richms: People say that but I've not had a problem with 20+ amps on the 12v line and nothing on the 5 or 3.3v.

really depends on the power supply, some do some dont

Jase2985:

 

mb82:

k1w1k1d:

 

The variable voltage is easy to do with one of these. I just unsoldered the blue 10k pot from the circuit board and ran wires to a 10-turn 10k pot on the front of the box.

 

 

 

http://www.ebay.com/itm/2PCS-150W-DC-DC-Boost-Converter-10-32V-12-35V-6A-Step-Up-Power-supply-Module-/400442480064?hash=item5d3c3b55c0

 

How do I calculate the current draw on the 12v rail to get 24v @ 5 amps on the output of the dc converter.

Also the pot you are referring to is that on the dc converter itself?

 

V=IxR or I=V/R or R=V/I

 

R=V/I , R=24/5, R= 4.8

I=V/R, I=12/4.8, I= 2.5    so 2.5Amp current draw at 12v to produce 24v @ 5 Amps?

I do not think I am doing it right?

mb82:

 

R=V/I , R=24/5, R= 4.8

 

I=V/R, I=12/4.8, I= 2.5    so 2.5Amp current draw at 12v to produce 24v @ 5 Amps?

 

I do not think I am doing it right?

 

wrong formulas, wrong way around.

The power is the same, P=V*I, so 24v * 5 is 120 watts.

I = P / V  so 120 / 12 = 10 Amps

Then 10A multilplied by the inverse of the efficiancy so say 85% means

10 * (1/0.85) = 11.77 A

I would also not recommend any of the "boost/buck" converters for making an adjustable power supply. I got one and found the current limit became very very erratic when your in and out voltage was about the same, I cooked some 10w LEDs that way with it limited to 900mA fine when it was shorted or into a load that it clamped at well below 12v, but the series voltage of 3 LEDs is right around 12v and I had them start to pulse really brightly and blow after some time. 2 converters and about 6 LEDs later I decided to just use a buck only one and it didnt happen again, asked around and a few other people had seen the same thing happen.