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Author Topic: 12v down to 5.8 volts at 1A? *updated x2 - last post  (Read 6818 times)

Daneel

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12v down to 5.8 volts at 1A? *updated x2 - last post
« on: December 24, 2004, 06:34:47 AM »

Using the x24's led calc, an input of 12v with 5.8 out at 1 amp gives me a 6 ohm resistor..  is that even possible?

if i'm messed in the head please tell me..
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Hak Foo

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12v down to 5.8 volts at 1A? *updated x2 - last post
« Reply #1 on: December 24, 2004, 05:08:41 PM »

Well, it's plausible, but it will require a fairly high wattage resistor.

You're dropping 6 volts, and power is voltage times amperage as I recall-- so you'd output six watts... pretty hefty resistor.

Why not try using a 7806 voltage regulator to get you to 6 volts, then a little small resistor to drop the last 0.4 volts, if it's even critical
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PIYIRIO

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12v down to 5.8 volts at 1A? *updated x2 - last post
« Reply #2 on: December 24, 2004, 06:12:23 PM »

your figuring for 1 amp, we usually measure in milliamps for leds.
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viridius

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12v down to 5.8 volts at 1A? *updated x2 - last post
« Reply #3 on: December 25, 2004, 12:55:41 AM »

Resistors are really only appropriate for small current draw devices.  Anything much above a few hundred mA and you'll probably be paying more for a power resistor than you would for a voltage regulator.  Hak Foo's got the right idea.  A 7806 would give you a relatively reliable 6 volts at max one amp.  You'd need to sink the regulator, but I'm sure you're not a stranger to heatsinks.  If 5.8 is crucial, you could get an adjustable voltage regulator.  With a pair of resistors, you can get pretty much any voltage you want.  In a TO-3 package, I think you can regulate a few amps.
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Daneel

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12v down to 5.8 volts at 1A? *updated x2 - last post
« Reply #4 on: December 26, 2004, 05:11:31 AM »

Ok, so i have to go voltage reg it seems..

this thing is so i can power my new logitech MX Duo reciever/dock from a molex plug instead of a wall socket, so yeah, 5.8v at 1 amp is needed.

so i need a 7806 voltage reg, what resistor (incude wattage) would i pair with this to give me the 5.8 volts?

and with the sink for it, how hot would it get?  i need a rough guess because i need to know where to put it in my briefcase.
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Hak Foo

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12v down to 5.8 volts at 1A? *updated x2 - last post
« Reply #5 on: December 26, 2004, 01:40:29 PM »

I'd suggest the variable regulator, tuned to deliver 5.8.  I'm betting that has a wide range of current draws (charging much higher than recieving, for example), and the drop you get from a resistor depends on the draw.

If it's 1A constant, I'd think 0.2 ohms (!), in a half-watt format, would do the job, but I bet it's not a constant 1A.

If the case is metal, you may get away with sinking the regulator on the case.  A hole, a little thermal paste, and a bolt to mount it.  Added benefit:  it won't rattle around loose.
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Matt_T_Bratt

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12v down to 5.8 volts at 1A? *updated x2 - last post
« Reply #6 on: December 26, 2004, 01:44:14 PM »

You might check the voltage coming off the power supply for the Duo.  If it says 5.8V my guess would be it actually puts out 6.3 or so (something higher, anyway)
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Daneel

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12v down to 5.8 volts at 1A? *updated x2 - last post
« Reply #7 on: December 26, 2004, 09:42:27 PM »

:idiot: hak foo pointed it out, i just realised that there wouldn't be a constant current draw, so the resistance would vary as the draw changed for 1 amp to 50mA when not charging.

looks like variable voltage regulator is the only way to go.

At the moment i'm looking at an LM317K or an LM317T, are these the right thing?

please recommend something.

thanks
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Daneel

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12v down to 5.8 volts at 1A? *updated x2 - last post
« Reply #8 on: December 28, 2004, 12:13:27 AM »

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viridius

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12v down to 5.8 volts at 1A? *updated x2 - last post
« Reply #9 on: December 28, 2004, 04:46:04 AM »

They're pretty much the same thing.
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