How to Convert 5V DC to 12V DC?

Discussion in 'General Electronics Discussion' started by GeorgeBurgess, Nov 4, 2012.

1. GeorgeBurgess

7
0
Nov 4, 2012
I have a power supply with a different connector, and output (on one of 24 pins) to the circuit input I am attempting to use it with, so basically I got an electronics engineer I know to make an adapter up for me, he did a great job in doing so however I have one issue, one of the pins of this adapter's input side is of 5V DC but I need it to be 12V DC on the output, the engineer used a simple 0-240V to 12V transformer however this doesn't appear to be working.

What is the best way to achieve this 5V DC to 12V DC conversion? Thank you in advance.

2. (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,500
2,840
Jan 21, 2010
The best method is using a 5V to 12V DC-DC converter.

What currents are involved?

It sounds like your friend made a separate 12V power supply. Does this not work at all?

3. GeorgeBurgess

7
0
Nov 4, 2012
Hi, Thanks for the quick reply,

The 5V input should be at least 2A
and I require the output to be of at lease 1.3A

I'm pretty certain he didn't make a separate power supply, he just hooked up the transformer between the input and output.

4. davennModerator

13,894
1,969
Sep 5, 2009
Hi George

what you are saying is a little confusing

1) This unit you have takes 12V DC at @1.5A or so correct ?
2) why do you need a 5V DC to 12V Dc converter ?
3) what is your 5V DC power source --- battery ? mains power ?
4) your initial explanation of what your mate did was to give you a 240V AC to 12V (hopefully DC) plugpack is that correct ?
but you then say in your second post that he didnt .... just trying to sort out the contradiction

Dave

5. (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,500
2,840
Jan 21, 2010
Well, a transformer won't work for DC, certainly not one with a 240V primary. Can you show us a photo of it?

I don't understand what you're saying about current. Does your application require 2A or 1.3A at 12V?

How much excess current capacity do you have on your 5V rail (i.e. what is the difference between its rated current and the current drawn by whatever uses the 5V)?

(I'm assuming that you need the 5V and you also need a small amount of power at 12V. If this is not true then a DC-DC converter is not what you require. Maybe your EE friend made a 12V power supply for that reason)

6. GeorgeBurgess

7
0
Nov 4, 2012
Sorry for not being very clear,

To clarify, my application requires 12V DC at 1.3A, however I only have a 5V DC rail for this use, the 5V DC rail should be good for 2 or more amps.

For the 5V DC I am using a computer power supply as I am attempting to make an adapter for a proprietary motherboard connector.

My engineer friend didn't give me a plugback, he simply hooked up a 0-240V to 12V transformer.

From what I can gather a DC-DC converter would be best? As nothing else will be sharing this 5V rail.

I have attached a picture of what I believe to be the exact transformer.

File size:
5.2 KB
Views:
280
7. CocaCola

3,635
5
Apr 7, 2012

You want 12V @ 1.3A that is 15.6 Watts
You need 5V @ about 4A to make this viable

If you only have about 5v @ 2A it is not going to happen...

But, if your friend used a 12V transformer where did this 12V vanish to why is it all the sudden 5V?

We really need more details and clarification...

The transformer you pictured might take 240VAC to 12VAC but you keep calling it a 0-240V to 12V and that simply makes no sense, as transformers like the one pictured have fixed ratios...

Last edited: Nov 5, 2012
8. GeorgeBurgess

7
0
Nov 4, 2012
Well I guess that is why it is not working, I am calling it a 0-240V to 12V transformer as that is how he described it, so assuming my 5V DC output is capable of 4A how would I go about getting/making this DC-DC converter?

9. (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,500
2,840
Jan 21, 2010
You can't.

The transformer is connected to 240V, and you get 12VAC from the secondary. You then rectify, filter, and regulate the output to get a stable 12V rail.

10. CocaCola

3,635
5
Apr 7, 2012
Can we get some clear pictures of what your friend built for you? It will be worth 1000 words here...

11. GeorgeBurgess

7
0
Nov 4, 2012
Sure, I will try an upload some tomorrow as it's quit late here now.

12. GeorgeBurgess

7
0
Nov 4, 2012
Here we go, It's not much to look at, but is basically "reroutes" the pins except one also has the transformer attached.

File size:
138.2 KB
Views:
326
File size:
144.1 KB
Views:
331
File size:
143.1 KB
Views:
360
13. davennModerator

13,894
1,969
Sep 5, 2009
OK, so that guy calls himself an electronics engineer huh ?? Thats a real abortion of a construction LOL
I wouldnt rely on him for any other info
I dont see any diodes so the voltage out is AC NOT DC
You wanted DC go buy yourself a 240V plugpack that has 12V DC output and an amp or 2 more than what your circuit requires.
OR use the 12V output from the computer PSU if you want to. Its capable of several amps or so

There's absolutely no need to do any conversion from 5V to 12V when you already have 12V present from the PSU

Job Done

Dave

Last edited: Nov 5, 2012
14. CocaCola

3,635
5
Apr 7, 2012
Wow, to be blunt, what a haphazard mess... I have no idea what that transformer is in there for, we would need the pin out of both devices to even hazard a legitimate guess... Also a picture of any identifying numbers on the transformer...

With the bare metal of the transformer (mostly the mounting tabs) and bare wires hanging out of the screw downs you are asking for something to short out and go *POOF*...

IMO that entire thing should be redone from ground zero, all wires soldered and heat shrunk, get ride of the gobs of tape and screw down connectors... As well as figure out what the transformer is and get it more securely mounted as a complete unit... As it sits I would advise against it's use in anything or in anyplace you care about, especially if any of those are mains level lines...

***EDIT**

I fully agree...

15. GeorgeBurgess

7
0
Nov 4, 2012
Thanks for all your help but I can assure you that this guy is a perfectly competent electronics engineer with years of experience, the problems were most probably down to my relay of information.

I can't use the PSUs 12V rail as I need to up the standby voltage to 12V from 5V and of course the 12V rail won't be active while the computer is on standby; I would also preffer not to use a plugpack as not doing so will save on space and plug usage.

What would be the best way of making an appropriate DC-DC converter?

16. CocaCola

3,635
5
Apr 7, 2012
Nothing personal but it's not a matter of relayed information, the entire thing looks like it was tossed together by a child with a roll of tape... Honestly that looks like stuff I did as child (pre-teen), that is far from what one would consider 'professional' by any means imaginable... If the guy is a professional as you claim he should be ashamed of that, it's clearly a potential safety hazard even if you ignore how ugly it is...

**EDIT**

I also want to add what is the purpose of the transformer in that mess? If that is a common computer supply source like it looks ATX, maybe? Then there is no AC current, thus the transformer is useless...

Last edited: Nov 6, 2012
17. (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,500
2,840
Jan 21, 2010
Yeah. The phrase "Wheeties packet" came to mind.

I don't for a moment think anyone with experience with electronics put that together unless:

1) There's a lot you haven't told us.

2) Someone is pulling someone's leg.

I'm not putting a lot of weight on (1)

edit: Just take a look at what is being done with the ground leads!

Last edited: Nov 6, 2012
18. davennModerator

13,894
1,969
Sep 5, 2009
deleted my comment... CC and steve have said what I was thinking
George, seriously, you need to find some one who you can work with directly that IS competent

OK mate, have a look at this datasheet LT1173. It should happily supply ~ 1.5 Amp at 12V using the reccomended power diode IN5818

cheers
Dave

Last edited: Nov 6, 2012