# Computer Power Supply to bench top theory

I want to use my Corsair 430CX as a benchtop power supply to do small circuit work. I am fuzzy on the theory however when it comes to the amount of amperage that the rail can support. It worries me, the amount of current possible that can come out of these rails and i want to know what I am dealing with in terms of working with small circuits.

I understand ohms law to the extent that the voltage should be the determining factor in whether or not i release magic smoke, I mean this in the sense that wattage or volts * amperage does not come into play when working with circuits. I may be entirely wrong on this front. I picture myself connecting a 1/8 watt or 1/4 watt 1MOhm resistor across a 50aH 12v car battery and can't believe in the math. I feel like I would release magic smoke and endanger myself.

Can someone validate the theory or enlighten me because all that I have read so far makes me think I can create small circuits with a 12v 30amp rail and 300+ watts and be fine when I picture it being very different from working with a small a23 12v battery. Surely these cannot be the same things, can they?

• If a 1/4 watt megohm resistor would blow up, so would you car radio, your headlamp, etc. Ohm's law works. If you put 12 volts across that resistor Ohm's law (E=I*R --> I=E/R) says the current will be 12/1,000,000 = 12 micro amps. However, as @PeterJ points out, the danger is what happens when you accidentally short out the power source. Shorting out a car battery is much more dangerous than shorting out a small 9V battery. Commented Mar 7, 2015 at 6:03
• @DoxyLover sorry brother, you are wrong. All of those things you listed still condition the power they need. They don't just accept it. Go connect it against a car battery if you don't believe me. Commented Mar 7, 2015 at 19:05
• sorry, no. Let's ignore the radio for now and focus on purely resistive loads, such as the 1megohm resistor and the headlamp. These don't care if the power source provides 12 volts at 10 amps or 1,000 amps. The current drawn is limited by the resistance of the load, as determined by Ohm's Law. (This assumes the power source can supply enough power for the load, which is why I specified 10 amps for the headlamp. For the resistor, it doesn't matter if the source is 8 AAA cells in series or 8 car batteries in parallel, it still draws only 12 micro amps and dissipates 144 micro W.) Commented Mar 7, 2015 at 19:39
• @DoxyLover and that's why you ignored the radio, it probably conditions the power. Commented Mar 7, 2015 at 22:11
• The same basically holds true for the radio, it's just harder to understand, which is why I skipped it. In any case, my point is that the resistor you brought up is no different than anything else. It doesn't matter how much current is "offered" by the power source, a load will only consume as much as it "wants" and, in the case of a resistive load, that "want" is dictated by Ohm's Law and is therefore, proportional to the voltage, only. Commented Mar 8, 2015 at 1:16

The math works and there's a good question / answer Choosing power supply, how to get the voltage and current ratings? that should help reassure you on that side of things. Powering a small circuit from a power supply is not an issue with one exception I can think of:

The power supply will still be capable of delivering a large amount of current in short circuit and other unexpected conditions that present as a low resistance. I have a 40A bench supply and recently while testing something used some low-current test leads and fried those in an instant when I accidentally shorted them. Some circuit failures can also be more dramatic / dangerous when a large amount of current is available.

For that reason for use as a general bench supply I'd recommend adding a fuse, it depends on what you're doing but perhaps a 500mA - 1 A fuse would be a good choice because that should blow before test leads and component leads that short have a chance to melt.

• Thank you very much! Polite and respectful answer! I appreciate your help tonight sir! Commented Mar 7, 2015 at 5:35
• Again, I just finished reading the link you provided and definitely can see the wisdom in the inline fuses. I will definitely be using them!! Thank's again! Commented Mar 7, 2015 at 5:40
• I tried using the 12v car battery in inline with a 100 Ohm resistor to truly test this theory that all would be fine but it started to smoke. I had my ammeter in line and it said it was drawing some small amount of current. I can't remember exactly since I was so overly focused on safety. I had a 1k Ohm in earlier and it was fine. I feel like this is the same inherent danger that I am dealing with with a pc benchtop power supply. Not sure where to go from here. Current limiting the powersupply somehow? I like the power supplys online that can only do 2Amps max since I feel like if I make a Commented Mar 7, 2015 at 19:01
• mistake, I wont be punished as bad for it. Any ideas as to why it smoked? Commented Mar 7, 2015 at 19:02
• Edit, Actually I see it happening in multisim right now. It keeps blowing the resistor as well because it is 1/4 watt. Does this mean it would blow with a 12v a23 battery as well? Commented Mar 7, 2015 at 19:04