Hello, i searched but couldn't find the answer to my question so im going to ask it now 
.
What happens when you put a lower psu than what you need, i have 2 power supplies a 250v and 185v (i know they are old)
To know requires a 3.5 digit multimeter. Then measure each critical voltage with computer at idle and under maximum load (multitasking to all peripherals simultaneously).
I assume you meant watts - not volts. Watts says little to nothing that is useful. In fact, a supply rated by one at 350 watts is also rated by another as 500 watts. And neither has lied.
Relevant is current for each DC voltage. Each critical voltage will appear on same colored wires. Measure any one of purple, orange, red, and yellow. For example, three voltages could have more than enough power (current or wattage). And the fourth might be undersized – on a supply with higher watts. Worse, you would not know. Normal is for an undersized power supply to still boot a computer. To learn, you need numbers from the meter.
Only way to learn if each voltage has enough current: buy a meter. Meters are only sold in stores that limit their customers by IQ - such as Kmart, Sears, Lowes Radio Shack, etc. Best prices will probably be less than $18 in Wal-Mart. Get a meter if your objective is to learn. And then learn more from the few who better know this stuff by posting those numbers. No numbers (no meter) means your responses will be limited to wild speculation.
