12volt - 5volt dc/dc converter

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[wizard]

Limp Gawd
Joined
Oct 30, 2001
Messages
376
I'm attempting to run a dual cold cathode kit and 2 different 5volt led array's to one 12 volt switch. Spliting the voltage at the switch is no problem, one leg to the CCFL and one to a DC/DC converter... any ideas on how I can build one of these? I searched the forums and google for ideas but came up dry. I think I saw one someone designed a few weeks back but I cant find it now.

Pics and/or wiring diagram at request. Thanks in advance...
 
Go to Radio Shack and pick up a 7805, the diagram on the back of the package is all you need.
 
Find out how much those parts draw before you just slap a 7805 in circuit.

The 7805 is only rated at 1A, with "good" heatsinking. And with a Tja of 60deg C / watt (with no heatsink), it can only dissipate about a watt or so. With 7V across the part, this means you're limited to under 200ma unless you heatsink it.

For a LED array, you can probably just use a power resistor. Got an ammeter? measure how much current the array draws at 5V, then use ohms law to figure out a resistor that drops 7V at the same current.
 
This is generally assumed, but ... are you using this inside a computer?
 
If this is indeed in a computer, then I would just use a dual pole switch with 12V on one side and 5V on the other. If there is no 5V available, I would just use a resistor.
 
there is no reason to use a regulated 5v supply... it would work tho, but it be realy pointless.

a big resistor is your best bet, the wirewound ones can dissapate a lot of heat
 
theshadow27 said:
there is no reason to use a regulated 5v supply... it would work tho, but it be realy pointless.

a big resistor is your best bet, the wirewound ones can dissapate a lot of heat
Click to expand...


Problem is, both of these solutions give off heat. You're putting this in your computer. I realize that it's not a lot of heat, but the point still remains. If you're trying to cool your computer, you probably don't want some wirewound resistors heating things pointlessly.

Internally, a much more efficient solution would be either the dual-throw switch suggested above, or a relay (mechanical or solid state).
 
my bro-n-law suggested a relay. what am i looking for in a relay?

i have a multimeter, dont think i can messure draw these led's have...there is 10 LED's in an array...basically all connected to teh same 2 power wires... how do i know what size resistor to get? :confused:

i took electronics a few years ago and i still have all of my resistors laying at home from my little led projects...just never did anything like this. :p

edit: because of the specs of the case mod, a two throw switch will not do. :(
 
[wizard] said:
edit: because of the specs of the case mod, a two throw switch will not do. :(
Click to expand...
Why? Could you use a dual pole relay with both 12V and 5V? Or is 5V just not available? Actually, if you do have 5V available, you could get by with a single pole relay. Have the switch control one voltage, which will power that device plus the relay, which will switch the other voltage.

edit: We're not talking about a double throw switch. We are talking about dual pole, single throw switch.
 
jpmkm said:
Why? Could you use a dual pole relay with both 12V and 5V? Or is 5V just not available? Actually, if you do have 5V available, you could get by with a single pole relay. Have the switch control one voltage, which will power that device plus the relay, which will switch the other voltage.

edit: We're not talking about a double throw switch. We are talking about dual pole, single throw switch.
Click to expand...

thanks for the suggestion but i dont understand how a relay will solve this problem...

i meant dual pole single throw switch. i have a particlar switch that matches my mod that i'd like to use... it is only a single pole switch... a dual pole would be ideal but it's not currently an economical option.
 
[wizard] said:
thanks for the suggestion but i dont understand how a relay will solve this problem...
Click to expand...
I don't understand what the problem is. Do you not have 5V available?
 
i understand that you can only use a SPST switch -- thats where the relay (+no resistor) comes in. if you have +5 avalaible:
relayish.jpg
 
theshadow27 said:
relayish.jpg
Click to expand...

this requires a dual pole switch...

i have 5 volt lines available but i need both a 12 volt device and 2 5volt device to be powered on and off with a single switch. if a relay will allow me to just use a single 12 volt in and a 5 volt in, i can do that as long as i only have one "power" line running through my switch. can i just put my sources on thier respective power inputs and run the switch in a single ground line or does this raise all of the voltages to 17volts?
 
i think you misread my crude diagram. the thing on the left is the relay, the thing on the right is your single pole switch. switches dont have coils ;)

EDIT: i have spotted the confusion... the thing under "12v relay" is an arrow pointing to the left. sorry bout that, i know i drew it in 30 seconds in paint
 
[wizard] said:
this requires a dual pole switch...

i have 5 volt lines available but i need both a 12 volt device and 2 5volt device to be powered on and off with a single switch. if a relay will allow me to just use a single 12 volt in and a 5 volt in, i can do that as long as i only have one "power" line running through my switch. can i just put my sources on thier respective power inputs and run the switch in a single ground line or does this raise all of the voltages to 17volts?
Click to expand...
No, that can be accomplished with a single pole switch. Just have the switch run to the relay. Doesn't matter what voltage. When you turn on the switch, the relay actuates. The relay is double pole, so you can switch both the 12V line and the 5V line with it. Turn on the switch and both turn on.

Alternatively, you could do it like I stated earlier. Use the switch to switch 12V. The 12V will go to your 12V devices and also go to a relay. The relay will switch the 5V line. When you turn the switch on, your 12V devices will turn on and the relay will actuate, turning on your 5V devices.

If you want to put the switch on the ground line, you will have to put diodes on your 12V and 5V lines. Otherwise, when the switch is off, you will have a 7V potential and current will flow. Keep in mind that diodes will drop the voltage a bit.
 
jpmkm said:
If you want to put the switch on the ground line, you will have to put diodes on your 12V and 5V lines. Otherwise, when the switch is off, you will have a 7V potential and current will flow. Keep in mind that diodes will drop the voltage a bit.
Click to expand...


7V potential across what? I'm going to spend a few minutes and whip up a real diagram to make everything clear.

Edit: No i'm not. This stupid computer here does not have any programs installed where you can rotate a element in a diagram. WTF.

My question still stands. Where are you going to get current flow due to a 7V potential?
 
fat-tony said:
7V potential across what? I'm going to spend a few minutes and whip up a real diagram to make everything clear.

Edit: No i'm not. This stupid computer here does not have any programs installed where you can rotate a element in a diagram. WTF.

My question still stands. Where are you going to get current flow due to a 7V potential?
Click to expand...
Let me try to diagram this.
Code: 
  12VDC    5VDC
    |       |
    |       |
12V dev.   5V dev.
    |       |
     \     /
      \   /
       \ /
        V
	|
	|
	\  Switch
	|
	|
       GND
What I believe was proposed was something like this. Put a switch on the ground and use that as a common ground for both 12V and 5V. That works just fine when the switch is closed. However, when the switch is open, we have a situation like this:
Code: 
   12VDC
     |
     |
12VDC device
     |
     |
     |
 5VDC device
     |
     |
    5VDC
Which gives us 12VDC - 5VDC = 7VDC potential. A couple diodes, though, should prevent this, but will obviously result in a voltage drop. Depending on how the led array is configured, the array itself may prevent this current flow. I don't know. It's late and I don't really feel like thinking it through all the way.
 
That's an interesting way of doing it. Depending on the device though, many have chassis grounds that would result in them being powered anyways.
 
yeah that could work... i would never have thought of that...

the relay is 100% guaranteed to work, the "Y" split is like 98%
also, with the relay as i drew it you could have the load go thugh the relay and not the switch, which will increase the lifespan of the switch's contacts (if your drawing a lot)
 
thanks for everyones input. i threw in a relay and got it wired up and she works beautifully. i've learned alot. pics to come at the end of the build.
 
relay.JPG


my case feet are so dim they are imposible to see. all joints and y's are connected via buttconnectors... could the shared grounding be the issue? the jumper wire i used to connect everything was a larger guage than what the devices came with. the led's are all connected to one single power and ground wire... is there a problem with my schem?
 
are you sure the case feet arent pre-made to run off 12v? ive never seen a set of them that run off 5. even if it is a red wire, it is still prob 12v. that would be my first guess.

all your wireing is exactly as it should be; except there realy is no need to connect both ground wires on the molex, they both go to the same place. butt connectors are just as good as soldering, so i doubt thats the problem.

i assume that the other LEDs and CCFLs work as expected?
 
the others work beautifully.

the plug the case feet where using originally had them plugged into the 5v lead on the molex... but upon looking....

newegg link to my case feet

12volt... once again you guys prove why this site is [H]ard....
 
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