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Old 13-11-07, 09:19 PM
JN JN is offline
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12v > 5v Resistor value...

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?

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Old 13-11-07, 10:17 PM
Ham Ham is offline
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Get a few different level variables/rheostat?

Ill prod the old man... 1 sec
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Old 13-11-07, 10:24 PM
JN JN is offline
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Quote:
Originally Posted by name='Ham'
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.
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Old 14-11-07, 12:09 PM
PV5150 PV5150 is offline
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Could you use a couple of diodes in series to drop the voltage? You can find diodes that drop 1.2V, 3V, 5V etc
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Old 14-11-07, 12:21 PM
JN JN is offline
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Quote:
Originally Posted by name='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
Oooh that sounds like a plan. Got any ideas of exactly what ones I'd be looking for?
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Old 14-11-07, 12:21 PM
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Rastalovich Rastalovich is offline
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Quote:
Originally Posted by name='XMS'
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 +/-%
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Old 14-11-07, 12:34 PM
PV5150 PV5150 is offline
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Quote:
Originally Posted by name='XMS'
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
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Originally Posted by name='Jim'
"Jonathan 'Fatal1ty' Wendel may be 12-time world gaming champ, but how does he cope inside a 50c hot box sucking on a heavy load?":rocker::rocker:
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  #8  
Old 14-11-07, 01:06 PM
K404 K404 is offline
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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
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Old 14-11-07, 01:35 PM
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Rastalovich Rastalovich is offline
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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.
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Old 14-11-07, 01:42 PM
BigFatCat BigFatCat is offline
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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.
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