12v > 5v Resistor value...

[JN]

Hi guys,

I need to drop 12v down to 5v using a resistor. The power consumption at the 5v end is 320ma, so through use of various online calculators I worked out that I needed a 22ohm resistor capable of handling around 2w.

I found one of these on ebay, and tried it out tonight, however its giving me 7.2v. Any ideas?

 

[Ham]

Get a few different level variables/rheostat?

Ill prod the old man... 1 sec

 

[JN]

Get a few different level variables/rheostat?

Ill prod the old man... 1 sec

Cheers dude.

I've got some trimmers here, but nothing that will handle this i don't think.

 

[PV5150]

Could you use a couple of diodes in series to drop the voltage? You can find diodes that drop 1.2V, 3V, 5V etc

 

[JN]

Could you use a couple of diodes in series to drop the voltage? You can find diodes that drop 1.2V, 3V, 5V etc

Oooh that sounds like a plan. Got any ideas of exactly what ones I'd be looking for?

 

[Rastalovich]

Hi guys,

I need to drop 12v down to 5v using a resistor. The power consumption at the 5v end is 320ma, so through use of various online calculators I worked out that I needed a 22ohm resistor capable of handling around 2w.

I found one of these on ebay, and tried it out tonight, however its giving me 7.2v. Any ideas?

Off the top of my head, is it the way u`ve introduced the resistor ? Just thinking 12-5=7 +/-%

 

[PV5150]

Oooh that sounds like a plan. Got any ideas of exactly what ones I'd be looking for?

Just having a look for you now mate

 

[K404]

VRM:

12V in, 5V out:

L78S05.

Using resistors to drop the voltage through a voltage divider rule can be hit and miss as you design for one, set, load resistance. Its not so versatile.

K

 

[Rastalovich]

I`m not sure about using the 22ohm resistor, if I`m thinking the way I would do it.

All u really need to break down voltages over such a small scale is to take the 22ohm resistor u already have and put a 30.8 (30/31) ohm resistor in series with it and connect one end to the 0v and the other to the 12v.

Connecting to the 0v and the middle of the connection will give u 7 or 5v to use depending on which way around u connect the resistors.

All ur doing really is setting up a ratio over the 12v to 0v supply, the ratio being 22:30.8. Mathematically u could pick better resistor values tho.

 

[BigFatCat]

If you can power off a molex, connect the -ve to either black and the +ve to the red, to give you 5v. And you can also get 7v and 12v by going red-yellow and black-yellow respectively.

 

[JN]

VRM:

12V in, 5V out:

L78S05.

Using resistors to drop the voltage through a voltage divider rule can be hit and miss as you design for one, set, load resistance. Its not so versatile.

K

You sir, are a gem

Turns out the local maplins has 2 in stock as well

Just one Q...how do you know what pin is +12v, +5v, 0v ?

 

[K404]

looking at that pic, the legs are: output, ground, input.

Thats upside down, but thats neither here nor there....

You might wanna get a heatsink for it- Im using one to power 3 fans and its a bit burny burny.

Here to help

K

 

[JN]

looking at that pic, the legs are: output, ground, input.

Thats upside down, but thats neither here nor there....

You might wanna get a heatsink for it- Im using one to power 3 fans and its a bit burny burny.

Here to help

K

Cheers dude. I'll only be pulling 320mA thru it so hopefully it wont get too smokey.

I might use some thermal adhesive to glue it to the metal bracket of this fan controller tho.

 

[ionicle]

VIR is the formula i learnt in physics

voltage = current x resistance

which when rearranged equals voltage / current = resistance...

just for future reference