ATX power supply 5V load resistor for better 12V regulation

4R7 Ohm 10 watt wire wound resistor attached to 5 volt pins on ATX power supply molex connector

4R7 Ohm wire wound load resistor on 5V line of ATX power supply

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.

Result?

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.

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7 Responses to ATX power supply 5V load resistor for better 12V regulation

  1. Pete says:

    Hi there, instead of worrying about dissipating waste heat from the ballast resistor, I used it to heat the bed, meaning I could use a lower power 12V heater – double win. Details here: http://tatmonkey.blogspot.co.uk/2013/03/reprap-heated-build-platform.html

  2. Notry says:

    Useless … because the components are not greedy for a fixed +12Vcc, anything even lower is good, they regulate it anyway to an even lower voltage.

    • I should have replied to this ages ago, because it is just wrong! While the heat bed and nozzle don’t care what voltage they get, they can’t heat up fully if they don’t have enough electrical power.

      Power = Voltage X Current. For a fixed resistance (which the heat bed element and the nozzle heater wire both are), if you reduce the voltage you reduce the power dissipated (that is, the heat). That means it takes longer to reach the precise temperature which is essential for melting the filament correctly.

      In some cases, with an ATX power supply that is only borderline powerful enough, losing one volt from the 12 volts of your heat bed supply may mean that the bed will never make it up to operating temperature. The ballast resistor on the 5V line ensures you get as much voltage (and therefore as much heating power) as your supply is capable of producing.

      And as for the bit about ‘they regulate it anyway to an even lower voltage’, we are talking here about a resistor (the heat bed element), not the operating voltage of a microcontroller. There is no ‘lower voltage’ involved.

  3. heat bed says:

    I was able to keep my heat bed powered on with just 50-100ma current draw or a 100-200 ohm resistor. less waste, smaller (250ma) resistor. still gets hot, ouch touch.

    • Did you mean 250mW resistor? 5 volts at 0.1 amps is 0.5 watts, so a quarter watt resistor is well outside its comfort zone!

      Sure, the heat bed will get some power, even without a load on the 5 volt line. What I’ve read suggested you needed to draw more like an amp from the 5 volt lines to get the ATX voltage regulation working optimally. Still, if it ain’t broke don’t fix it, I guess.

  4. nald says:

    what happen if i put 25 watts 15 ohms to the red wire…???

    • Basic Ohm’s Law stuff declares that I = E / R, and P = I^2 x R. Using a 15 Ohm resistor instead of my 4.7 Ohm resistor would draw I = E/R = 5V / 15 Ohm = 333 milliamps, and dissipate P = I^2 x R = 0.111 * 15 = 1.6W,

      So a 15 ohm 25 watt resistor will stay comfortably cool, but drawing only a third of an amp of current might not help much with your 12 V regulation. If you happen to have a 15R 25W resistor in your parts bin, you could try it out and see if it makes any difference. If you have to buy something specially for this, I’d suggest a 4R7 like mine.

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