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Old 02-06-2007, 09:14 AM
betheballdanny is offline
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Re: FrankenPower! Building a 12V Power Supply

I already worry about my LiPo's burning my house down, I don't want to worry about a PS doing the same.

It because of the Wattage (Power) rating on the R. Any PS loaded will drop some, but not much.

Lets say you use the 1W 10k R (10000 Ohm) Resistor

So, V=I * R ---> I=5 / 10000 --->.0005

P=V * I

So, P=0.0005 * 5 ----> .0025

So, a 1W 10k R would work well

...even a 1/4 W would be fine

…and not heat up.
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Old 02-06-2007, 09:16 AM
betheballdanny is offline
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Re: FrankenPower! Building a 12V Power Supply

I think we have covered the bases here, just make sure you R doesn't heat up too much - if you can't hold it in your hand - use another line.

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Old 02-06-2007, 09:58 AM
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Re: FrankenPower! Building a 12V Power Supply

That works, but I wonder why I've done over 20 of these and have never had a resistor get too hot...pure luck?
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Old 02-06-2007, 10:10 AM
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Re: FrankenPower! Building a 12V Power Supply

Craig, I measured the voltage on the sandbar last night while charging up all my lipos...stayed right around 5V. Didn't jump at all. Resistor stayed warm, but not hot.
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Old 02-06-2007, 01:14 PM
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Re: FrankenPower! Building a 12V Power Supply

FWIW,

Whether a 10 ohm 2.5w resistor or a 10 ohm 10w resistor is used, they will both generate the same heat (wattage) when connected to the same 5v supply. The difference is that the smaller wattage resistor is also smaller in size, so a smaller surface area must dissipate the same heat and will therefore have a higher surface temperature. And, as with most things electronic, the hotter it gets, the shorter the potential life span. If you can afford it and have the room for it, use the 10w resistor.

As for whether to connect the resistor to a 2A or a 20A 5v circuit, it does not matter. Just because the circuit is capable of producing 20 amps does not mean it will supply 20 amps to any load you connect to it. It depends on the load. Just like using your lipo charger. It may be rated for 10A input, and a car battery can produce several hundred amps, does not mean you are going to blow your charger if you connect it to your car battery.

I was in a field service class years ago, and the equipment we were being trained on had a 5v 80 amp output. Some guy in the class said "you sure don't want to grab that 5v line" and I said "5v is not enough to shock anybody" and he said "it ain't the voltage, its the current that kills you". Sigh.

I have a couple of old computer power supplies, probably 300 to 500 watts. I'm going to try this when I get a chance. Should not need the particular power supply that has been mentioned, just that it is nice to see the actual mods that are needed to get this one to work. I may have to do a little figuring to make a different one work.

Bob R.
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Old 02-06-2007, 01:36 PM
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Re: FrankenPower! Building a 12V Power Supply

While I'm not an EE, I do have a level of comfort of the volitility of a PC PS as compared to a lipo battery. Don't forget folks, there is a fuse in these things. Great thread BTW!
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Old 02-06-2007, 01:54 PM
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Re: FrankenPower! Building a 12V Power Supply

Too Bad I already Have this http://www.millionbuy.com/pyrps14k.html or I would be building one of these.

Has anybody tried the ATX computer power supply with a triton? A typical car battery puts out over 13 volts and My Power supply puts out 13.8 volts. I know when I connect the Triton to a car battery and the car battery approaches 12vdc the Triton errors out and states low voltage.
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Old 02-06-2007, 02:02 PM
Mike Bargman is offline
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Re: FrankenPower! Building a 12V Power Supply

I've got a friend that uses his Triton with his ATX Supply and it's fine.
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Old 02-06-2007, 03:18 PM
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Re: FrankenPower! Building a 12V Power Supply

Quote:
Originally Posted by Dangerous Dan View Post
Has anybody tried the ATX computer power supply with a triton? A typical car battery puts out over 13 volts and My Power supply puts out 13.8 volts. I know when I connect the Triton to a car battery and the car battery approaches 12vdc the Triton errors out and states low voltage.
You may have some voltage loss in the cables running from the battery to the charger, causing the low voltage error. Shorter, larger gauge cables should help.

If you check the voltage of the battery while the car is running, you will probably find the voltage is closer to 13.8v, that is the nominal charging voltage for lead-acid 12v batteries. That is why most of the "12v" power supplies like yours are actually 13.8v, that is the power usually seen by products that plug into your cigarette lighter while you are driving, and that is usually the design voltage for normal operation. A fully charged 12v battery, at rest (no load or charger applied to it for some period of time) should be around 12.6 to 12.7v.

Bob R.
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Old 02-06-2007, 06:03 PM
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Re: FrankenPower! Building a 12V Power Supply

This is what I have to use with the power supply. This is from Deans.

Wet Noodle
The ultimate wire for full race applications. 12 Gauge silicone wire with an unbelievable 1660 strands, it is so flexible that it feels like a wet noodle! The highest grade of copper and pure silicone insulation provide maximum power and easy soldering.

Wet noodle is available in three colors: red, black, & blue.

http://www.horizonhobby.com/Products...ProdID=WSD1410
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Old 02-06-2007, 08:32 PM
Mike Bargman is offline
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Re: FrankenPower! Building a 12V Power Supply

I thought Wet Noodle meant something else...
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Old 02-07-2007, 01:05 AM
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Re: FrankenPower! Building a 12V Power Supply

does every desktop computer have one of these, i need something to do and there is a old computer in my garage
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Old 02-07-2007, 09:23 AM
Mike Bargman is offline
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Re: FrankenPower! Building a 12V Power Supply

Yes...pretty much every desktop computer has one. Remove it and look at the specs on the side and post them here and I'll tell you if it'll work...
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Old 02-07-2007, 04:37 PM
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Re: FrankenPower! Building a 12V Power Supply

it's 250 watts but.. im using it o charge lipos will it work? the lipos are all under 1000mah and i'll anly charge like say 4 at a time
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Old 02-07-2007, 10:56 PM
Mike Bargman is offline
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Re: FrankenPower! Building a 12V Power Supply

It should be fine. If you look on the side or bottom of the PS. There should be a sticker or label that tells you the Amperage rating of each voltage. For Example +12V = 24A
-12V = .8A
+5V = 4A..

Something like that...
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