Various people (Triffid_hunter in particular) and web sites (e.g. the reprap wiki) have explained that ATX power supplies are a bit cheap in their voltage regulation circuitry.
If you want to get the most power out of your 12V cables (for your extruder nozzle, motors and especially your heatbed), you have to draw some current from the 5V wires as well. Stupid, but that’s how the ATX is designed.
Triffid_hunter suggests using a 12 V 50 Watt halogen bulb, which has the added advantage of giving you a light source to watch your printer make pretty objects.
Choosing a load resistor
I chose a slightly simpler approach:- a 4.7 Ohm resistor. (The standard now says we write that as 4R7, in case the printing of the decimal point is unreadable.)
The one I bought for less than $2 from my local Jaycar store is a 4R7 Ohm 10 Watt wire-wound resistor with a tolerance value of 5%. You can see it in the photo.
If my maths is correct, 5 volts into a 4R7 ohm (+/- 5%) resistor draws between 1.0 and 1.12 amps, which is just about what I wanted. The power dissipation should be around 6.27 watts. That is safely under the resistor’s nominal power dissipation of 10 watts, but it will get hot. You could run two 10 ohm 10 watt resistors in parallel, to spread the heat out.
Dealing with the heat
Left out in still air, my 4R7 gets hot enough that touching it is painful, but doesn’t raise blisters. Taking a hint from the reprap wiki, I physically wired it to the exhaust air ventilation grill of the power supply, as shown in the photo.
This keeps it in a constant stream of moving air, and allows even more cooling through physical contact with the large metal case. It may not be child-safe, but then the whole printer is not child-safe 🙂
Wiring it up
Note that the resistor is not electrically connected to the ATX case. The only electrical connections are to the 5V (red) pin on the molex connector, and the 0V (black) connector next to it.
I bought the white molex connector in the photo from Jaycar too – it’s a standard PC part. You can see how I labelled the connector before attaching the crimps to the resistor.
Don’t accidentally attach it to the yellow (12V) pin, as 12V will cause the resistor to carry more than its rated current, and won’t help you with your regulation at all.
I used heat-shrink tubing to insulate the resistor leads. The leads don’t seem to get hot, so hopefully the heatshrink won’t overheat.
From an unloaded voltage of 11.65 from my 12 volt connectors without the load resistor, I’ve now got 12.5 volts unloaded.
More importantly, with my nozzle and heatbed both running, the 12 volt lines now stay closer to 12 volts. With less voltage drop I’ve got more power going to the nozzle and heatbed, so they heat up faster. If I switch to using ABS instead of PLA at some point, that extra available power could be very important.
I’ve read that some ATX power supplies will shut down completely if too much current is drawn from the 12 volt lines without at least an amp drawn from the 5 volt line too. From first-hand reports, that tends to happen part-way through long prints. I’m pleased to think that shouldn’t happen to me.