FrankenPower! Building a 12V Power Supply

[betheballdanny]

Wooaaaahhh, hang on everybody!

Ckoerner, thanks for the Ohms Law review, but there is more to this than V=I*R

Mike, Whoever chose the 10W 10 Ohm Resistor knew what he was doing (was this you?), because he knew to sink 2 Amps on a 5V line.

P=V*I

10W=5V*2A

The 10W 10 Ohm R can only sink 2A. If you use the 5V 30A line, you would need a 150W 10 Ohm R, or you will blow the 10W 10 Ohm R by turning it into a heater.

SO, you have to be sure to use the 5V 2A line across the 10W 10 Ohm R to ground, otherwise the 10W 10 Ohm R will act as a heater if you try to sink 30 A with it, aka the wrong red wire...enough to burn a hole in the plastic lid.

I should have the PS this week - I'll post again once I break into it and see for myself.

[ckoerner]

5V line with a 10ohm resistor =.5Amps load.
Wattage=V*I or I^2*R 5V*.5A=2.5W,,,,,,.5V^2*10=2.5W

10W is what the resistor can handle.............How much heat it can disapate.


You put a 5W resistor on it, it still will draw the same current, but will be a little warmer

[ckoerner]

It would take a 2.5ohm resistor to pull 2A

[Mike Bargman]

It's not going to matter how much current is running thru the resistor, it's going to depend on the voltage. Now, I'm going to charge up some batteries later on and see if the voltage on the resistor increases. The resistor does not get hot at all. It gets a little warmer to the touch, but not hot by any means. I've built over 20 of these PS's and never had any problems...

[Chris Puckett]

This thing is going to be all tested up by the time I get it. :-( LOL

[ckoerner]

	Quote: Originally Posted by betheballdanny
	P=V*I 10W=5V*2A

The power supply is rated for 10W. 5V*2A=10W is correct, but when you put a 10ohm resistor you take the 5V\10ohms=.5Amps. So the resistor will put a .5amp(2.5W) load on the 5V power supply. The resistor really only needs to be rated over 2.5W. But it would be pretty hot.

[Mike Bargman]

So a 20W resistor would stay fairly cool then, right?

[ckoerner]

Yep!

[madmax]

Whoa! Hey fellas, we're not making a reinventing the Jarvik heart, or improving on space shuttle components here.. Plug the sucker in and see if it works, and Mikey..... THANK YOU for finally doing this. Very cool article, post, and incredibly usefull information.

[Mike Bargman]

I'm not worried about it, Max. They're legit questions, but I've done a ton of these and have never had problems. I'm gonna do the 5V mod tonight and see how it works!

[Wayne]

Would it possible to take one of these, power a Li-po charger and charge Li-Poly batteries?

[betheballdanny]

Mike, thanks again for taking the lead on this - the pay off is great. We tested with an older 200W PS and checked all the 5V lines (all red), but since we couldn't determine which one was the .73A line, we were taking spears. Using the Sandbar 10W10 O R, it reached 400 Deg F within a min after power on - so we shut OFF, so it does make a difference - folks doing this should know.

You want to find/use the line VS, the S in the Safety circuit. As for this PS it unloads at 0.72A - we must have picked the 22A line that superheated the Sandbar R.

So, it's trail and error if the lines are not labeled on your PS - until you find a line that doesn't heat up the R - preferably the VS line

OR

Use a 1W 10k Ohm R, my EE buddy said that should do the trick on any line and not heat up.