2, 2009
12,234
3,282
1) Get your units right. 5Ah is not a current rating. 5A is a current rating.

2) Get your series/parallel configuration right. Batteries in series add voltage. Batteries in parallel do not add voltage. (The 12V battery in parallel with the 6V battery will destroy the 6V battery).

Your are confused on both accounts.

Last edited: Nov 16, 2014
7. ### djsfantasi AAC Fanatic!

Apr 11, 2010
2,715
791
First, the 10Ah rating of the battery is a measure of how much current the battery can supply over time. It is not a measure of how much current is used. That is determined by the load. If the load draws 5A, then that is all that will come from the battery. Even if it could supply 10A. A lot of people get this wrong.

Secondly, your Wii specs. probably don't say 5Ah. Loads are specified in amperes or A; not Ah. So let's assume the Wii draws 5A. And you want to drop 15V as before.
15vdc/5A=3ohms

Next, you didn't want to use a regulator because you've been told it gets hit. Well, that 3 ohm resistor is going to get pretty darn hot! It will have to dissipate 75W. Think of how hot a 75W incandescent light bulb gets.

So how do you beat this heat problem. A switching power supply might help, but 15vdc is a lot of volts to waste. Why not use a 12V and 6V batteries in series (not use the second 12V battery). Then you only have to drop 3vdc.

Then a linear regulator or switching supply is feasible. I didn't say so earlier, but a resistor in line is a terrible way of dropping voltage in a power supply, because as the circuit operates, the resistor will change how many volts it will drop, as current requirements change.

8. ### Bob Obo Thread Starter New Member

Nov 16, 2014
7
0
Oh sorry about that still trying to remember it, looked it up earlier today. That's good to know though that a higher voltage battery will short a lower voltage battery in parallel, is that the same for series? Something that might have caused some confusion is that the tablet and the console are separately powered. Also my stepdad suggested i use a variable "pot" resistor, like the ones on lights but for dc. Would this work?

9. ### MrChips Moderator

Oct 2, 2009
12,234
3,282
1) Find out how much current and voltage you need to power whatever it is you want to power. (And please do not respond with a Ah rating.)

2) Using a resistor or pot will waste energy.

3) Use a single 12V battery with a boost converter to get the desired voltage.

4) As an alternative, use a 12VDC to AC inverter and plug in the AC adapter. Problem solved.

May 21, 2014
1,239
524
11. ### Bob Obo Thread Starter New Member

Nov 16, 2014
7
0
Thanks for explaining a bit more, i think i was unclear on what the others were trying to explain because of my noobiness. I also never knew that about resistors getting really hot, did you figure out the watts by voltage*resistance?
If i were to hook up the batteries like so:
-[12vdc}+{6vdc]-[12vdc}+
(sorry for the bad schematic but i don't know how else to draw one on here) Wouldn't the output voltage still be 18 or is this not safe?

12. ### Bob Obo Thread Starter New Member

Nov 16, 2014
7
0
Would i just find one for the amount of voltage i want it to drop and the max amperage i want to go through it, and then plug the converter regulator directly into the system?

13. ### ISB123 Well-Known Member

May 21, 2014
1,239
524
Adjust it to 5V and the Wii will draw the current it needs.

14. ### MrChips Moderator

Oct 2, 2009
12,234
3,282
No. Your 6V battery is toast.

15. ### Bob Obo Thread Starter New Member

Nov 16, 2014
7
0
Alright cool i think i've got what i need to do this project now:
1) don't use resistors to drop mass voltage
2) it's a not ah, and the difference of them
3) i should probably use just one 12vdc battery or the 12vdc and 6vdc battery so that i only have to boost or drop 3 vdc
4) don't hook up batteries in reverse to drop voltage
5) the console will draw the current it needs
6) figure out what exactly a switching supply is
Sorry i know this has been a messy post because i would often miss an answer while replying to another just because they were coming in so fast. Also sorry for my incorrect usage of current as "ah", and for my other misunderstandings. Thanks for all of your speedy help!
Also Mrchips thanks for the link to the "Ohm's Law for Noobies" article, it's very helpful.

Last edited: Nov 16, 2014
16. ### MrChips Moderator

Oct 2, 2009
12,234
3,282
You're welcome.

Last edited: Nov 17, 2014
17. ### djsfantasi AAC Fanatic!

Apr 11, 2010
2,715
791
Ditto. You're welcome. It is easy to work with someone who can listen and learn!

18. ### djsfantasi AAC Fanatic!

Apr 11, 2010
2,715
791
By the way, I missed these questions.

I figured out the watts by the equation:
Watts = Amps x Volts

To add a schematic, draw it in paint, use a schematic program, like ExpressSCH, or sketch it and take a picture. Then when posting, there is a button labelled "Upload a File". Click it and enter the picture file with your schematic. Once the file is uploaded, there are options to insert it in your post as an image or a thumbnail. Large pictures are best as a thumbnail.

Hope that helps!

Bob Obo likes this.